Alumni

Select year
Degree Date: December, 2022
Benjamin Foust's picture
Benjamin Foust
Thesis Advisor: Karsten Heeger
Postdoctoral Researcher
Pacific Northwest National Laboratory
Thesis Title: Precise Measurement of the U-235 Antineutrino Energy Spectrum
Thesis Abstract: Neutrino experimentation is an important pathway to physics beyond the standard model, raising questions on how neutrinos obtain their vanishingly small but non-zero mass, what that mass is, and if they are their own antiparticle. Antineutrinos, as an abundant product of nuclear decay chains, are also important to advancing our understanding of nuclear physics and verifying nuclear databases. In the field of reactor antineutrinos, there are two outstanding anomalies experiments are attempting to address. Previous antineutrino experiments at low enriched uranium power reactors have shown a global flux deficit at various baselines from reactors, hinting at a possible new type of neutrino called a sterile neutrino. Additionally, measured reactor neutrino spectra have a poor overall fit to theory, and observe an excess of events in the 4-6 MeV prompt energy range. Due to the existence of multiple isotopes present in the reactors, the source of the discrepancy is unclear. By making a precision spectrum measurement at a research reactor composed of highly enriched Uranium, the PROSPECT experiment helps clarify both of these anomalies. PROSPECT’s latest published spectrum result is composed of over 50000 candidate antineutrino events with a signal to background ratio greater than one. Future work with an improved spectrum measurement will further constrain antineutrino flux calculations and lead to a more refined eV-scale sterile neutrino prediction or exclusion. In this thesis, I give an overview of the current state of reactor neutrino physics, the PROSPECT detector design and analysis, and recent results. I next discuss my work in making a joint analysis of the U-235 neutrino spectrum with the STEREO collaboration. This work finds good compatibility between the two experiments’ spectrum measurements. The prompt measurements are combined in a process called joint unfolding to remove the effects being contributed by the separate detectors and reactors. The resulting joint spectrum constitutes the most precise pure U-235 antineutrino measurement to date. The PROSPECT experiment and this thesis work have made major strides in the search for sterile neutrinos and the source of the reactor antineutrino spectral shape discrepancy. Further, a path towards the resolution of both these anomalies is set with future efforts from the PROSPECT collaboration.  
Degree Date: May, 2022
Emma Castiglia's picture
Emma Castiglia
Sarah Demers
Research Data Scientist
Meta
Machine Learning for Tau Leptons and the Search for the Associated Production of a Higgs Boson with a Vector Boson, with the Higgs Boson Decaying to a Tau Pair at ATLAS
After the discovery of the Higgs boson in 2012, many particle physicists have focused on measuring its various production methods and decay modes. One channel that has previously only been measured at well below the observation threshold is the associated production of a Higgs boson with a vector boson, where the Higgs boson decays to a pair of taus. In this thesis, I describe an ongoing search for that channel using data from proton-proton collisions taken during Run 2 at the Large Hadron Collider with the ATLAS detector (2015-2018), with a recorded integrated luminosity of 139$fb^{-1}$ at a center of mass energy $\sqrt{s}=13$TeV. In the analysis, the W boson must decay to a light lepton (e/$\mu$) and a neutrino, while the Z boson is required to decay to a pair of electrons or muons. At least one of the taus from the Higgs boson must decay hadronically. The background from misidentified electrons, muons, and taus is modeled by a data-driven Fake Factor method. The remaining backgrounds are primarily from dibosons, and these are modeled with Monte Carlo simulation. Machine learning has become an integral tool across many experiments in particle physics, including the ATLAS experiment. In this thesis, I describe how neural networks and decision trees are used to improve tau identification and the calibration of the tau energy, as well as ongoing studies for future improvements. Machine learning algorithms are also instrumental in separating out a complicated signal from a very similar background. In the Higgs boson analysis detailed in this thesis, the score from a neural network trained to target the diboson background is used as the final discriminating variable.  
Emily Kuhn's picture
Emily Kuhn
Laura Newburgh
NASA Postdoctoral Program (NPP) Fellow
NASA Jet Propulsion Laboratory (JPL)
Calibration Instrumentation for the Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX)
The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) is a 21 cm neutral hydrogen intensity mapping experiment to be deployed in the Karoo Desert in South Africa. It aims to improve constraints on the dark energy equation of state through measurements of large-scale structure at high redshift, while doubling as a state-of-the-art fast radio burst (FRB) detector. This dissertation focuses on two aspects of the HIRAX instrument characterization: (1) optimizing the signal-to-noise of antennas, through the design and implementation of a custom test-bed for determining the noise temperature of radio antennas operating between 400-800MHz, and (2) mapping the HIRAX telescope beam pattern with a custom drone calibration system. The work described is critical to HIRAX’s development, both by informing final antenna design and providing the tools to generate beam maps that will factor into all cosmological analysis.    
Michael Oliver's picture
Michael Oliver
John Harris
Postdoctoral Associate (Caines)
WL-W 303
Measurement of Correlations Between Neutral Pions and Charged Hadrons in Heavy Ion Collisions with ALICE
In Ultra-Relativistic Heavy Ion collisions, such as those done at the Large Hadron Collider (LHC) and Relativistic Heavy Ion Collider (RHIC), the high energy densities create an exotic state of matter not seen since the first 10-4 seconds past the Big Bang, a Quark Gluon Plasma (QGP) where quarks and gluons are not confined into hadronic bound states. The properties and evolution of this matter can be studied using a naturally existing probe: the hard QCD (Quantum Chromo-Dynamics) jets that are produced in partonic hard scatters at the beginning of the collisions. Similarly to the x-rays in medical Computed Tomography, the escaping jets reflect the transverse structure of the medium. However, this analogy breaks down in two key ways. The QGP, unlike the human body, is rapidly evolving on the same timescale of the jet’s passing through of the medium. Additionally, the interaction of the jet with the QGP is not fully understood and may modify the structure of jets beyond a simple attenuation. The field of studying these jet-medium interactions, called jet tomography, is advanced by the research in this thesis using correlations high momentum p0 mesons and hadrons arising from the same jet-producing hard scatter process. The focus in this study is on experimentally varying the path-length traversed by the involved jets by examining the correlations with respect to the reaction plane of the colliding ions. This is done using Pb-Pb collisions measured by ALICE detector at the LHC at √SNN = 5.02TeV.
David Stewart's picture
David Stewart
Helen Caines
Postdoctoral Associate
Wayne State University
Jet to Event Activity Correlations in Small System Collisions at STAR
Heavy ion collisions at the LHC and RHIC produce a quark gluon plasma (QGP), in which quarks and gluons are deconfined into an extended medium. This “fourth phase” of matter is also believed to have been the first material phase of the universe following the Big Bang. In experiment, high energy partons scatter at short time scales and may subsequently lose energy, or are “quenched”, via interactions with the QGP. We approximate these high energy partons by “jets,” the combined four-momenta of collimated sprays of final state particles, and use these jets to probe for QGP formation and properties. We present semi-inclusive jet measurements from √sNN = 200GeV proton+gold (p+Au) collisions recorded by STAR at RHIC in 2015, which are the first semi-inclusive jet measurements for small system collisions (p/d/He3+A) at RHIC kinematics . These measurements demonstrate that, within the precision allowed by statistics and systematic uncertainties, there is no jet quenching in these small system collisions. We also present measurements of significant, and previously unexpected, correlations between the semi-inclusive jet momentum spectra and collisional event activity. We demonstrate how these results support the conclusion that correlations between jet formation and EA in small systems offer an opportunity to probe not the QGP, but rather the early initial stages of these collisions.  
Joseph Sullivan's picture
Joseph Sullivan
Meng Cheng
Postdoctoral Fellow
University of British Columbia
Quilting topological phases of matter with quantum thread: a Luttinger liquid love letter
Kicked off by the discovery of the quantum Hall effect in the early 1980s, the study of topological phases of matter has captured the attention of the condensed matter physics community for over four decades. With topologically ordered phases, symmetry-protected topological phases and, most recently, fracton phases, examples of states of matter beyond the Landau-Ginzburg symmetry breaking paradigm abound. One approach for constructing these novel states of matter is to employ a layered approach; 2-dimensional phases can be built by coupling together 1-dimensional “wires”, 3-dimensional phases can be built by coupling together 2-dimensional “layers” and/or 1-dimensional “wires” and so on. Two major advantages of this approach are its analytical tractability and its ability to describe chiral phases. In this talk we will make use of these constructions to study several examples, both new and old, of exotic strongly coupled quantum phases of matter.
William Thompson's picture
William Thompson
Reina Maruyama
Postdoctoral Associate
Harvard
Searching for Dark Matter with COSINE-100
For more than two decades, the DAMA collaboration has claimed a direct discovery of dark matter. This claim comes in the form of an annual modulation in the event rate of a sodium iodide-based dark matter detector that has persisted for more than two decades. Currently, this modulation is observed at a significance of 12.9 σ. While such a discovery would be a milestone in physics, significant doubt has been cast on the validity of DAMA’s discovery claim, as it is in conflict with the null results reported by other direct detection experiments. Despite this tension, a direct test of DAMA’s claim has not previously been possible, as no other dark matter experiment utilized the same target material, sodium iodide. To resolve this stalemate in the field, the COSINE-100 collaboration is operating a NaI(Tl)-based dark matter detector to perform a definitive test of DAMA’s claim of dark matter discovery. In this dissertation, we present new constraints on the annual modulation signal from a 173 kg·yr exposure with COSINE-100, acquired over the first 2.82 yr of data-taking. This new result features an improved event selection that allows for both lowering the energy threshold to 1 keV and a more precise time-dependent background model. In the 1–6 keV and 2–6 keV energy intervals, we observe best-fit values for the modulation amplitude of 0.0067±0.0042 and 0.0050±0.0047 counts/(day·kg·keV), respectively, with a phase fixed at 152.5 days. In addition, we present the results of two enabling experiments for current and future NaI(Tl)-based dark matter searches. First, we detail results from a measurement of the sodium quenching factor across multiple NaI(Tl) detectors. We find no significant variation in the quenching factor between detectors, affirming the COSINE-100 and ANAIS-112 experiments as direct tests of DAMA’s dark matter discovery claim. We also report results from a measurement of the activation rate of cosmogenic radioisotopes in NaI(Tl) detectors performed using artificially activated detectors. The results of this measurement will enable the future deployment of lower-background NaI(Tl) dark matter detectors.  
Chris Wang's picture
Chris Wang
Robert Schoelkopf
Grainger Postdoctoral Fellow
University of Chicago
Bosonic quantum simulation in circuit quantum electrodynamics
The development of controllable quantum machines is largely motivated by a desire to simulate quantum systems beyond the capabilities of classical computers. For investigating intrinsically multi-level model bosonic systems, using conventional quantum processors based on two-level qubits is inefficient and incurs a potentially inhibitive mapping overhead in the current “near-intermediate scale quantum” (NISQ) era. This motivates the development of hybrid quantum processors that contain multiple types of degrees of freedom, such that one can leverage an optimal one-to-one mapping between the model system and simulator. Circuit quantum electrodynamics (cQED) has emerged as a leading platform for quantum information processing owing to the immense flexibility of engineering high fidelity coherent interactions and measurements. In cQED, microwave photons act as bosonic particles confined within a nonlinear network of electromagnetic modes. Controlling these photons serves the basis for a hardware efficient platform for simulation of naturally bosonic systems. In this thesis, we present two experiments that encapsulate this idea by simulating molecular dynamics in two different regimes of electronic-nuclear coupling: adiabatic and nonadiabatic. In the first experiment, we implement a boson sampling protocol for estimating Franck-Condon factors associated with adiabatic photoelectron spectra. Importantly, we fulfill the scalability requirement by developing a novel single-shot number-resolved detector for microwave photons. In the second experiment, we develop and employ a model for simulating dissipative nonadiabatic dynamics through a conical intersection as a basis for modeling photochemical reactions. We directly observe branching of a coherent wave-packet upon passage through the conical intersection, revealing the competition between coherent evolution and dissipation in this system. The tools developed for the experiments in this thesis serve as a basis for implementing more complex bosonic simulations.  
Luna Zagorac's picture
Luna Zagorac
Nikhil Padmanabhan & Richard Easther (University of Auckland)
Postdoctoral Fellow
Perimeter Institute of Theoretical Physics
Research Website
A Light in the Dark: UltraLight Dark matter Phenomenology in Simulation
Of the outstanding problems in astronomy, the nature of dark matter is certainly one of the most mysterious. Containing five times more energy density than its luminous counterpart, dark matter has been shaping the large-scale structure of our Universe for billions of years. The expansion of accessible and accurate cosmological simulations has revolutionized how we visualize the imprint of dark matter in the structure of our Universe. In my Ph.D., I contributed to this revolution through the development and implementation of a new code, CHPLULTRA: a parallel, portable, and efficient tool for HPC simulations of a promising dark matter candidate, Fuzzy or UltraLight Dark Matter (ULDM). ULDM is a well-motivated axion-like dark matter candidate whose incredibly small mass results in naturally cored profiles, thus ameliorating many of the small-scale problems of cold dark matter (CDM) while maintaining the same robust large-scale results. When unperturbed, the lowest energy solution of the ULDM system is a spherical “soliton” structure with a known mass density profile. A ULDM dark matter halo is formed through collisions of these solitons and has two characteristic parts: a central soliton core, and an NFW “skirt” surrounding it. In order to investigate ULDM dynamics, I calculated the full spectrum of eigenstates for ULDM systems with approximately stationary potentials, thus allowing me to 1) link the qualitative behavior of soliton cores in ULDM simulations with superpositions of specific modes and 2) decompose CHPLULTRA simulations of ULDM halos into individual eigenstates. Using this formalism, I investigated the formation of halos through soliton collisions and the dependence of the final halo product on initial parameters. Crucially, this allowed me to explore how halo cores form and explain discrepancies in the literature surrounding the core-halo mass relation: a key prediction of ULDM. I was also able to comment on the composition of the halos’ skirts, including their qualitative behavior and eigenstate makeup, as a function of initial binary parameters. Finally, I sketched out some of the exciting future directions for understanding ULDM through the language of its eigenstates; these include combining my work on ULDM with my previous work on primordial black holes, which is included as part of this dissertation.
Degree Date: December, 2021
Han Aung's picture
Han Aung
Daisuke Nagai
Postdoctoral Associate
Hebrew University of Jerusalem
Research Website
Cosmology and Astrophysics with Dark Matter and Gaseous Halos
Multi-wavelength astronomical surveys promise to provide unprecedented data on dark matter halos from galactic to cluster scales in the coming decade. One of the new frontiers lies in studies of intracluster medium (ICM) and circumgalactic medium (CGM) for cosmology and galaxy formation. In this work, we will combine cosmological and idealized simulations to study the non-linear growth of structures and the effects of small scale astrophysics in the realistic cosmological settings. Specifically, we will investigate the connection between the dark matter halos and the cosmic web structures, focusing on the outer boundaries of dark matter and gaseous halos around cluster-size dark matter halos and the filamentary cold gas accretion feeding highredshift galaxies. We show that the edge of the phase space structure and accretion shock radii denote the outer boundaries of the halos and have the potential to advance cluster cosmology. We also investigate the roles of hydrodynamic and gravitational instabilities on the survival of cold gas streams feeding high-redshift galaxies. My research highlights the importance of understanding the growth of dark matter halos and their interactions with the cosmic web structures for advancing cosmology and astrophysics in the era of multi-wavelength astronomical surveys.
Kelly Backes's picture
Kelly Backes
Steve Lamoreaux
Quantum Sensor and Security Specialist
MITRE Corporation
A quantum-enhanced search for dark matter axions
Almost a century after the dark matter problem was first posed, dark matter’s expectedly minute experimental signature continues to elude direct detection and remains one of the most profound mysteries in fundamental physics. The QCD axion serves as a potential solution to the dark matter problem as well as an entirely unrelated problem in fundamental physics: the strong charge-parity (CP) problem of quantum chromodynamics. The detection of these dark matter axions is made difficult by their extremely feeble coupling to regular matter, specifically the photon. The axion-photon conversion power is dwarfed by quantum uncertainty, which manifests as a fundamental noise source limiting the measurement of the quadrature observables used for detection. A promising route to finding the axion is through the use of cutting-edge quantum measurement technologies. Specifically, quantum squeezed states have long held the potential to enhance searches for new fundamental physics by allowing an experiment to circumvent the standard quantum limit. This thesis reports on the first use of quantum squeezed states in a search for axion dark matter with the Haloscope at Yale Sensitive to Axion Cold dark matter (HAYSTAC) experiment. This technique doubles the search rate for axions by preparing a microwave-frequency electromagnetic field in a squeezed state and near-noiselessly reading out only the squeezed quadrature. The results presented in this thesis exclude axions within the 16.96-17.12 and 17.14-17.28 μ eV/c2 axion mass windows at a theoretically interesting level of sensitivity and demonstrate that delicate quantum technologies can be run effectively in the constrained environment of a particle physics detector.
Daniel Berkowitz's picture
Daniel Berkowitz
George Fleming/Vincent Moncrief
Consultant
The Development of Mathematical Methods for Tackling Problems in Non-Perturbative Quantum Field Theory, Cosmology, and Gravity
We have extended two recently developed theoretical methods, the Quantum Finite Elements (QFE) and the Euclidean-signature semi-classical method (ESSCM). The QFE is a technique for constructing lattice field theories (LFTs) on curved Riemannian manifolds. We extended the applicability of the QFE to formulating LFTs on certain three dimensional Riemannian manifolds such as the 3-sphere. This was done by first constructing a novel simplicial approximation to the 3-sphere. Then, after correctly computing the weights of the links and vertices that make up this simplicial complex, we defined a Laplacian on it, whose low lying spectrum was observed to approach the known continuum limit as we further refined our complex. To facilitate a comparison between the QFE and the bootstrap, we calculated an estimate of the fourth-order Binder cumulant using CFT data extracted from the conformal bootstrap. The ESSCM is a methodology for facilitating the use of already known mathematical theorems/results to approach Lorentzian signature problems in bosonic field theory and quantum gravity in terms of their Euclidean-signature analogs. We further developed this method by applying it in a novel fashion to quantum cosmological models with matter sources. In particular, for the Taub models, we proved for the first time the existence of a countably infinite number of well behaved ‘excited’ state solutions when a cosmological constant is present. Both methods are promising and have applications for field theory, beyond standard model physics, and quantum gravity.  
Stephen Eltinge's picture
Stephen Eltinge
Sohrab Ismail-Beigi
Postdoctoral Associate
Yale University, Department of Applied Physics
Ab Initio Insights Into Substrate Effects, Structural Identification, and Excitonic States in 2D Materials
The 21st century has seen enormous growth in the study of two-dimensional (2D) materials, beginning with the isolation of graphene but rapidly expanding to include a wide variety of other compounds. Due to their size, 2D materials have immediate appeal for applications in nanoscale electronics. At the same time, uniquely low-dimensional phenomena such as the quantum spin Hall effect, quantum confinement, and 2D superconductivity are of interest to basic physics researchers. This dissertation presents ab initio investigations of three 2D materials. First, we discuss the binding of stanene on various substrates. Stanene, the buckled monolayer form of tin, is predicted to be a 2D topological insulator with symmetry-protected helical edge states. We investigate the effects of strain, chemical functionalization, and substrate–overlayer interactions on the topological band structure of stanene, showing that Al2O3 is an ideal substrate for synthesizing a potential quantum spin Hall insulator. Next, we examine the polymorphic structure of borophene sheets, the monolayer form of boron. We report on research that revealed the complex atomic structure of borophene on the Cu(111) and Cu(100) surfaces, including the crucial role played by simulated scanning tunneling microscopy (STM) data. We discuss the effect of modulation by the substrate on the occurrence of Dirac cones in the borophene band structure. Finally, we discuss the potential for Mg2TiO4 films to host long-lived, strongly bound interlayer excitons. At the DFT level, we obtain the band structure of Mg2TiO4 films grown on MgO and show how the polar films have a band offset favorable for interlayer exciton formation. Motivated by this work, we present GW and GW-BSE calculations of quasiparticle energies, exciton binding energies, and optical absorption spectra. These calculations more clearly characterize the suite of excitons that exist in Mg2TiO4 and shed light on the importance of film thickness in controlling their relative binding energies. The materials studied in this dissertation are diverse in chemical identity and properties, but are unified by their 2D structure and the crucial role played by their growth substrates, which are discussed throughout.
Paul Fanto's picture
Paul Fanto
Yoram Alhassic
Research Staff Member
Institute for Defense Analyses -- System Evaluation Division
Statistical properties of nuclei: beyond the mean-field approximation
The statistical model of compound-nucleus reactions has important applications in fundamental nuclear science, nuclear astrophysics, and nuclear technology. This model relies on two theoretical areas: (i) statistical reaction theory, which describes the compound nucleus with the Gaussian orthogonal ensemble (GOE) of random-matrix theory; and (ii) statistical properties of nuclei, i.e., nuclear structure observables that determine statistical-model predictions of reaction rates. The GOE statistical theory predicts that the partial widths of compound-nucleus resonances follow the Porter-Thomas distribution (PTD) and that total γ-decay widths have a narrow distribution. However, recent experiments measured width distributions that were broader than statistical-model predictions. We study these results with resonance-reaction models based on the GOE. Nuclear level densities are important statistical properties of nuclei and inputs to the statistical model. Mean-field methods are widely used to calculate level densities microscopically but neglect important correlations. We introduce two novel methods for symmetry projection after variation in the finite-temperature mean-field approximation and calculate nuclear state densities with exact particle-number projection. Moreover, we calculate state densities in the configuration-interaction shell model framework using the static-path plus random-phase approximation (SPA+RPA). The SPA+RPA includes static fluctuations and small-amplitude time-dependent quantal fluctuations beyond the mean field. We find that the SPA+RPA state densities agree with exact shell model Monte Carlo (SMMC) state densities and improve significantly over mean-field state densities in heavy lanthanide nuclei.
Jeremy Gaison's picture
Jeremy Gaison
Karsten Heeger
Linus Pauling Postdoctoral Fellow
National Laboratory
Measurement of the Reactor Antineutrino Spectrum of U-235 by PROSPECT and Daya Bay
Since their experimental discovery over 60 years ago, neutrinos have proven to be a fascinating means of exploring the physical universe. Through a variety of both natural and man-made sources, physicists have discovered many unusual features about these particles from their oscillation between their different flavor states to their particularly small mass. There are still many questions to answer regarding these fundamental particles, though. Among these questions is whether a possible fourth type of neutrino exists, a sterile neutrino, which could resolve a range of discrepancies between recent measurements and predictions at a variety of different energies and baselines. Precision neutrino measurements may also help to solve questions in nuclear physics and resolve previously measured spectral distortions. The PROSPECT experiment is a 4 ton, 6Li-loaded liquid scintillator detector at Oak Ridge National Laboratory operating >10m from the High Flux Isotope Reactor, a research reactor highly enriched in 235U. The segmented design of the detector allows for unprecedented background rejection at the Earth’s surface. Here I will describe the design, construction, data taking, and analysis of PROSPECT towards its measurement of over 50,000 neutrino events and the results from both its sterile neutrino search and its precision measurement of the 235U antineutrino spectrum. Further, I will describe the analysis that combines results from the PROSPECT and Daya Bay 235U measurements and present the most precise measurement of the 235U antineutrino energy spectrum to date.
Sheridan Green's picture
Sheridan Green
Frank van den Bosch
Quantitative Strategist
Susquehanna International Group
Research Website
The tidal evolution of dark matter substructure and the significance of halo-to-halo assembly history variance
Accurately predicting the abundance and structural evolution of dark matter subhaloes is crucial for understanding galaxy formation, large-scale structure, and constraining the nature of dark matter. Due to the nonlinear nature of subhalo evolution, cosmological N-body simulations remain its primary method of investigation. Subhaloes in such simulations have recently been shown to still be heavily impacted by artificial disruption, diminishing the information content (at small scales) of the simulations and all derivative semi-analytical models calibrated against them. In this dissertation, we build an improved semi-analytical model of dark matter substructure evolution and use it to assess various limitations of current-generation cosmological simulations. This work begins with the development of a state-of-the-art model of the mass evolution of individual subhaloes, which captures the joint effects of tidal heating and stripping on the subhalo structure. We incorporate this model into our recently released SatGen framework for studying the build-up and evolution of populations of dark matter subhaloes. We use SatGen to generate independent predictions for key quantities in small-scale cosmology, such as the evolved subhalo mass function (SHMF) and radial profiles. By comparing our results to cosmological simulations, we show that artificial disruption affects the SHMF at the 10-20 percent level, ameliorating previous concerns that it may suppress it by as much as a factor of two. We show that the resolution limit of N-body simulations, rather than artificial disruption, is the primary cause of radial biases in the subhalo number density found in dark matter-only simulations. Simulations have shown that the formation of a central disc can drastically reduce the abundance of substructure compared to a dark matter-only simulation, which has been attributed to enhanced destruction of substructure due to disc shocking. We use SatGen to examine the impact of discs on the substructure of Milky Way-like haloes with unprecedented statistical power. We find that the overall subhalo abundance is relatively insensitive to properties of the disc aside from its total mass. Furthermore, we show that the halo-to-halo variance in subhalo abundance dominates the mean impact of a Milky Way-like disc. We argue that the disc mainly drives excess mass loss for subhaloes with small pericentric radii and that the impact of disc shocking is negligible.  
Connor Hann's picture
Connor Hann
Steven Girvin
Research Scientist
Amazon Web Services Center for Quantum Computing
Practicality of Quantum Random Access Memory
Quantum computers are expected to revolutionize the world of computing, but major challenges remain to be addressed before this potential can be realized. One such challenge is the so-called data-input bottleneck: Even though quantum computers can quickly solve certain problems by rapidly analyzing large data sets, it can be difficult to load this data into a quantum computer in the first place. In order to quickly load large data sets into quantum states, a highly-specialized device called a Quantum Random Access Memory (QRAM) is required. Building a large-scale QRAM is a daunting engineering challenge, however, and concerns about QRAM’s practicality cast doubt on many potential quantum computing applications. In this thesis, I consider the practical challenges associated with constructing a large-scale QRAM and describe how several of these challenges can be addressed. I first show that QRAM is surprisingly resilient to decoherence, such that data can be reliably loaded even in the presence of realistic noise. Then, I propose a hardware-efficient error suppression scheme that can further improve QRAM’s reliability without incurring substantial additional overhead, in contrast to conventional quantum error-correction approaches. Finally, I propose experimental implementations of QRAM for hybrid quantum acoustic systems. The proposed architectures are naturally hardware-efficient and scalable, thanks to the compactness and high coherence of acoustic modes. Taken together, the results in this thesis both pave the way for small-scale, near-term experimental demonstrations of QRAM and improve the reliability and scalability of QRAM in the long term.
Naim Goksel Karacayli's picture
Naim Goksel Karacayli
Nikhil Padmanabhan
Postdoctoral Scholar
The Ohio State University
Improving the Cosmic Statistics of Neutral Hydrogen
The study of the large-scale cosmological structure seeks to understand the makings and the evolution of the universe. In this subject, I worked on improving current techniques and their application to the existing large, high-precision cosmological data sets. Specifically, my dissertation explores boosting power spectrum measurements at large scales for 21-cm intensity maps through reconstruction, and at small scales for Lyman-alpha Forest by developing and applying the optimal estimator to hundreds of high-resolution spectra. The cosmic tidal reconstruction is a novel technique for low redshift (z < 2) 21-cm intensity mapping surveys (e.g., CHIME and HIRAX) that recovers the lost large-scale line-of-sight signal from local small-scale anisotropies formed by tidal interactions. My thesis shows this algorithm is robust against redshift space distortions and can recover the signal with approximately 70 % efficiency for k < 0.1 h/Mpc using N-body simulations. I also introduce an analytical framework based on perturbation theory. The Lyman-alpha Forest technique can probe matter in vast volumes far into the past (2 < z < 5) and at smaller scales than galaxy surveys (r < 1 Mpc) through absorption lines in quasar spectra. The 1D power spectrum is shaped by the thermal state of the intergalactic medium (IGM), reionization history of the universe, neutrino masses and the nature of the dark matter. Using synthetic spectra, I show the optimal quadratic estimator for P_1D is robust against quasar continuum errors and gaps in spectra, and can improve accuracy beyond FFT-based algorithms for the upcoming DESI data. I also apply this estimator to the largest data set of high-resolution, high-S/N quasar spectra, which yields the most precise P_1D measurement at small scales that can improve the warm dark matter mass constraints by more than a factor of 2.
Sangjae Lee's picture
Sangjae Lee
Charles Ahn
Postdoctoral Assoicate
Seoul National University
Physics of the Electronic Structure and Collective Excitations in Transition Metal Compounds
In condensed matter systems novel phenomena such as superconductivity and magnetism can emerge from an intricate interplay between the materials lattice, spin, charge, and orbital degrees of freedom. Understanding how the structural and electronic degrees of freedom are coupled in transition metal compounds is crucial not only from the viewpoint of fundamental condensed matter physics, but also to establish a potential groundwork for substantial technological advancement through the discovery of exotic phases. In this dissertation, I illustrate how the electronic structure and collective excitations can be manipulated by controlling the dimensionality and interfacial structure using molecular beam epitaxy (MBE). Specifically, three studies, each based on a different transition metal system, are presented. First, I demonstrate a significant modification to the charge and orbital configurations of LaCoO3 by utilizing dimensional confinement and interfacial charge transfer in a bi-layer heterostructure. The change in the electronic structure results in a unique magnetic ground state, which is inaccessible in bulk cobaltates. In the second study, I investigate the spin excitation in dimensionally confined ferromagnetic Fe thin films. The dispersion of the spin excitation evolves anisotropically as the thickness of the Fe layers is reduced. This physics of this scaling is accurately captured by a simple Heisenberg model based on spin exchange interactions. In the last study, I investigate how the spin exchange pattern in the antiferromagnetic phase of NdNiO3 can be adjusted by changing the component layer thicknesses in heterostructures. Using the atomic layering capability of MBE, I also demonstrate control of the exchange interaction, which changes the quantum and classical dynamics of collective magnetic excitations. In all three studies, synthesis of high-quality thin films, advanced characterization techniques involving synchrotron x-rays, and theoretical calculations are harmoniously integrated to reveal the fascinating physics in transition metal systems. The results presented in this thesis provide a pathway to realize new emergent electronic and magnetic properties by manipulating the underlying electronic interactions in thin film heterostructures.
Jingping Li's picture
Jingping Li
Walter Goldberger
Postdoctoral Assoicate
Carnegie Mellon University
The Wordline Effective Field Theory of Spinning Gravitational Sources
The advent of gravitational wave physics has raised great interest in efficient calculations of gravitational dynamics. In particular, the worldline effective field theory (EFT) has proven to be powerful for describing the dynamics of compact binary inspirals. In this thesis, we report on progress in this method on including rotating gravitational sources. It has been shown that a connection exists between the radiative amplitudes from spinless classical sources in Yang-Mills theory and dilaton-gravity theory, inspired by the double copy construction in the scattering amplitude community. We generalize this result to spinning sources and find that an additional axion channel is necessary for the connection to be established. The spectrum coincides with that of the low energy limit of string gravity, and we deduce that the worldline EFTs correspond to the low energy limit of classical string theories. Furthermore, we show that tidal effects also admit a double copy structure. On the other hand, there has been new progress on incorporating dissipative effects into the worldline EFT. We generalize this construction to describe rotating objects and apply it to describe the absorptive effects of Kerr black holes by matching with graviton absorption probabilities calculated by Teukolsky equations. Using the resulting EFT, we reproduce the correct mass and spin absorption rates under general backgrounds. We demonstrate the utility of this EFT by computing the power and angular momentum dissipation in non-relativistic binaries.
Andrew Shim's picture
Andrew Shim
Owen Miller

Undecided
Probing the Upper Limits to Light–Matter Interactions: from Spontaneous Emission to Superresolution
As newly emerging optical devices and metamaterials inundate the photonics community, there is a growing need to understand the limits to device performance. Are current state-of-the-art designs already near the frontiers of what is possible, or can they be further improved with better technology? Addressing such broad, foundational questions calls for a new paradigm, a novel approach to explore the unimaginably vast, physically allowed design space. In this thesis, we will establish fundamental limits to light–matter interactions in the context of wave propagation in free space via beam shaping, scattering phenomena involving designable structures, and intrinsic properties of optical materials. First, we derive upper bounds to free-space concentration of light, mapping out the limits to the maximal intensity for any spot size and beam-shaping device. We use “inverse design” to discover metasurfaces operating near these limits, achieving up to 90% of maximum possible intensity. We then establish a sum rule for spontaneous emission, a prototypical near-field response, which relates integrated response over all frequencies to a simple electrostatic constant. Going further, we develop an analytical framework to derive upper bounds to near-field response over any bandwidth of interest and material platform. Finally, we derive fundamental limits to the refractive index of any transparent material. To this end, we use the theory of composites to identify metal-based metamaterials that can exhibit small losses and sizeable increases in refractive index over the current best materials.
Degree Date: May, 2021
Soner Albayrak's picture
Soner Albayrak
David Poland
Postdoctoral Researcher
University of Amsterdam
Analytic Studies of Fermions in Conformal Bootstrap
In this thesis, we analyze unitary conformal field theories in three dimensional spaces via analytic techniques of conformal bootstrap program through correlation functions of nonscalar operators, in particular Majorana fermions. Via the analysis of these correlation functions, we access several sectors in the spectrum of conformal field theories that have been previously unexplored with analytic methods, and we provide new data for several operator families. In the first part of the thesis, we achieve this by the large spin expansions that have been traditionally used in conformal bootstrap program for scalar correlators, whereas in the second part we carry out the computations by combining several analytic tools that have been recently developed such as the Lorentzian inversion formula and the weight shifting op-erators with the harmonic analysis for the Euclidean conformal group. We compare these methods and demonstrate the superiority of the latter by computing correction terms that are inaccessible in the for-mer. A better analytic grasp of the spectrum of fermionic conformal field theories can help in many directions including making new pre-cise analytic predictions for supersymmetric models, computing the binding energies of fermions in curved space, and describing quantum phase transitions in condensed matter systems with emergent Lorentz symmetry.
Tyler Lutz's picture
Tyler Lutz
John Wettlaufer
Wissenschaftlicher Mitarbeiter/postdoc
Universiät Magdeburg
Frictional, Large-Deformation Poroelastic Flow: Theory and Experiments
Fluid flow through deformable, porous materials is seemingly ubiquitous in the natural world---spanning length scales from the cellular to the planetary---and offers a phenomenologically rich setting in which to study the generally nonlinear coupling between solid- and fluid-mechanics in multiphase materials. As much as we might like to study such flows in strict isolation from their environment, this thesis argues that properly accounting for forces that arise on the boundaries of such flows is essential to understanding the behavior of realistic soft porous media flows. Building on an experimental program initiated more than half a century ago, we demonstrate a novel empirical method for simultaneously measuring the pore pressure and medium deformation profiles alongside the volume flux in uniaxial porous media flow. We perform a suite of experiments studying the flow-compaction of a foam sample in a regime in which the friction between the sample and boundaries of the experimental cell cannot be ignored. By opposing the motion of the foam, the wall friction leads to a demonstrable hysteresis in all of the measured quantities, a path-dependence which is difficult to account for in conventional theoretical models of large-deformation poroelasticity. Our experimental measurements constrain the material constitutive relations of our foam sample and thus enable us to formulate a mathematically closed theory of its poroelastic dynamics. Informed by these closures, we develop a particle-based theoretical framework that accounts for both static and kinetic frictional effects, and we demonstrate that our model quantitatively captures the full friction-induced phenomenology evinced in our experiments.
Ryan Petersburg's picture
Ryan Petersburg
Debra Fischer
Senior Professional Staff 1
Johns Hopkins Applied Physics Laboratory
Exoplanet Measurement to the Extreme: Novel Methods of Instrumentation and Data Extraction for Radial-velocity Spectrographs
The current generation of radial-velocity spectrographs are at the precipice of discovering a multitude Earth-like exoplanets orbiting in the habitable zones of nearby stars. Such detections require Doppler precision of approximately 10 cm s^{-1), an order of magnitude better than the typical best-case measurement from the previous generation of instruments. Therefore, the radial-velocity community requires research and innovation from all angles to push our technology over the brink. This thesis present multiple contributions to this field—ranging from the development of precision laser equipment to the implementation of advanced statistical data analysis algorithms—all in support of the EXtreme PREcision Spectrograph with the goal of improving instrument precision and exoplanet detection capability.
Mariel Pettee's picture
Mariel Pettee
Sarah Demers
Chamberlain Fellow
Lawrence Berkeley National Lab
Interdisciplinary Machine Learning Methods for Particle Physics
Following the discovery of a Higgs boson-like particle in the summer of 2012 at the Large Hadron Collider (LHC) at CERN, the high-energy particle physics community has prioritized its thorough study. As part of a comprehensive plan to investigate the many combinations of production and decay of the Standard Model Higgs boson, this thesis describes a continued search for this particle produced in association with a leptonically-decaying vector boson (i.e. a W or Z boson) and decaying into a pair of tau leptons. In Run 1 at the LHC, ATLAS researchers were able to set an upper constraint on the signal strength of this process at μ = σ/σ_SM < 5.6 with 95% confidence using 20.3 fb^-1 of collision data collected at a center-of-mass energy of √s = 8 TeV. My thesis work, which builds upon and extends the Run 1 analysis structure, takes advantage of an increased center-of-mass energy in Run 2 of the LHC of √s = 13 TeV as well as 139 fb^-1 of data, approximately seven times the amount used for the Run 1 analysis. While the higher center-of-mass energy in Run 2 yields a higher expected cross-section for this process, the analysis faces the additional challenges of two newly-considered final states, a higher number of simultaneous interactions per event, and a novel neural network-based background estimation technique. I also describe advanced machine learning techniques I have developed to support tau identification in the ATLAS High-Level Trigger as well as predicting and analyzing the dynamics of many-body systems such as 3D motion capture data of choreography.
Daniel Seara's picture
Daniel Seara
Michael Murrell
Postdoctoral Fellow
University of Chicago
Energetics of biological mechanics and dynamics
Living matter is a class of soft matter systems that microscopically consumes energy to drive essential life processes such as replication, migration, and shape change at the scale of both single cells and multicellular tissues. While much work has been done to understand the molecular processes underlying such behaviors, we lack a general understanding of how the microscopic breaking of detailed balance translates to large-scale cellular behaviors and materials properties. Using the tools of stochastic thermodynamics, this thesis uses energy dissipation to understand dynamical and mechanical phase transitions driven by force generation, binding kinetics, and (dis)assembly seen in experiments, simulations, and theoretical models of biological materials. We focus on the actomyosin cytoskeleton, a network composed of polymeric proteins (actin) subject to forces molecular motors (myosin), as our model system. At the subcellular level, analysis of actin filament motions in experiments shows that energy dissipated through bending controls the transition between stable and contractile steady states. Using simulations, we show that mechanosensitive binding kinetics of molecular motors controls a fluid-solid phase transition characterized by thermodynamic quantities with opposite symmetries under time-reversal. At the cellular level, we develop new tools for measuring irreversibility in spatiotemporal dynamics to analyze the energetic costs of oscillations and synchronization of a model biochemical oscillator inspired by (dis)assembly driven actomyosin dynamics. Throughout this work, we illustrate that a cell’s distance from equilibrium, quantified by energy dissipation, tunes its mechanical properties and dynamics. This provides a framework to unify disparate biological functions through the lens of non-equilibrium thermodynamics.
Olivier Trottier's picture
Olivier Trottier
Joe Howard
Undecided
Morphogenesis of class IV neurons in Drosophila melanogaster
The establishment of the neuron’s morphology is essential to its function. The class IV neurons of the Drosophila melanogaster larva are two-dimensional sensory neurons that develop a complex dendritic arbor sensitive to mechanical stimuli. The fully-developed dendritic tree results from a multitude of stochastic processes including dendritic tip growth, branching and self-avoidance. However, it is yet unknown how the microscopic dendritic growth processes produce the macroscopic morphology of the class IV neurons. In this study, we aim to bridge this gap by formulating multi-scale models of neuronal dendritic morphogenesis. We begin by analyzing the tip dynamics and branching process of class IV dendritic trees. We find that the tip growth dynamics can be described by a Markov process that transitions between three velocity states: growing, paused and shrinking. Driven by the results of our analysis, we propose two types of model of morphogenesis. First, we use the mean-field approximation to formulate dendritic tree growth as a system of reaction-diffusion equations with two kinds of species, dendrites and tips. This coarse-grained approach predicts that the dendritic tree grows by the propagation of a density wave whose tail stabilizes to a steady-state. Second, we construct an agent-based model of morphogenesis that implements the stochastic rules of microscopic tip growth and branching whose combined effects lead to the development of the dendritic tree. Within the limitations of the model, this more fine-grained approach predicts morphometrics that agree with the measured values. In summary, our results characterize the development of class IV neurons and provide a framework to understand how the large-scale morphology of the class IV neuron dendritic tree emerges from the local stochastic growth of its branches.
Christian Weber's picture
Christian Weber
Keith Baker
Research Associate Physics
Brookhaven National Lab
New Search for H→ZZ_d→4l using pp collision data at √s=13 TeV with the ATLAS detector
In 2012 the ATLAS and CMS experiments both reported the discovery of a new particle in the remnants of high-energy proton-proton collisions. The particles properties were consistent with the ones of the Standard Model Higgs boson. Its discovery, 58 years after its postulation, marked the completion of the Standard Model of Particle Physics.   Subsequent data taking at both experiments continued to record Higgs boson decays. With this increased dataset, we are now able to probe for new physics by studying the Higgs boson itself in ever greater detail. Specifically, the unique properties of the Higgs boson, it being chargeless, having sping-0, and coupling to mass, allow us now to probe previously inaccessible sectors of particle physics. That is, potential new particles whose interaction with Standard Model particles is otherwise forbidden or suppressed by lack of standard model charge or other symmetry considerations.   In this presentation I will detail the search for a beyond Standard Model gauge boson, that is not directly charged under the standard model. Specifically, the search for 'dark' Z-boson via the exotic Higgs boson decay H→ZZ_d→4l. I will give an overview of the machinery used to produce Higgs boson as well as the detector used to measure its decay products, the LHC and the ATLAS detector. In addition to that I will present limits on the cross section H→ZZ_d→4l and detail the statistical methods used in inferring the cross section limits from the recorded data.
Sisi Zhou's picture
Sisi Zhou
Liang Jiang
IQIM Postdoctoral Scholar
California Institute of Technology
Error-corrected quantum metrology
Quantum metrology, which studies parameter estimation in quantum systems, has many applications in science and technology ranging from frequency spectroscopy to gravitational wave detection. Quantum mechanics imposes a fundamental limit on the estimation precision, called the Heisenberg limit (HL), which bears a quadratic enhancement over the standard quantum limit (SQL) determined by classical statistics. The HL is achievable in ideal quantum devices, but is not always achievable in presence of noise. Quantum error correction (QEC), as a standard tool in quantum information science to combat the effect of noise, was considered as a candidate to enhance quantum metrology in noisy environment. This thesis studies metrological limits in noisy quantum systems and proposes QEC protocols to achieve these limits. First, we consider Hamiltonian estimation in Markovian master equations and obtain a necessary and sufficient condition called the “Hamiltonian-not-in-Lindblad-span” condition to achieve the HL. When it holds, we provide ancilla-assisted QEC protocols achieving the HL; when it fails, the SQL is inevitable even using arbitrary quantum controls, but approximate QEC protocols can achieve the optimal SQL coefficient. We generalize the results to parameter estimation in quantum channels, where we obtain the “Hamiltonian-not-in-Kraus-spa” condition and find single-letter formulas for asymptotic estimation precision by showing attainability of previously established bounds using QEC protocols. All QEC protocols are optimized via semidefinite programming. Finally, we show that reversely, metrological bounds also restrict the performance of error-correcting codes by deriving a powerful bound in covariant QEC.
Yuqi Zhu's picture
Yuqi Zhu
David DeMille
Postdoctoral Associate (Reina Maruyama)
WLC-254C
Experiments with 87Rb: Towards Co-trapping 88Sr19F and 87Rb
Polar molecules can interact via the anisotropic and long ranged electric dipole-dipole interaction owing to their large permanent electric dipole moments. Being able to study them in the ultracold temperature regime and at high density would allow us to study strongly correlated physics, and expand the toolbox for quantum computation. In order to reduce the temperature of SrF beyond the sub-Doppler temperature (~10 μK) achieved with laser cooling by polarization gradients, we hope to implement sympathetic cooling of SrF and Rb. Pursuing this direction requires advancements in experimental capabilities. This thesis describes the experimental efforts towards co-trapping SrF and Rb, which is the starting point of studies atom-molecule collilsions in the ultracold temperature regime. The thesis reports on some technical aspects of conservative trapping: a high-power circuit was developed to facilitate current switching in the context of magnetic quadrupole traps, and depths of optical dipole traps were estimated in anticipation of complexities due to a strongly state-dependent trapping potential. The thesis also details the construction and performance of an apparatus for laser cooling and trapping Rb. The apparatus consists of a 2D MOT that functions as a high-flux beam source of Rb, and an RF MOT. The 2D MOT produces a flux on the order of 109 atoms/s. With 500-ms loading, the RF MOT is able to collect ~109 atoms at temperature ~1 mK. Compared to the molecule number typical in a SrF MOT, the atome number in the Rb RF MOT is 104 higher, suggesting good prospects of observing atom-molecule collisions in a subsequent conservative trap. In addition, we established a comparison between dc and RF MOT of Rb in the same apparatus. We found that in an RF MOT, the number is significantly lower and the temperature significantly higher, while the loading time constant and lifetime are similar to those of a dc MOT.
Degree Date: December, 2020
Stephen Albright's picture
Stephen Albright
Charles Ahn
Program Manager
New York Academy of Sciences
Ultra-thin Film Growth of Chalcogenides to Realize Novel Electronic Phenomena
Chalcogenides, compounds containing the elements S, Se, or Te, are a class of materials that includes those with two-dimensional materials properties that have been shown to exhibit novel electronic phenomena, including superconductivity and non-trivial topology. These properties are both intrinsically valuable to our continuing fundamental understanding of electronic behavior in materials and potentially useful in developing next-generation electronic devices. In this dissertation I describe the techniques necessary to synthesize ultra-thin films of four chalcogenide systems of sufficient quality to display their unique properties. In my first study, I find careful substrate surface preparation as the key to growing FeSe, a monolayer superconductor that requires a double-layer TiO2 terminated surface of SrTiO3. In my second study, of Bi2Te3, a layered topological insulator that requires reconstructed surfaces of Si or Ge substrates, I demonstrate a proce dure to protect chalcogenide films from oxidation by capping with amorphous Se or Te. My third study, of stanene, a Sn-based analog of graphene that is predicted to be a topological insulator, identifies the chemical reactions that occur when Sn is deposited on Bi2Te3 that prevent formation of a stanene monolayer. Finally, in my fourth study, I present the steps necessary to synthesize uniform and continuous topological crystalline insulator SnTe films on SrTiO3 and I characterize the topological states present at the SnTe/SrTiO3 interface. I utilize molecular beam epitaxy deposition processes to synthesize each chalcogenide ultra-thin film and use a suite of complementary techniques, including diffraction, spectroscopy, electrical transport measurements, and microscopy to characterize the deposited films. The findings presented here demonstrate the strong dependence of chalcogenide superconductors and topological insulators on their interfacial structure. Correspondingly, the results highlight the sensitivity of chalcogenides to growth conditions, and thus the care necessary to achieve uniform ultra-thin films appropriate for measurement and functionalization. The precise nature of the interplay between electronic properties and interfacial structure in chalcogenides provides a critical foundation for understanding the origins of these phenomena and how to adapt these materials into functional devices.
Supraja Balasubramanian's picture
Supraja Balasubramanian
Bonnie Fleming
Postdoctoral Associate
Fermi National laboratory
A Differential Cross-Section Measurement of Muon Neutrino-Induced Charged Current Neutral Pion Production in MicroBooNE
MicroBooNE is a liquid argon time projection chamber (LArTPC) on the Booster Neutrino Beam at Fermi National Laboratory. Its primary goals are to conduct a νe appearance search to investigate the nature of the anomalous low-energy excess of electromagnetic events observed by MiniBooNE, study neutrino-argon interactions, and conduct research and development for future large-scale argon based detectors such as DUNE. A significant background for MicroBooNE’s νe appearance measurement are photon showers from neutral pion decay, which can mimic the electromagnetic cascade signature of electrons from νe’s. Studying neutral pion production in the MicroBooNE detector provides the opportunity to better understand electromagnetic shower topologies in LArTPCs. Furthermore, conducting experimental neutrino-argon cross sections is crucial for constraining modeling systematics and calculating the neutrino energy and flux accurately for future accelerator-based neutrino oscillation experiments. This thesis presents a measurement of νμ charged current single π0 differential production cross sections on argon in the MicroBooNE detector. This will be the first of its kind on argon.
Estella Barbosa de Souza's picture
Estella Barbosa de Souza
Reina Maruyama
Consultant
Boston Consulting Group
A Model-Independent Search for Dark Matter-Induced Annual Modulation Signal with the COSINE-100 Experiment
Joshua Burt's picture
Joshua Burt
John Murray
Undecided
Large-Scale Organization of Microcircuit Specialization in Human Cortex
Neural circuit dynamics across a range of spatiotemporal scales endow the brain with specialized computational capabilities that subserve human cognition and behavior. Technological advances and big data initiatives in recent years have revolutionized our understanding of the brain’s multi-scale architecture. Yet there remains a major disconnect in linking large-scale dynamics of networked neural systems to their underlying circuit mechanisms. Biophysically-based computational modeling of neural systems provides a uniquely powerful framework for mechanistically linking these levels of analysis. This dissertation develops an extensible computational framework that integrates multi-modal brain data with neural systems modeling. We leverage this approach to link regional physiological differences in brain microcircuitry to the large-scale specialization of brain function. We further demonstrate how this modeling framework can generate and test predictions for the large-scale functional impacts of molecular perturbations, with relevance to psychiatry.
Shany Danieli's picture
Shany Danieli
Pieter van Dokkum
NASA Hubble and Carnegie-Princeton Postdoctoral Fellow
Institute for Advanced Study
Research Website
Clues to the Nature of Dark Matter from Low-Mass Galaxies Outside the Local Group
The primary motive for this thesis is the highly uncertain nature of dark matter, presumably the dominant substance in our universe. From an astronomical perspective, perhaps the perfect laboratories for investigating the nature of dark matter on small scales are low surface brightness, low mass galaxies. They are the most abundant type of galaxy in the universe. In the Local Group, they are found to be dark matter dominated and thus have larger portions of dark matter for their total mass. And, most importantly, their low number of stars per unit area means that these galaxies provide a nearly unobstructed view of the dark matter skeleton of galaxies. In this thesis, the census and properties of low mass galaxies beyond the Local Group, in clusters, groups, and in isolation, are studied. Specifically, the focus is on informing the unconstrained relation between the stellar content of galaxies at the faint end of the luminosity function and their host dark matter halos. Novel observational tools and techniques are utilized to overcome long-standing barriers for detecting and characterizing such faint galaxies. First, dwarf galaxies in nearby groups are studied as part of the Dragonfly Nearby Galaxies Survey. Using follow-up HST observations of low surface brightness galaxies discovered in the vicinity of the nearby spiral galaxy M101, it is found that the M101 satellite luminosity function is remarkably similar to those of Milky Way and M31, with an even flatter slope for fainter magnitudes. Next, a series of studies of ultra-diffuse galaxies (UDGs) and their galactic dynamics show that they challenge previously derived tight scaling relation between galaxies' sizes, luminosities, and dark matter content. From a study of a size-limited sample of galaxies the Coma cluster, it is shown that red, large galaxies (r_eff>2 kpc) have a fairly uniform distribution in the size-luminosity plane: there is no peak at the absolute magnitude implied by the canonical size–luminosity relation. It is also inferred that for large galaxies, size is not an indicator of the halo mass. At the extreme end of the distribution, two remarkable UDGs, NGC1052-DF2 and NGC1052-DF4, were discovered and studied. From high-resolution imaging with HST and spectroscopic follow-up with the Keck Cosmic Web Imager, it is inferred that both galaxies seem to have very little to no dark matter. These galaxies are very far off the canonical relation between the dark matter content and baryonic content in galaxies. These discoveries are just the tip of the iceberg in this new exciting journey of exploring previously unknown structures and phenomena in the universe. Finally, the prospects of constructing a complete census of relatively isolated "field" dwarf galaxies in the Local Volume is studied. A model is presented for calculating the predicted detection rates of dwarf galaxies between 3 and 10 Mpc using integrated light surveys. It is shown that surveys with surface brightness sensitivity of 30 mag/arcsec^2 should be able to detect galaxies with stellar masses down to 10^4-10^5 Mpc, out to 10 Mpc. Motivated by the results of this study, the Dragonfly Wide Field Survey was initiated and carried out. The survey, covering 330 deg^2 in the Stripe82 and GAMA fields, was designed with the goal of better characterizing the faint end of the galaxy population outside of the Local Group. The preliminary data characterization reveals that it reaches 1 sigma depth of mu_g ~ 31 mag/arcsec^2 on arcminute scales and thus pushes the current limits of the field galaxy mass function by 2-3 orders of magnitude. Moving forward, a complete photometric catalog of all low surface brightness galaxies in the survey footprint will be constructed and follow-up observations will be carried out to provide further observational constraints on dark matter models by studying the dark matter halos substructure in those galaxies.    
Max Hays's picture
Max Hays
Michel Devoret
Postdoctoral Associate
Realization of an Andreev spin qubit: Exploring the sub-gap structure of Josephson nanowires using circuit QED
A weak link between two superconductors hosts discrete, fermionic modes known as Andreev levels. They govern the physics of the weak link on the microscopic scale, ultimately giving rise to macroscopic phenomena such as the Josephson supercurrent. Conventional superconducting quantum circuits crucially rely on the nonlinearity of the supercurrent in Josephson tunnel junctions, which arises from the ground-state properties of millions of Andreev levels acting in concert. Yet fundamentally, each Andreev level is itself a fermionic degree of freedom, able to be populated by the spin-1/2 quasiparticle excitations of superconductors. In this thesis, we explore how individual Andreev levels can be measured and manipulated in special weak links known as Josephson nanowires. In addition to the typical Andreev physics associated with weak links, these highly-ordered semiconductor nanowires possess spin-orbit coupling and enhanced g-factors. The interplay between these superconducting and semiconducting properties unlocks the Andreev degrees of freedom in ways that are impossible in conventional weak links. Using the microwave techniques of circuit QED, we achieve coherent manipulation of the Andreev levels and probe their interactions with the environment. Finally, by taking advantage of a spin-orbit-induced spin-splitting, we realize the Andreev spin qubit: a two-level quantum system formed from the spin states of an individual quasiparticle.
Judith Hoeller's picture
Judith Hoeller
Nicholas Read
Postdoctoral Associate
Topological quantization of Berry phases in quantum and classical systems
Berry phases are known to occur for a spin 1/2 in a slowly varying magnetic field in quantum mechanics, and for the polarization of light in a gently twisted optical fiber in classical optics. Such Berry phases take on continuous values. I describe two systems in which Berry phases are topologically quantized to discrete values: (1) cold atoms in accelerated optical lattices, and (2) dissipative two level systems coupled to radiation with a slowly varying phase offset. (1) Cold atoms in accelerated optical lattices have been used to simulate Bloch oscillations, which would occur for electrons in solids with applied electric fields but usually their relaxation times are too short.  Bloch oscillations occur because of the translational symmetry of the lattice, which guarantees that the wave function in one energy band is periodic in quasimomentum. However, crystallographic symmetries of the lattice may entangle multiple energy bands, in which case it becomes questionable if the wave function is periodic at all. I will discuss that it can be--due to a symmetry-quantization of Berry phases--which then results in period-multiplied Bloch oscillations. (2) A dissipative two level system coupled to radiation could describe two optomechanically coupled normal modes of a dielectric membrane or a microwave-driven superconducting quantum bit. By modeling the dynamics with a non-Hermitian Hamiltonian, we find that for a certain driving frequency and strength, the Hamiltonian is no longer diagonalizable. At such an exceptional point, striking differences to Hermitian dynamics take place. After a transient time, the unique eigenvector locks the relative amplitudes between the two-level states. Additionally, if the phase of such a drive is slowly varied around a cycle, a Berry phase of exactly π is acquired.
Scott Jensen's picture
Scott Jensen
Yoram Alhassid
Postdoctoral Associate
University of Illinois, Urbana-Champaign
Lattice Auxiliary-Field Quantum Monte Carlo Studies of the Unitary Fermi Gas
The Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensate (BEC) crossover has been experimentally realized using Feshbach resonance techniques with ultracold atomic Fermi gases of 6Li and 40K. In the middle of this crossover is the unitary Fermi gas (UFG), in which the s-wave scattering length diverges and interactions are strongest. The UFG has many interesting properties including a particularly high critical temperature for superfluidity Tc ~ 0.15 TF (in units of the Fermi temperature TF) and a nearly perfect fluid behavior approaching the Kovtun-Son-Starinet bound on the ratio of shear viscosity to entropy density. It has applications to dilute neutron matter in the crust of neutron stars and is relevant to QCD plasmas. The UFG serves as a paradigm of strongly correlated Fermi superfluids with a proposed pseudogap regime above Tc and below a pairing temperature scale T*. This regime is characterized by a strong suppression of the single-particle spectral weight near the Fermi surface in the absence of coherent pairing and off-diagonal long-range order. The extent of this pseudogap regime in the UFG has been debated extensively and to date remains an open and controversial topic. A less controversial but similarly significant open problem has been the temperature dependence of Tan’s contact in the UFG. The contact is a fundamental property of systems with short-range interactions, which appears in the tail of the momentum distribution and in the high-energy tail of the shear viscosity. Various theories lead to widely different temperature dependence of the contact. We address the possible existence and extent of a pseudogap regime in the UFG and present results for the temperature dependence of the contact. The UFG is a strongly correlated system and the understanding of its properties requires non-perturbative methods. We have implemented lattice finite-temperature auxiliary-field quantum Monte Carlo (AFMC) methods for the UFG in the canonical ensemble to calculate its thermodynamic properties including an energy-staggering pairing gap, the condensate fraction, heat capacity, spin susceptibility, thermal energy and its associated Bertsch parameter, and the contact. We also present novel AFMC optimizations which have allowed us to perform controlled simulations with large lattices and extrapolate to the continuum limit, thus removing systematic errors associated with the finite lattice spacing. Taking the continuum limit in particular has proven crucial in obtaining the correct temperature dependence of the contact. Our AFMC results for the contact are in remarkable agreement with the latest precision ultracold gas experiments across the superfluid phase transition. To address pairing correlations and pseudogap physics without the need for numerical analytic continuation, we calculate the temperature dependence of a model-independent energy-staggering pairing gap which requires the use of the canonical ensemble. Our work provides the first finite-temperature calculation of this observable for the homogeneous UFG. We do not see significant signatures of a pseudogap above our estimated critical temperature of Tc=0.153(7)TF and find an upper bound on the pairing temperature scale of T*~0.17 TF suggesting a significantly narrower pseudogap regime than previously proposed.  
William Sweeney's picture
William Sweeney
A. Douglas Stone
Data Scientist
VideaHealth
Electromagnetic Eigenvalue Problems and Nonhermitian Effects in Linear and Saturable Scattering
In this thesis we address a series of new problems in non-hermitian optical scattering – linear and nonlinear – with increasing degrees of complexity. We develop the theory of reflectionless scattering modes (RSMs), introducing a novel and broad class of impedance-matched eigenproblems: for a given structure, find the incident wavefronts and frequencies which are not partially reflected at all, but are instead transmitted through the scatterer, or dissipated within it. The RSM framework includes critical coupling and coherent perfect absorption (CPA) as specific instances, and does not necessarily require intrinsic gain or loss to realize in most cases. We analyze the symmetry properties of the RSMs, and find that they support exceptional points (EPs) which can be directly probed with steady-state excitation, and are accompanied by a quartic flattening of the reflection lineshape. These are distinct from the often-studied resonant and scattering EPs, and can be engineered in hermitian cavities with no gain or loss, in which case the transmission lineshape is also flattened. We study degenerate coherent perfect absorption (CPA EP), which is a specific example of the new kind of RSM EP mentioned above. Here, two perfectly absorbed states are brought together by tuning parameters of the scatterer. In the case of a ring-resonator, it is known that resonant EPs take on a chiral flavor; we show that the same holds true for the CPA EP, and use this fact to design a patterned ring resonator which either predominantly absorbs or reflects light, depending on the direction of incidence. We extend CPA from cavities with a linear dielectric response to include the saturating nonlinearity and dispersion of a two level absorbing medium. By using the CPA theorem, which relates CPA in a lossy cavity to lasing in an amplifying cavity, augmented to account for both saturation and dispersion, we show that the SALT algorithm in the single-mode regime can also be used to find the saturable CPA modes through a simple mapping. This demonstrates that between a lower and upper threshold for loss one can maintain CPA by continuously adjusting the pump strength. We also clarify the bad-cavity limit of dispersive but linear CPA, identifying new modes that are hybrids of the cavity and atomic degrees of freedom, with a strongly dispersive response to changes in the pump. We present and solve the general problem of scattering from an arbitrary cavity with a saturable two-level amplifying or absorbing medium, generalizing the known phenomenology of bistability that has mainly been studied in structures with little or no spatial complexity. Unlike with lasing or CPA, these solutions have both incoming and outgoing flux. We carefully analyze the validity of approximations used for isolated resonances, and find that the previously used inverse single-pole approximation requires some modification for the case of scattering.  
Hao Yan's picture
Hao Yan
Simon Mochrie

Positional Fluctuations in Synthetic and Living Polymer Systems
Chromatin organization is inextricably linked to its dynamics. The loop extrusion factor (LEF) model provides a framework for how topologically associating domains (TADs) arise: cohesin or condensin extrude DNA loops, until they encounter boundary elements, namely CTCF. However, a characteristic subdiffusive behavior of MSD is observed in fission yeast on the seconds timescale and experiments show that cohesin or condensin largely constrains chromatin mobility. Such finding is inconsistent with prior LEF model. I develop a new LEF model in which LEF loading depends on the underlying architecture of transcriptional units and chromatin remodeling is identified as the essential ATP-dependent activity that drives chromosome motion. I demonstrate that this model predicts TADs, including TAD boundaries lacking CTCF binding sites, comparably well to prior CTCF-dependent LEF models for the mouse genome, and also successfully predicts TADs in the CTCF-lacking model, S. pombe. Lastly, I perform polymer dynamic simulations and show that the DNA-looping by cohesin and condensin largely constrain chromatin mobility, which is in agreement with experiments.
Mengzhen Zhang's picture
Mengzhen Zhang
A. Douglas Stone
Postdoctoral Associate
University of Chicago
Properties and Applications of Gaussian Processes
Gaussian states, operations, and measurements are central building blocks for continuous-variable quantum information processing which paves the way for abundant applications, especially including network-based quantum computation and communication. To make the most use of the Gaussian processes, it is required to understand and utilize suitable mathematical tools such as the symplectic space, symplectic algebra, and Wigner representation. Applying these mathematical tools to practical quantum scenarios, we developed various schemes for faithful quantum transduction, interference-based bosonic mode permutation and ultra-sensitive bosonic sensing. Quantum transduction, proposed for transferring quantum information between different bosonic platforms, will enable the construction of large-scale hybrid quantum networks. We demonstrated that generic coupler characterized by Gaussian unitary process can be transformed into a high-fidelity transducer, assuming the availability of infinite squeezing and high-precision adaptive feedforward with homodyne measurements, all of which are Gaussian operations, measurements or classical communication channels and can be easily analyzed using symplectic algebra. To address the practical limitation of finite squeezing, we also explored the potential of interference-based protocols. It turns out that these protocols can let us freely permute bosonic modes only assuming the access to single-mode Gaussian unitary operations and multiple uses of a given generic multi-mode Gaussian process. Thus, such a scheme not only enables universal decoupling for multi-mode bosonic systems, which can be useful for suppressing undesired coupling between the system and the environment, but also efficient and faithful bidirectional single-mode quantum transduction. Moreover, noticing that the Gaussian processes are appropriate theoretical models for optical sensors, we studied the quantum noise theory for optical parameter sensing and its potential in providing great measurement precision enhancement. We also extended the Gaussian theories to discrete variable systems, with several examples such as quantum (gate) teleportation. All the analyses and conclusions originated from the fundamental quantum commutation relations, and therefore are widely applicable.
Degree Date: May, 2020
Robert Blum's picture
Robert Blum
Sean Barrett
Postdoctoral Associate (Barrett)
SPL 14
Applying novel NMR techniques to many-body spin systems, and novel reconstruction techniques to NMR data
This experimental thesis focuses on two distinct themes: developing NMR techniques to probe many-body spin systems, and extracting the maximum amount of information from the minimum amount of data. For the first theme, I describe the first NMR observations of discrete time crystal signatures. A discrete time crystal (DTC) is a many-body quantum state where a driven system exhibits discrete time translational symmetry breaking. A surprising aspect of our DTC signatures is that they were detected in an ordered spatial crystal of ammonium dihydrogen phosphate, and yet the signatures look quite similar to those detected in very different systems with more disorder. We use a novel DTC echo experiment to probe the coherence of the driven system. Finally, we show that interactions during the pulse of the DTC sequence contribute to the decay of the signal, complicating attempts to measure the intrinsic lifetime of the DTC. For the second theme, I discuss the acceleration of NMR experiments via non-uniform sampling and spectral reconstruction methods. Our lab previously developed a reconstruction method (called DiffMap) that we successfully applied to 2D NMR spectra. Here, we develop an in-depth understanding of the action of the DiffMap algorithm, identifying the factors that cause reconstruction errors and how to predict them. This improved understanding allows us to formulate a bottom-up approach to finding the sparsest sampling required to accurately reconstruct individual spectral features with DiffMap. I also describe the development of the coDiffMap reconstruction method, which modifies and extends DiffMap for the case of pseudo-3D NMR experiments, using the correlations across the 2D slices of the data set as additional information. Incorporating this information allows entire spectra (not just individual features) to be reconstructed accurately with very sparse sampling. I particularly focus on describing our efforts to understand coDiffMap's error sources and reconstruction behavior in an analogous way to how we analyze DiffMap. For both DiffMap and coDiffMap, we were able to push the amount of sparsely-sampled data required for an accurate reconstruction all the way down to a level comparable to the "information content" of the original, dense spectrum.
Luke Burkhart's picture
Luke Burkhart
Rob Schoelkopf
Postgraduate Associate
Yale University
Error-Detected Networking for 3D Circuit Quantum Electrodynamics
Quantum machines have the potential to serve as groundbreaking tools for scientific discovery in the coming decades. As the complexity of these devices increases, it may be necessary to borrow ideas from complex classical systems, and build them in a modular fashion, with independently designed, optimized, and tested components, networked together into a functioning whole. To build a modular machine from superconducting circuits requires the ability to perform operations between quantum bits housed in separate modules. For this, we must be able to move qubits between modules, or generate entanglement across the network, conveying information in the form of photons. In all implementations to date, photon loss in the links between modules is a dominant source of error, which must be overcome in order to build a scalable modular machine. We demonstrate two approaches for rapid and faithful quantum communication and entanglement between modules in a superconducting quantum network. Encoding information in cavity resonators allows application of strategies for error mitigation in harmonic oscillators to detect photon loss in the communication path. Using a low-loss communication bus, we transfer a qubit in a multi-photon encoding and track loss events to improve the fidelity. Furthermore, generating entanglement with two-photon interference and post-selection against loss errors produces a Bell state with half the error obtained in the single photon case. We discuss several routes towards high-fidelity operations in superconducting quantum networks based off these tools.
Christopher Davis's picture
Christopher Davis
Reina Maruyama
Senior Data Scientist
CarMax
Search for Neutrinoless Double Beta Decay with Majoron Emission in CUORE
This thesis describes a search performed at the Cryogenic Underground Observatory for Rare Events (CUORE) for Majoron-emitting neutrinoless double-beta decays. A discovery of any form of neutrinoless double-beta decay would be of immense importance to the field of physics, as any mechanism by this decays occurs would require physics beyond the Standard Model. In addition, neutrinoless double-beta decays, including type I Majoron models, would show that lepton number is not a conserved quantity and, along with other measurements, could explain the prevalence of matter over antimatter in the early universe.   Also described in this thesis is the experiment, CUORE, which is a neutrinoless double-beta decay experiment currently in operation at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy, along with a detailed description of one of the calibration systems used in the experiment.   Since April 2017, CUORE has been taking physics data using 988 TeO2 crystals arranged in 19 towers inside of a custom cryostat operating at approximately 10 mK. The results presented in this thesis correspond to a live-time of 216.36 kg yr in TeO2 and place lower limits (90\% C.L.) for majoron decays of spectral index 1, 2, 3, and 7, respectively. These results are the strongest limits in Te-130 to date, and limits produced for coupling constants of each majoron decay mode are comparable to that of other present experiments. Lastly, these searches for majoron decays continue to be an interesting phase space for research into neutrinoless double-beta decay, and serve as another promising decay mode to observe as detector technology and background reduction techniques improve.
Arpit Dua's picture
Arpit Dua
Meng Cheng & Liang Jiang
UQM-IQIM Postdoctoral scholar
Caltech
Structure of fracton stabilizer models
In recent years the study of topological phases of matter has moved to the forefront of theoretical condensed matter physics. This has been fueled in part by the theorized existence of exotic phases of matter that can serve as topological quantum memories and computers. The classification of topological phases of matter in two spatial dimensions in terms of anyon theories, or modular tensor categories, and chiral central charges forms the cornerstone of the subject. For the special case of 2D translation invariant topological stabilizer models this classification was established rigorously at the level of lattice Hamiltonians. In three spatial dimensions, recent steps have been taken towards a similar goal, with some success for topological phases that admit a topological quantum field theory (TQFT) description. However, the burgeoning field of fracton topological phases present a new and challenging facet of the classification problem, as practically all the familiar tools from the study of TQFTs no longer apply. We addresses the structure of a class of commuting projector models in three dimensions called fracton stabilizer models. Fracton stabilizer models were found either as examples of exactly solvable topological models with glassy dynamics or in search of quantum memories with a better memory time than TQFT stabilizer codes such as the toric code. The main property responsible for glassy dynamics and better memory time is that the excitations in fracton stabilizer models are either partially or completely immobile without creating further excitations. The properties of fracton stabilizer models have been understood model by model but there has been no unifying framework. We study properties of fracton stabilizer models using tools such as compactification, 3D generalizations of the S-matrix invariant and entanglement renormalization. Using such tools led to conjectures about the structure of these models and shed light on its interplay with the mobilities of the excitations in the model. For example, when a model has a particle which is mobile in two dimensions, a two-dimensional toric code can be extracted from it using a local unitary. We prove this and other conjectures about the structure of stabilizer models using tools from commutative algebra. 
Raymond Ehlers's picture
Raymond Ehlers
Helen Caines
Postdoctoral Associate
Oak Ridge National Lab
Jet-Hadron Correlations Measured in Pb--Pb Collisions at $\sqrt{s_{\text{NN}}} = 5.02$ TeV with ALICE
Quantum Chromodrynamics (QCD) describes the interactions of quarks and gluons. Due to asymptotic freedom, sufficiently high energy density can cause matter to transition to a deconfined state of matter known as the Quark-Gluon Plasma. As partons propagate through this QCD medium, which can be formed in ultra-relativistic heavy-ion collisions, they lose energy and the resulting jets are known to be modified in a phenomena known as jet quenching. This thesis investigates potential path length dependence of jet quenching via the measurement of azimuthal jet-hadron correlations with respect to the event plane orientation in Pb--Pb collisions at $\sqrt{s_{\text{NN}}} = 5.02$ TeV with the ALICE detector. Such studies also help constrain the large background underlying this measurement. The associated hadron yields and correlation widths associated with the trigger and recoil jets are compared as a function of event plane orientation. They are found to be predominately consistent between the different orientations within uncertainties, although there are suggestions of deviations at low associated particle transverse momentum. Indeed, theoretical predictions suggest that any deviations are expected to be small, which may be due to competing processes associated with jet quenching. I also discuss my contributions to ALICE Overwatch, a project to enable nearly real-time data quality monitoring and assurance using the capabilities of the High Level Trigger.
Luyao Jiang's picture
Luyao Jiang
Jack Harris

Wells Fargo
Nonreciprocal dynamics in a cryogenic optomechanical system
Nonreciprocity in various branches of physics has been studied for more than a century, e.g., from classical to quantum mechanics, and from particle to condensed matter. It is particularly interesting to consider nonreciprocal phenomenon in open (non-hermitian) systems. In this dissertation, I use a cryogenic cavity optomechanical system to demonstrate robust nonreciprocal interactions between two phononic resonators. The nonreciprocity, either transient or static, is realized via the cavity mediated optomechanical interaction. I will start with a pedagogical introduction to nonreciprocity as well as non-hermiticity in physics, followed by a brief review of optomechanics and a theoretical derivation of the nonreciprocity in optomechanical systems. Then I will introduce our experimental realization of the optomechanical system, i.e., the membrane-in-the-middle setup. Next I will present the main result of this dissertation, which includes the experimental demonstration of both transient and static optomechanical nonreciprocity. I will conclude with a discussion of our on-going study of high order exceptional points (the degeneracy in multi-level open systems).
Norman Lam's picture
Norman Lam
John Murray
Postdoctoral Associate
MIT
Inhibitory Regulation of Cognitive Functions in Cortical and Thalamic Circuits: Computational Mechanisms and Experimental Predictions
Cognitive behaviors fundamentally arise from the interactions of numerous neurons in the brain. However, while neuronal properties are well-characterized, how an ensemble of interconnected neurons acts is generally not apparent from the single-neuron dynamics. Computation in specific circuits of neuronal populations is known to reflect key variables in core cognitive functions, such as decision making, although the underlying mechanisms are not fully understood. There thus are explanatory gaps between the knowledge at the levels of single neurons, neural circuits, and behavior. Microcircuit-level modelling, which describes the dynamics of populations of biophysically-realistic model neurons, is a useful tool to bridge the aforementioned gaps. Constrained by neuronal dynamic and connectivity pattern, microcircuit model can demonstrate potential underlying mechanisms of neural computation relevant to cognitive function. Proposed mechanisms can generate dissociable predictions that can be directly tested in experiments. This dissertation entails multiple studies concerning the underlying neural mechanisms of core cognitive functions, including decision making, working memory, and attention. The studies primarily utilize computational modelling of microcircuits, and heavily collaborate with experimentalists for constraining and testing the models. Insights from theoretical models and dynamical systems analyses are also leveraged to aid in the interpretation of computational model results. Inhibitory neurons, which suppress the activity of other neurons, are crucial components of neural microcircuit. They maintain the microcircuit in the dynamical regime appropriate for neural computation, and can serve as focal ``control knobs'' to modulate cognitive processes based on demand. The dissertation explores both roles of inhibitory neurons. In particular, the balance between excitation and inhibition is a critical regime for cognitive computation, and its disruption is linked to deficit in cognitive functions induced across neuropsychiatric disorders. To understand how disruptions of excitation-inhibition balance result in cognitive deficit, we study circuit dynamics and computation under excessive or insufficient inhibition for several core cognitive functions. While disruptions in both directions can impair cognitive performance, they distinctly alter circuit dynamics, allowing specific tasks to dissociate the direction of disruption, especially in the context of neuropsychiatric disorders and perturbation experiments. This thesis aims to contribute to our understanding of the neural mechanisms underlying various cognitive behaviors, especially on the regulatory roles of inhibitory neurons on circuit computation, and on the effects of physiological disruptions in neuropsychiatric disorders.
Claudia Lau's picture
Claudia Lau
Charles Ahn

Structural characterization of epitaxial oxide heterostructures via X-ray scattering
The impact of atomic-scale structural distortions on the transport properties of epitaxial thin film oxides and their interfaces is investigated through high resolution surface x-ray diffraction. The crystalline thin films studied are the metallic oxides (La,Sr)TiO3 and LaNiO3, the high mobility oxide BaSnO3, the high dielectric oxide LaInO3, and the ferroelectric oxide Pb(Zr,Ti)O3. Films were grown on SrTiO3 and DyScO3 commercial substrates by the physical vapor deposition techniques off-axis RF magnetron sputtering, pulsed laser deposition, and molecular beam epitaxy. High resolution x-ray diffraction measurements were taken at Argonne National Laboratory and Brookhaven National Laboratory, using synchrotron radiation from the Advanced Photon Source and the National Synchrotron Light Source II. Crystal truncation rods are measured on epitaxial thin film (La,Sr)TiO3 and BaSnO3, both grown on SrTiO3 substrate and fully strained at thicknesses n≤10 unit cells. Three-dimensional electron density maps with picoscale resolution of the atomic positions are derived by the powerful phase retrieval method Coherent Bragg Rod Analysis. Supertetragonality, which is associated with interface-induced polarization, is observed in both films. The electron density map for the (La,Sr)TiO3 film additionally reveals polar displacements near both the film/substrate and film/vacuum interfaces. Deposition of a capping layer on the (La,Sr)TiO3 film suppresses polar displacement as well as lattice parameter expansion; the structural changes can be correlated to a measured enhancement in the material’s conductivity. The energy-dependent resonant x-ray scattering technique is used to probe the cation stoichiometry of thicker, relaxed La-doped BaSnO3 films, which have a range of high room-temperature carrier mobilities. Analysis of energy scans measured around weakly scattering odd-order Bragg peaks reveal a less than 1% difference in stoichiometry between the films. In-plane φ rocking curves measured using a rotating copper anode laboratory x-ray source likewise show minimal differences in crystalline disorder. The φ rocking curve full width half maxima in contrast to those of the ω rocking curves suggest that film disorder is a reflection of mosaic size in the DyScO3 substrate. Two epitaxial interfaces, the polar/nonpolar interface of LaInO3 with BaSnO3 and the ferroelectric/conducting interface of Pb(Zr,Ti)O3 with LaNiO3 are studied by a combination of crystal truncation rod and half-order Bragg peak analysis. Half-order Bragg peaks, the result of the perovskite unit cell doubling through rotations of oxygen octahedra, are isolated to the LaInO3 and LaNiO3 layers in these heterostructures. In the first system, mixed tilting and strong cation displacements remain robust, right up to the BaSnO3 interface for films as thin as three unit cells. Both the intrinsic polarization of the LaInO3 and the structural discontinuity across the LaInO3/BaSnO3 interface appear to contribute to the 10^4 enhancement in conductivity measured at the interface of the two oxides. In the second system, the ferroelectric functions as a gate with switchable polarization which modulates the conductivity of the LaNiO3 channel. As the applied voltage is swept through the coercive field of the ferroelectric, the oxygen octahedral rotations in the LaNiO3 exhibit a hysteresis loop, with larger rotations leading to reduced conductivity. The non-volatile switching and hysteresis behavior show promising potential for the future of all-oxide field effect devices. 
Catherine Matulis's picture
Catherine Matulis
Damon Clark
Engineer
Mathworks
Contrast adaptation in Drosophila direction-selective circuits
In visual systems, neurons adapt both to the mean light level and to the range of light levels, or the contrast. Contrast adaptation has been studied extensively, but it remains unclear how it is distributed among neurons in connected circuits, and how early adaptation affects subsequent computations. In this study, we investigated temporal contrast adaptation in neurons across Drosophila's visual motion circuitry. Several ON-pathway neurons showed strong adaptation to changes in contrast over time. One of these neurons, Mi1, showed almost complete adaptation on fast timescales, and experiments ruled out several potential mechanisms for its adaptive properties. When contrast adaptation reduced the gain in ON-pathway cells, it was accompanied by decreased motion responses in downstream direction-selective cells. Simulations show that contrast adaptation can substantially improve motion estimates in natural scenes. The benefits are larger for ON-pathway adaptation, which helps explain the heterogeneous distribution of contrast adaptation in these circuits.
Shantanu Mundhada's picture
Shantanu Mundhada
Michel Devoret
Quantum Engineer
Quantum Circuit, Inc.
Hardware-efficient autonomous quantum error correction
The promise of quantum speedup in information processing is not yet fulfilled in a useful quantum algorithm due to the susceptibility of quantum information to decoherence. This makes quantum error correction (QEC) a vital area of research. The vast majority of QEC protocols, however, come with an overwhelming hardware and software overhead. For superconducting quantum circuits, it is possible to minimize this overhead by hardware-efficient encoding in infinite dimensional Hilbert spaces of high-Q harmonic oscillators, and error mitigation using autonomous feedback. Autonomous quantum error correction (AQEC) is particularly challenging, since it requires specific nonlinear interactions between various modes of a quantum system. In this thesis, we explore the Hamiltonian engineering techniques, geared towards realizing the required interactions for a promising class of hardware efficient QEC codes, namely Schrödinger cat codes. A four-component Schrödinger cat code, which allows for first-order protection against all error channels, requires a highly nonlinear four-photon driven-dissipative process for autonomous stabilization of the decoherence free manifold. We propose a scheme for engineering such a process through a Raman-assisted cascading of readily available four-wave mixing interactions, and experimentally demonstrate the feasibility of this cascading. Furthermore, an improved four-wave mixing device that cancels unwanted always-on interactions is studied. We also propose an implementation of a new error-correction code, the pair-cat code, which offers autonomous protection against all the error channels using low-order parametric interactions.
Kyungjoo Noh's picture
Kyungjoo Noh
Liang Jiang
Quantum Research Scientist
Amazon Web Services
Research Website
Quantum computation and communication in bosonic systems
Quantum computation and communication are important branches of quantum information science. However, noise in realistic quantum devices fundamentally limits the utility of these quantum technologies. A conventional approach towards large-scale and fault-tolerant quantum information processing is to use multi-qubit quantum error correction (QEC), that is, to encode a logical quantum bit (or a logical qubit) redundantly over many physical qubits such that the redundancy can be used to detect errors. The required resource overhead associated with the use of conventional multi-qubit QEC schemes, however, is too high for these schemes to be realized at scale with currently available quantum devices. Recently, bosonic (or continuous-variable) quantum error correction has risen as a promising hardware-efficient alternative to multi-qubit QEC schemes. In this thesis, I provide an overview of bosonic QEC and present my contributions to the field. Specifically, I present the benchmark and optimization results of various single-mode bosonic codes against practically relevant excitation loss errors. I also demonstrate that fault-tolerant bosonic QEC is possible by concatenating a single-mode bosonic code with a multi-qubit error-correcting code. Moreover, I discuss the fundamental aspects of bosonic QEC using the framework of quantum communication theory. In particular, I present improved bounds of important communication-theoretic quantities such as the quantum capacity of bosonic Gaussian channels. Furthermore, I provide explicit bosonic error correction schemes that nearly achieve the fundamental performance limit set by the quantum capacity. I conclude the thesis with discussions on the importance of non-Gaussian resources for continuous-variable quantum information processing.     
Brooke Russell's picture
Brooke Russell
Bonnie Fleming
Owen Chamberlain Postdoctoral Fellow
Lawrence Berkeley National Laboratory
An Electron Neutrino Appearance Search in MicroBooNE with 5$\times$10$^{19}$ POT
MicroBooNE is a single-phase liquid argon time projection chamber (LArTPC) short-baseline accelerator neutrino experiment located at Fermi National Accelerator Laboratory in the Booster Neutrino Beam. MicroBooNE's foremost scientific objective is to definitively resolve the low-energy excess of single shower electromagnetic events seen by the precursor MiniBooNE experiment. This thesis examines a small fraction of early MicroBooNE data. By leveraging fine-grained drifted ionization charge signal from particle interactions, the LArTPC detector technology provides detailed topological and calorimetric information for neutrino-argon interaction analysis. The interplay of scintillation light and tomographic imaging of ionization charge signals is exploited for a charged current neutrino pre-selection. This pre-selection serves as the foundation for parallel inclusive charged current muon neutrino and electron neutrino selections. The charged current muon neutrino selection aims to constrain the expected intrinsic charged current electron neutrino events measured, such that an excess, if present, may be quantified. With approximately 5$\times$10$^{19}$ protons on target (POT) beam exposure, the low-energy excess of electron-like events is measured at 0.51$\sigma$.
Kyle VanderWerf's picture
Kyle VanderWerf
Corey O'Hern

Geometry and Contact Mechanics of Athermal Jammed Packings of Frictionless Spherical and Nonspherical Particles
I perform computational studies of quasistatically jammed packings of frictionless particles of a variety of shapes. Each of the four studies presented in this dissertation focuses primarily on the connection between the mechanical and geometric properties of these packings – specifically, how physical aspects such as force and torque balance, pressure, and shear and bulk moduli correlate with (a) constituent particle shape, (b) the degree of particle ordering in packings, and (c) the packings’ networks of interparticle contacts. I also investigate how each of these geometric properties affect each other. For example, in the first study, I examine the relationship between constituent particle shape, packing fraction, and contact number, and demonstrate a variety of correlations between these properties for two-dimensional jammed packings of nonspherical particles. In the second study, this analysis is extended to three dimensions, where an additional shape parameter is necessary to describe the packing fraction at jamming onset, and the effect of orientational ordering is considered. In the third study, I show that the scaling exponents of the ensemble-averaged shear modulus of jammed disk packings as a function of pressure are dependent on changes in the contact network that occur as the pressure increases, and demonstrate that jammed packings can unjam via isotropic compression. Finally, in the fourth study, I present the preliminary results of research into cyclically-compressed jammed disk packings, which are able to be trained into reversible states over time if the amplitude of the compression is sufficiently small, but which remain irreversible indefinitely if the amplitude of the compression is too large.
Shilo Xia's picture
Shilo Xia
David C. Moore
Chamberlain Fellow
LBNL
Search for Neutrinoless Double Beta Decay and Detector Physics Measurements with the Final EXO-200 Dataset
Liquid xenon (LXe) is employed in a number of current and future detectors for rare event searches. This work presents the latest results from the EXO-200 experiment, which searched for neutrinoless double beta decay (0$\nu\beta\beta$) in $^{136}$Xe between 2011 and 2018. With upgraded hardware, increased exposure and analysis improvements, the detector resolution, sensitivity and final data limit were also improved over time. Taking advantage of a single-phase, large detector with good purity and well-calibrated energy response, measurements of the absolute scintillation and ionization yields generated by MeV energy gamma sources over a range of electric fields was performed in EXO-200 and are presented in this thesis. These measurements are useful for simulating the performance of future 0$\nu\beta\beta$ detectors employing LXe, such as nEXO, which is a next generation 0$\nu\beta\beta$ experiment using $^{136}$Xe aiming to reach a half-life sensitivity $\sim10^{28}$ years. The development of high-bandwidth digital cable prototypes with sufficiently low radioactivity for use in nEXO is described in the end.
Degree Date: December, 2019
Tonima Tasnim Ananna's picture
Tonima Tasnim Ananna
Meg Urry
Postdoctoral Associate
Dartmouth College
ACCRETION HISTORY OF AGN: X-RAY POPULATION SYNTHESIS MODEL & MULTIWAVELENGTH PHOTOMETRIC REDSHIFTS
A new X-ray population synthesis model, its physical implications, along with detailed analysis of AGN X-ray spectral parameter space are presented in this work. This population synthesis model uniquely fits all the newest observed constraints from high energy X-ray bands such as NuSTAR and Swift-BAT, and predicts a Compton-thick fraction of 50−56% in the local Universe. The new X-ray population synthesis model was computed using a neural network. Given an input distribution of AGN spectral parameters, this neural network converges onAGN populations that can fit the cosmic X-ray background (CXB). Other observed quantities such as AGN number counts and Compton-thick fractions are then used for validation to choose the correct solution from all possible matches to the X-ray background. A measure of the average radiative efficiency and AGN ionization contribution based on the new population synthesis model are also presented in this work. We find somewhat higher values of radiative efficiency with our model than was estimated by previous models, due in part to higher space densities of obscured objects in the new model, as well as to using a newer (lower) reported value for the local black hole mass density. Recent results suggest that AGN in late stages of galaxy mergers tend to be heavily obscured. Therefore more obscured objects may spin up as a result of recent mergers, leading to higher radiative efficiencies. We find that some regions of the X-ray spectral parameter space can produce accept-able fits to the CXB, while others will overproduce or underproduce parts of the CXB independent of the underlying AGN population. A full exploration of acceptable spectral parameter spaces is done using a Bayesian technique. We identify regions of the AGN spectral parameter space that never produce a fit to the CXB and thus can be rejected. These findings are compared to observed AGN spectral parameter distributions in surveys. This work also presents a new multiwavelength study of AGN spectra in order to calculate photometric redshifts for Stripe 82X, one of the largest volume X-ray surveys to date. The calculation of these photometric redshifts informs us on the population types found in wide-shallow fields, which is necessary to understand the populations accessed in the wider layers of wedding cake surveys, and the techniques and AGN templates used are applicable to future wide-area surveys by LSST and eROSITA.
Charles D. Brown II's picture
Charles D. Brown II
Jack Harris
Assistant Professor of Physics
Starting at Yale in January 2023
Optical, Mechanical and Thermal Properties of Superfluid Liquid Helium Drops Magnetically-Levitated in Vacuum
The field of optomechanics studies the interactions between light and the motion of an object. One of the goals in this field is to generate and control highly non-classical motion of a massive mechanical oscillator. There has been progress in generating such non-classical motion via coupling the oscillator to a qubit, or by utilizing the non-linearity of single photon detection. However, interest still remains in generating non-classical motion directly via the optomechanical interaction itself. Doing so requires strong coupling between the light and the mechanical oscillator, as well as low optical and mechanical loss and temperature. The unique properties of superfluid helium (zero viscosity, high structural and chemical purity and extremely low optical loss) addresses some of these requirements. To exploit the unique properties of superfluid helium we have constructed an optomechanical system consisting entirely of a magnetically levitated drop of superfluid helium in vacuum. Magnetic levitation removes a source of mechanical loss associated with physically clamped oscillators. Levitation also allows the drop to cool itself efficiently via evaporation. The drop’s optical whispering gallery modes (WGMs) and its surface vibrations should couple to each other via the usual optomechanical interactions. In this dissertation we demonstrate the stable magnetic levitation of superfluid helium drops in vacuum, and present measurements of the drops' evaporation rates, temperatures, optical modes and surface vibrations. We found optical modes with finesse $\sim 40$ (limited by the drop's size). We found surface vibrations with decay rates $\sim 1$ Hz (in rough agreement with theory). Lastly, we found that the drops reach a temperature $T\approx 330$ mK, and that a single drop can be trapped indefinitely.
James Ingoldby's picture
James Ingoldby
Thomas Appelquist
Postdoctoral Researcher
ICTP in Trieste Italy
EFTs for Nearly Conformal Gauge Theories
The phenomena of electricity and magnetism, beta decay in radioactive nuclei, and the confinement of quarks within the proton can each be explained using the three different gauge theories which collectively make up the standard model. Alternative gauge theories display new kinds of exotic phenomena, which are not fully understood. For example, if the field content is chosen appropriately, a gauge theory is said to be in the conformal window, and can acquire conformal symmetry. In this case, all fundamental mass scales vanish from the theory and its states no longer correspond to collections of particles. This thesis focuses on nearly conformal gauge theories, which have a field content chosen to place them just outside of the conformal window. They exhibit confinement at low energies, but numerical studies indicate that their properties differ markedly from the familiar gauge theories of the standard model. In particular, an anomalously light composite scalar forms in these theories, which could be a Higgs boson candidate that would resolve the standard model hierarchy problem. Revealing the variety of phenomena exhibited by nearly conformal gauge theories is challenging because these theories confine, are strongly coupled and therefore cannot be analyzed using the standard tools of perturbation theory. Lattice gauge theory has been used to study these theories numerically. These studies have revealed a wealth of useful information about the nearly conformal gauge theories, including the presence of a light scalar composite state in the spectrum, but they required significant computer resources, so having complimentary theoretical tools would be advantageous. To this end, two effective field theories (EFTs) are developed in this thesis: A dilaton EFT in which the lightest scalar state is interpreted as a pseudo--Goldstone boson arising from the spontaneous breaking of conformal symmetry, and a linear sigma EFT, in which the lightest scalar belongs to a multiplet of states transforming linearly under an internal symmetry. These weakly coupled EFTs include only the lightest degrees of freedom present in the spectrum of the gauge theories and provide a simple, approximate description of the same physics, enabling extrapolation of the lattice results to regions of parameter space that are otherwise inaccessible. The dilaton EFT is fitted to lattice data with encouraging results.
Prashanta Kharel's picture
Prashanta Kharel
Peter Rakich
Device Lead and Founding Team Member
Hyperlight Corporation
Utilizing Brillouin Interactions for Optical Control of Bulk Acoustic Waves.
The interaction between light and mechanical motion has been harnessed for a variety of scientific and technological applications ranging from studies of decoherence to precision metrology and quantum information.  Building on these accomplishments, optomechanical systems show great potential for various classical and quantum applications, including ultra-low-noise oscillators and high-power lasers to quantum transducers and quantum memories. Central to these goals of optomechanics, and more generally of quantum information science, is harnessing long-lived phonons while minimizing thermal noise. Often times, this means achieving coherent control of high-frequency mechanical modes. Acoustic modes having high Q-factors and high frequencies are less sensitive to thermal noise as they are more decoupled from their thermal environment. A variety of microscale and nanoscale optomechanical systems use wavelength-scale structural control to access long-lived phonons at high frequencies. These GHz-frequency oscillators can be readily initialized in their quantum ground states using bulk refrigeration techniques, paving the way for impressive demonstrations ranging from non-classical mechanical states to remote entanglement between mechanical resonators. However, spurious laser absorption within these miniaturized systems (modal mass ~picogram), continue to threaten robust ground state operation. This is because even miniscule amounts of light absorbed at the material boundaries yield excess dissipation for light and add thermal noise. In this context, macroscale systems based on bulk acoustic wave (BAW) resonators are intriguing resources for optomechanics. At cryogenic temperatures, these resonators support long-lived phonons within devices geometries that mitigate surface interactions by orders of magnitude over their microscale counterparts. So far, electromechanical coupling has been used to access such long-lived phonons, enabling various scientific and technological applications ranging from tests of Lorentz symmetry to low-noise oscillators. However, if we could access such phonons with light it could open new avenues for sensitive metrology, materials spectroscopy, high-performance lasers, and quantum information processing. In this thesis, we demonstrate the optical control of long-lived, high-frequency phonons within BAW resonators.  We utilize Brillouin interactions to engineer tailorable coupling between free-space laser beams and high Q-factor phonon modes supported by a plano-convex bulk acoustic resonator. Analogous to the Gaussian beam resonator design for optics, we present analytical guidelines, numerical simulations, and novel microfabrication techniques to create stable acoustic cavities that support long-lived bulk acoustic phonons. For efficient optical control of bulk acoustic phonons, we utilize resonant multimode interactions by placing the bulk crystal inside an optical cavity. Resonant interactions permit us to dramatically enhance the optomechanical coupling strength. Utilizing enhanced optomechanical interactions in a system where we can select between Stokes and the anti-Stokes process, we demonstrate cooling and parametric amplification of bulk acoustic modes as a basis for ultra-low-noise oscillators and high-power lasers. Finally, we enhance the optomechanical coupling strength to be larger than the optical and mechanical decoherence rates, creating hybridized modes that are part light and part sound. Deterministic control of long-lived bulk acoustic phonons with light in this so-called strong coupling regime opens the door to applications ranging from quantum transduction to quantum memories.
Stefan Krastanov's picture
Stefan Krastanov
Liang Jiang
Postdoctoral Associate
TBD
Research Website
New Approaches to Control, Calibration, and Optimization of Quantum Hardware
In this dissertation I present a number of techniques that form building blocks for the quantum manipulations spanning various levels of the quantum technology stack. We begin at the very bottom, with techniques for the universal control of a quantum harmonic oscillators. Oscillators like microwave and optical cavities are among some of the more promising physical systems on top of which to implement quantum logic. However, most of our technology until recently has been focused on manipulating pseudo-classical states of light, restricted to operations that preserve the Gaussian profile of the quantum states. Much more general unitary operations are necessary to unlock the computation power of quantum mechanics and we will see a number of protocols enabling such operations.   However, the precise control and evaluation of the hardware requires a well calibrated model of the dynamical laws governing it. Methods like state and process tomography permit such calibration in principle, but they require a very large number of measurements and dealing with the noise inherent to the hardware makes them fragile. Instead of these methods, we will see how tools borrowed from compressed sensing and machine learning provide for cheaper, more robust, and higher fidelity calibration procedure.   Going one step higher the technology stack, we need to use these control techniques to actually prepare non-classical resources for use in quantum computation. One of the most ubiquitous such resource is quantum entanglement. We will see how one can optimize the entanglement distillation circuits for the error model of the actual hardware. The optimized circuits perform substantially better than a general distillation circuit by virtue of being optimized for the particularities of the hardware -- this way the results from the previously discussed calibration procedure inform the design of upper layers of the technology stack.   We can continue this optimization journey at still higher layers of the stack. On top of the physical qubits we have created we still need to implement an error correcting code that can asymptotically suppress errors. We will study a black-box decoder for such codes, based on a neural network architecture, that permits us to work with advanced codes for which individual decoders have not been designed yet. This enables us to both optimize the decoder for the particular hardware and even optimize the code structure itself, without worrying that the new code will be difficult to decode.
Danielle Norcini's picture
Danielle Norcini
Karsten Heeger
KICP and Grainger Fellow
University of Chicago
A search for eV-scale sterile neutrinos and precision measurement of the U-235 antineutrino spectrum with the PROSPECT experiment
Reactor experiments have been devoted to establishing the properties of the weakly-interacting neutrino. Recent neutrino oscillation experiments at low-enriched uranium (LEU) reactors suggest a disagreement between the observed electron antineutrino flux and energy spectrum when compared to leading model predictions. The ~6% flux deficit, known as the Reactor Antineutrino Anomaly, measured by detectors with baselines <500 m can be explained by the addition of a beyond-the-Standard-Model particle, an eV-scale sterile neutrino. This new type of matter would have a profound impact on fundamental physics and cosmology. The spectral deviations may be attributed to an incomplete understanding of antineutrino emission from fissile material. However, the insufficiencies in the nuclear models and the specific isotopic contributions are not yet clear. PROSPECT is a newly constructed, short-baseline experiment observing antineutrinos from the 85 MWth highly-enriched uranium High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory. Positioned 7-9 m from the HFIR core, the main science goals are to unambiguously search for eV-scale sterile neutrino oscillations and to precisely measure the antineutrino emission from 235U fission. PROSPECT uses the inverse beta decay interaction to identify neutrino events, but must do so in a near-surface environment with harsh reactor-related and cosmogenically-induced backgrounds. To overcome this challenge, PROSPECT developed a novel, compact 6Li-loaded scintillator detector with efficient neutron tagging and particle identification capabilities. These features, combined with event topology reconstruction made possible by optical segmentation, allow an unprecedented level of background suppression to observe antineutrinos with high energy resolution within a few hours of reactor-on operation. This thesis details the detector design, instrumentation development, and construction of the PROSPECT experiment at the Yale Wright Laboratory from 2014--2018. The detector calibration, event reconstruction, and performance are also discussed. With 33 (28) days of reactor-on (off) data at HFIR, the first search for eV-scale sterile neutrinos is presented. No sterile neutrino oscillations are observed within the sensitive (Δm412, sin22ϴ14) parameter space and PROSPECT excludes the best-fit sterile neutrino hypothesis to the Reactor Antineutrino Anomaly at 2.2σ confidence level. Using a slightly larger data set containing >31,000 antineutrino candidates, PROSPECT has produced the world-leading measurement of the 235U spectrum with a signal-to-background of 1.7 and 5%/E(MeV) energy resolution. The experimental data shows discrepancies with the leading Huber reactor model, with further tests disfavoring 235U as the sole isotope responsible for the spectral deviations observed by LEU experiments at 2.1σ confidence level.
Susan Pratt's picture
Susan Pratt
Simon Mochrie
Law School Student
Georgetown University
Development and implementation of a reversibly-interacting TRAP-peptide pair as a live-cell imaging strategy
The need to study proteins in living cells to acquire accurate information pertaining to their spatiotemporal dynamics drives the development and improvement of tools for visualization. Unfortunately, there does not exist a 'one size fits all' imaging strategy, leading to the continuous pursuit of new labeling and optical methods, with the goal of developing more widely applicable approaches that minimally disrupt the biological systems studied. Tetratricopeptide repeat affinity proteins (TRAPs) are unique in their ability to recognize and bind to short peptide sequences that consist of about five amino acids. These binding reactions are reversible, and the pairs can be engineered to have a variety of potential binding affinities. Specific TRAP-peptide pairs can be selected for, that present with no cross-reactivity, and are capable of being utilized in live-cell systems, such as the bacteria Escherichia coli and budding yeast Saccharomyces cerevisiae. Capitalizing on these features, we turn the TRAP-peptide pairs into a versatile tag-probe imaging strategy for in vivo protein studies. The TRAPs are designated as probes labeled with a fluorophore, while the peptides are genetically fused to the C-terminus of the proteins of interest. We conduct a proof-of-principle experiment in E. coli demonstrating the TRAP-peptide pair's utility as an imaging strategy, and later implement the system in budding yeast where we show that our reversibly-binding tag-probe system properly and reliably localizes Pma1 to the plasma membrane, unlike direct-fusions of Pma1 with fluorescent proteins. Using fluorescence recovery after photobleaching experiments (FRAP), we characterize our tag-probe system's binding kinetics, and proceed to use single particle tracking photoactivated localization microscopy (sptPALM) measurements under total internal reflection fluorescence (TIRF) illumination to analyze the behavior of Pma1. Remarkably, we find significant differences in the diffusional dynamics of Pma1, imaged using our novel labeling methodology, and Pma1, directly fused to a fluorescent protein. 
Jared Rovny's picture
Jared Rovny
Sean Barrett
Princeton Quantum Institute Postdoctoral Fellow
Princeton University
New NMR Applications and Methods: Discrete Time Crystalline Signatures and Accelerated Acquisition using Iterated Maps
This thesis describes two distinct research endeavors linked by nuclear magnetic resonance (NMR) experiments. One is related to the nascent study of discrete time crystals (DTCs) in driven quantum systems. In this thesis I describe the first NMR studies of DTC signatures in solids, characterizing the DTC signatures we observed in a crystal of ammonium dihydrogen phosphate, where we addressed nuclear spins of phosphorus 31P and hydrogen 1H. I also describe the methods we used to demonstrate quantum coherence in the resulting spin state, the lifetime of observables of the driven system, and the important experimental factors that play a recurrent role in the spin physics. The other topic pertains to accelerating NMR experiments, by acquiring less data than usual and reconstructing the resulting data sets. The reconstruction is performed using variants of the Difference Map (DiffMap) algorithm, whose properties we study extensively. I also describe a new method that was created during this work, based on DiffMap and useful for data sets possessing correlations among parts of the data, called coDiffMap. I demonstrate how coDiffMap allows us to reconstruct a pseudo-3D NMR data set very quickly with high fidelity, and discuss various aspects of this new algorithm’s implementation.
Tong Shu's picture
Tong Shu
James Rothman
Postdoctoral Associate
Institute of System Genetics in NYU School of Medicine
Molecular Mechanisms of Regulated SNARE Assembly Revealed by Single-Molecule Force Spectroscopy
Neurons in the brain communicate with each other by release of neurotransmitters at synaptic junctions. Neurotransmitter release is mediated by three membrane-anchored SNARE proteins (soluble N-ethylmaleimide sensitive factor attachment protein receptors) and various regulatory proteins, including Munc13-1, Munc18-1, Synaptotagmin, complexin, NSF (N-ethylmaleimide sensitive factor) and SNAP (soluble NSF attachment protein). By forming proper SNARE complex, SNARE proteins couple their folding and assembly to membrane fusion in a regulatory protein-dependent manner. To achieve sub-millisecond synaptic vesicle fusion, SNARE assembly pathway needs to be highly regulated in several aspects, promoting fast and accurate initial SNARE assembly, clamping half-zippered SNARE complex before Ca2+ influx and accomplishing full SNARE zippering for membrane fusion within sub-millisecond upon Ca2+ influx. However, the exact molecular mechanisms of these processes are poorly understood despite decades of researches. Here, we addressed one of the processes using high-resolution optical tweezer. We found that Munc13-1, Munc18-1, and two SNAREs – Syntaxin-1 and VAMP2 associate into a weak tetrameric complex. The third SNARE protein SNAP-25B rapidly binds the two SNAREs in the complex to form a proper ternary SNARE complex, which likely displaces the two regulatory proteins from the resultant four-helix bundle. Therefore, Munc13-1 and Munc18-1 cooperatively chaperone SNARE assembly, a process required for neurotransmitter release. In summary, the findings demonstrate new evidences in understanding the molecular mechanism of sub-millisecond fast Ca2+-triggered synaptic vesicle exocytosis.
Clarke Smith's picture
Clarke Smith
Michel Devoret
Postdoctoral Fellow
Inria
Design of Protected Superconducting Qubits
Controllable quantum systems that are shielded at a Hamiltonian level from the random fluctuations of their environments could provide a valuable resource for quantum information science. While these "protected qubits" promise unprecedentedly low error rates, this might come at the expense of ease of physical implementation. This thesis focuses on overcoming this apparent design problem in protected qubits within the context of superconducting circuits and their quantized electromagnetic fields. We describe some of the essential design tools: quantization of nonlinear lumped element circuits, approximation by an effective Hamiltonian, energy levels and matrix elements from numerical diagonalization, Hamiltonian verification via spectroscopy, and noise characterization using time-domain measurements. At each stage, examples are given from the following systems: the fluxonium artificial atom, the double fluxonium artificial molecule, and the cos2φ qubit. We validate the principle of designed protection with numerical predictions of the insensitivity of the cos2φ qubit to all expected decoherence mechanisms.
Savannah Thais's picture
Savannah Thais
Sarah Demers
Postdoctoral Associate
Princeton Institute for Computational Science and Engineering
Utilizing Electrons in the Search for Associated Higgs Production with the ATLAS Detector: Higgs decaying to a tau pair and vector boson decaying leptonically
The Higgs boson was discovered by the ATLAS and CMS collaborations in 2012 using data from $\sqrt{s}$=8TeV proton-proton collisions at the LHC. Since the initial discovery of the $H\rightarrow 4l$ and $H\rightarrow \gamma\gamma$ decays, multiple other Higgs analyses of production modes and decay channels have reached discovery significance. This thesis describes the search for the still unobserved vector boson ($V=W^{\pm},Z$) associated Higgs production with the Higgs decaying to a tau lepton pair using 46.9 fb$^{-1}$ of $\sqrt{s}$=13 TeV proton-proton collision data collected by the ATLAS detector in 2017. This analysis requires the vector boson to decay leptonically in order to reduce background contributions from hadronic activity in the detector and this thesis focuses primarily on the usage of final state electrons in the analysis.\\ The primary background in all analysis categories are misidentified (or `faked') objects; these contributions are estimated using a data-driven technique which relies on Machine Learning (ML) for object identification and reconstruction. ML is used broadly in High Energy Physics analyses and this work is introduced, with a focus on techniques for improving electron identification through image processing.
Degree Date: May, 2019
Salvatore Aiola's picture
Salvatore Aiola
John Harris
Postdoctoral Research Fellow
Istituto Nazionale di Fisica Nucleare (National Institute for Nuclear Research).
Jet and heavy-flavor measurements in pp and Pb–Pb collisions with ALICE
Quantum Chromo-Dynamics (QCD) is the field theory that describes the nuclear interactions, responsible for holding together quarks and gluons inside the atomic nuclei. Despite significant progress made in over four decades of experimental and theoretical work since QCD was established, there remain many open questions, especially regarding the fragmentation process of quarks and gluons and the behavior of QCD in the high-temperature regime. Jets are produced from hard-scattered quarks and gluons as a consequence of the confinement property of QCD, that banishes the possibility of free quarks at ordinary temperatures. However, at high energy densities, like those achieved in ultra-relativistic heavy-ion collisions, QCD predicts a transition to a phase of free quarks and gluons, called the Quark-Gluon Plasma (QGP). This thesis presents two measurements performed with the ALICE detector at the CERN Large Hadron Collider (LHC). Charm jets were tagged using D0 mesons. Their cross section and fragmentation function in pp collisions at the center-of-mass energy of 7 TeV were measured and compared with theoretical predictions obtained with Monte Carlo generators based on QCD. A general agreement was found, confirming that we understand the general features of jet and heavy-flavor production in the framework of QCD. Another measurement was performed in Pb-Pb collisions at a center-of-mass energy per nucleon of 2.76 TeV. Jets were reconstructed and their yield compared to a pp reference measurement. A strong suppression was observed, which is interpreted as caused by energy loss of hard-scattered quarks and gluons in the QGP.
Elizabeth Boulton's picture
Elizabeth Boulton
Daniel McKinsey

Applications of Two-Phase Xenon Time Projection Chambers: Searching for Dark Matter and Special Nuclear Materials
Over the past four decays, Liquid xenon has emerged as a popular medium for direct detection of dark matter. The Large Underground Xenon (LUX) experiment utilized a 300 kg two-phase xenon detector to set a world-leading limit on the WIMP spin-independent cross-section and WIMP mass parameter space with 332 live-days of data collection. This analysis proved especially challenging due to non-uniform, time-evolving drift fields during the WIMP search run, and it required the use of novel calibration systems and innovative analysis techniques. The LUX detector was also used to investigate properties of liquid xenon in order to further research beyond dark matter. Specifically xenon’s response to low- energy recoils O(< 5 keV) is not thoroughly understood, and yet it is vitally important to the search for axions, neutrino-less double beta decays, and the neutrino magnetic moment. To better understand this low energy regime, an 37Ar source (which produces low-energy, mono- energetic recoils) was developed and used to calibrate the detector. These measurements were compared to the 37Ar measurements in another two-phase xenon detector: the Particle Identification in Xenon at Yale (PIXeY) detector. Liquid xenon detectors excel at detecting rare events. In addition to searching for dark matter, this makes them potentially well suited for detecting the low decay rates of special nuclear materials (SNM). The PIXeY detector and the Compton-imaging Detector in Xenon (CoDeX), were used to investigate the feasibility of creating a two-phase xenon Compton-imager to detect SNM.
Dandan Ji's picture
Dandan Ji
Eric Brown

Modeling of the dynamics large-scale coherent structures in the system of Rayleigh-Benard Convection
We test the ability of a general low-dimensional model for turbulence to predict geometry-dependent dynamics of large-scale coherent structures, such as convection rolls. The model consists of stochastic ordinary differential equations, which are derived as a function of boundary geometry from the Navier-Stokes equations. The model describes the motion of the LSC in terms of diffusion in a potential. We test the model using Rayleigh-Bénard convection experiments in a cubic container, in which there is a single convection roll known as the large-scale circulation (LSC). The model predicts an oscillation mode in which the LSC oscillates around a corner. To characterize the dynamics of the large-scale circulation in a cube, we report values of diffusivities and damping time scales for both the LSC orientation $\theta_0$ and temperature amplitude, and of the mean temperature amplitude, for $8\times10^7 \le Ra \le 3\times 10^9$. We show that the potential for the orientation $\theta_0$ and slosh angle $\alpha$ has quadratic minima near each corner, with curvature about a factor of 2 smaller than predicted. The potential is nearly symmetric in $\theta_0$ and $\alpha$ near the corners as predicted, indicating the 2 parameters are uncoupled. The prediction of the natural frequency $\omega_r$ of the potential was too large by factor of 2.5, which is typical of this model. We observed a new oscillation mode around corners of the cell above a critical Ra $=5.\times10^8$. This critical Ra, appears in the model as a crossing of an underdamped-overdamped transition. The oscillation period, power spectrum, and critical Ra for oscillations, are consistent with the model if we adjust the model parameters by about a factor of 2. The uncertainty of a factor of 2 in model parameters is too large to correctly predict whether resonance exists. The structure of the oscillation mode consisted of oscillations in $\theta_0$ and $\alpha$ each around the same corner. The structure of the oscillation of $\hat \theta_0$ is consistent with the predicted $n = 1$ advected oscillation mode, the oscillation in $\hat \alpha$ is not quite that of the prediction. The observed oscillation in $\alpha_b$ is consistent with the prediction of $n = 1$, oscillation in $\alpha_t$ is having the consistent phase shift but the opposite correlation as the prediction, and there is oscillation in $\alpha_m$ while the prediction is 0. The observed oscillation is more complex. Since the model was developed in a cylindrical geometry, the continued success of the model at predicting the potential and its relation to other flow properties in a cubic geometry -- which has very different flow modes -- still suggests great promise for the potential to predict the dynamics of large-scale coherent structures in complicated geometries. However, the failure to predict the complexity of the oscillation structure indicates the need for a more complex model. The model also predicts that in the cubic container, where there are 4 potential wells created by the geometry, the orientation of a convection roll crosses a potential barrier stochastically. We observe stochastic switching between diagonals of the cubic cell. The measured switching rate is within 30\% of the prediction $Ra = 4.8\times10^8$. We also investigate the tilt-induced potential in the cubic cell. We find that even a change of $\0.03\pm0.02^\circ$ in the tilting angle could cause the breaking of the symmetry of in temperature profile. We successfully model the shape of the tilt-induced potential, although the magnitude of the prediction is 1.7 orders of magnitude smaller than the measurement. We further extend the model to the two coupled cubic cell. Using the predicted the functional form of the interaction-induced potential, we measure the magnitude from the experiment and then add the interaction-induced potential to the model. We show that there are two stable states, counter- and co- rotating state. The counter- and co-rotating states are not perfectly two-dimensional, but in each case there are two preferred orientations symmetrically distributed around the centerline, which the system can stochastically switch between. The transition between two states is measured within the range of $\beta = 1^\circ$ and $\beta = 2^\circ$, which is an order of magnitude smaller than the prediction. Reversal from co-rotating state(preferred) to counter-rotating state(unpreferred) and back to co-rotating state is observed in the experiment at $\beta = -1.42 ^\circ$. We find that from the preferred state to the unpreferred state, $\delta$ of the cell which switches the direction decreases, which is similar to the reversal event observed in the cylinder. We compare the switching rate of the measurement to the prediction by the conditional potential, we find for the double-well potential, the measurement is consistent with the prediction within the error bar; for the single-well potential, the measurement rate is an order of magnitude off from the prediction. It's possible that Kramers's approximation is no longer valid in the single-well calculation. The measured dependence of the LSC orientations in the two cells on $\beta$ is qualitatively agree with the prediction for both the counter and co rotating state. We test the measurement of the strength of the LSC $\delta(\beta)$ against the prediction. For the counter-rotating state, the measurement agrees with the model prediction, in the case of the co-rotating state, the slope on $\beta$ dependence from the measurement is 3 times smaller, and $\delta_1$ is bigger than $\delta_2$ by at least $0.0274 \pm 0.0005 ^\circ C$, which disagrees with the model. As to the width of the distribution of the strength $\sigma(\beta)$, the measurement is 4.7 times smaller than the prediction. The success of the model in the switching rate prediction and the general shape prediction indicate the model is promising for applications in coupled-cell geometries. The failure in the prediction on the magnitude suggests that the coupled-cell may have extra source of potentials that have not been considered, such as the location of where the interaction of the two LSC happens. 
Zack Lasner's picture
Zack Lasner
David DeMille
Postdoctoral Researcher
Harvard University
Order-of-magnitude-tighter bound on the electron electric dipole moment
The electron's electric dipole moment (eEDM) is a time-reversal- (T-) violating interaction that is generically predicted to have a magnitude near or above the bounds of current experimental sensitivity in extensions to the Standard Model. We have completed an improved measurement of the electron's electric dipole moment with an order-of-magnitude greater sensitivity than the previous best measurement. The result is consistent with no interaction, |de|<1.1*10^-29 e cm. This upper bound is a factor of 8.6 smaller than the previous bound and correspondingly probes for new particles with masses at ~3-30 TeV, which is 3 times higher than previously explored in eEDM experiments. In this work, we describe the second-generation ACME experiment, models for and suppression of systematic errors, sources of phase noise, and preliminary work toward a third generation of the ACME apparatus.
Junjiajia Long's picture
Junjiajia Long
Thierry Emonet

From Individual to Collective Behavior: The Role of Memory and Diversity in Bacterial Navigation
What is the best strategy to search in an unknown environment? Navigation with only local information available is a ubiquitous problem in nature, especially when local directional information is unreliable due to limited detection range and accuracy. Facing this challenge, many natural systems, including the chemotactic bacteria Escherichia coli, navigate by registering past information “in memory” and making temporal comparisons to bias their random walk up gradients of signal. In groups, these bacteria can migrate collectively as they consume nutrients to create a gradient that moves along with them, traveling greater distances than a single cell could have. Previously, most research has made simplifying assumptions to reach analytical predictions. Individual memory for signal has been assumed to be short compared to changes in signal so that the past has little effect on present and future behavior. In populations, all individuals have been assumed to be identical. However, cells navigate in all kinds of environments, including ones where their memory is long compared to other time scales, and biological organisms always exhibit phenotypic diversity. Here, we restore these missing components to examine their effects on bacterial navigation. In individual navigation, we show that memory controls the balance between the positive feedback between behavior and sensed signal and the negative feedback of forgetting past signals. When the positive feedback dominates, a cell can quickly turn around after moving in the wrong direction and can extend motion in the right one, achieving “ratchet-like” gradient climbing behavior. In collective migration, we show that the diversity controls the inevitable leakage of cells off the back of the migrating group, which in turn shapes the group’s diversity. During migration, phenotypes that climb gradients too slowly are purged so that the rest of the population stays coherently traveling for longer times. In conclusion, we explicate the role of memory for individuals and the role of diversity for groups, and provide insights on how general biased random walk strategies, from individual to collective behavior, could be exploited.
James Mulligan's picture
James Mulligan
John Harris
Postdoctoral Associate
Inclusive jet measurements in Pb-Pb collisions with ALICE
Droplets of deconfined quarks and gluons, known as the quark-gluon plasma, are produced experimentally in ultrarelativistic heavy-ion collisions. Studying this deconfined matter may allow insight into a variety of open questions about the high temperature regime of QCD and the emergent behaviors of QCD. One major effort to probe the quark-gluon plasma is the study of high-momentum jets produced in an initial high momentum-transfer scattering of a heavy-ion collision. Measurements have demonstrated that by traversing the dense plasma, jets are modified in several ways, including that jet yields are suppressed in heavy-ion collisions relative to proton-proton collisions. The ALICE detector at the Large Hadron Collider reconstructs jets with high-precision tracking of charged particles combined with particle information from the electromagnetic calorimeter, achieving a unique kinematic range of jets extending to low jet momenta. This thesis describes inclusive jet measurements in Pb–Pb collisions at √sNN = 5.02 TeV with ALICE, which constitute the first such full jet measurements at low transverse jet momentum at this collision energy. These measurements are compared to several theoretical predictions, and will help constrain models of jet energy loss.
Michela Paganini's picture
Michela Paganini
Paul Tipton
Postdoctoral Researcher
Facebook AI Research
Machine Learning in High Energy Physics: Applications to Electromagnic Shower Generation, Flavor Tagging, and the Search for di-Higgs Production
This thesis demonstrate the efficacy of designing and developing machine learning algorithms to selected use cases that encompass many of the outstanding challenges in the field of experimental high energy physics. Although simple implementations of neural networks and boosted decision trees have been used in high energy physics for a long time, the field of machine learning has quickly evolved by devising more complex, fast and stable implementations of learning algorithms. The complexity and power of state-of-the-art deep learning far exceeds those of the learning algorithms implemented in the CERN-developed ROOT library. All aspects of experimental high energy physics have been and will continue being revolutionized by the software- and hardware-based technological advances spearheaded by both academic and industrial research in other technical disciplines, and the emergent trend of increased interdisciplinarity will soon reframe many scientific domains. This thesis exemplifies this spirit of versatility and multidisciplinarity by bridging the gap between machine learning and particle physics, and exploring original lines of work to modernize the reconstruction, particle identification, simulation, and analysis workflows. This contribution documents a collection of novel approaches to augment traditional domain-specific methods with modern, automated techniques based on industry-standard, open-source libraries. Specifically, it contributes to setting the state-of-the-art for impact parameter-based flavor tagging and di-Higgs searches in the γγbb channel with the ATLAS detector at the LHC, it introduces and lays the foundations for the use of generative adversarial networks for the simulation of particle showers in calorimeters. These results substantiate the notion of machine learning powering particle physics in the upcoming years and establish baselines for future applications.
Meredith Powell's picture
Meredith Powell
Meg Urry
Porat Fellow
Stanford
Research Website
The Environments of Accreting Supermassive Black Holes
The details of black hole-galaxy coevolution can be revealed by studying the multi-scale environments of accreting supermassive black holes (SMBH). Using state-of-the-art multiwavelength surveys of complementary depths, volumes, and resolutions, I studied the galaxies and cosmic environments that host Active Galactic Nuclei (AGN) to test current models of supermassive black hole fueling and feedback. On galactic scales, I investigated the morphology vs. galaxy properties of 5000 galaxies and AGN hosts from the CANDELS/GOODS survey, in order to examine the role of major mergers in their evolution at z~1. The results were consistent with a scenario in which mergers are associated with rapid quenching in galaxies and some of the black hole accretion. However, I found that the majority of the AGN hosts were disk-dominated and therefore not triggered by a major merger event. I also constrained AGN feedback models by searching for hot outflows in the form of extended X-ray emission in two nearby AGN and comparing this emission with their spatially-resolved ionized gas kinematics. The lack of a detection indicated that either the coupling efficiencies between the AGN and ambient medium are small (< 5%), or the density of the medium through which the outflow travels is low ($N_H < 1$ cm$^{-3}$ at a distance of 100 pc). The significance of major mergers in triggering AGN was further investigated by measuring the halo-scale environments of AGN, which are determined via clustering analyses. By measuring the spatial correlation function of hard X-ray selected AGN from the Swift/BAT AGN Spectroscopic Survey (BASS) and modeling it by populating dark matter halos from the Bolshoi-Planck simulation with empirical halo models, I constrained the halo occupation statistics of local AGN. I found that, on average, the AGN occupy halos consistently with inactive galaxies of the same stellar mass distribution. This suggests that the AGN are dominantly triggered by in-situ processes rather than environmental processes like mergers. However, when disaggregating by column density, I found that obscured AGN reside in denser environments than unobscured AGN, despite no significant differences in their luminosity, redshift, stellar mass, or Eddington ratio distributions. I showed that this could be due to their host halos having statistically different assembly histories. Lastly, I calculated the host halo masses of luminous X-ray AGN around the peak epoch of SMBH growth at z~1.8, which were found to be similar to previous studies of moderate-luminosity X-ray AGN. This suggests that selection biases are the cause for the slight clustering disparity observed between optical and X-ray AGN, rather than luminosity differences expected from distinct triggering mechanisms. The independent investigations comprising this thesis work are consistent with the scenario where major mergers are not the dominant triggering mechanism for AGN from low-to-moderate redshifts, and instead are mainly triggered by secular processes. Additionally, while AGN feedback may play an important role for some objects, hot quasar winds are not ubiquitous among highly accreting AGN.
Matthew Steinecker's picture
Matthew Steinecker
David DeMille
Graduate School Student Year 6 (DeMille)
SPL 23
Sub-Doppler Laser Cooling and Magnetic Trapping of SrF Molecules
In recent years, there has been growing interest in methods for producing gases of ultracold polar molecules, driven by proposals to employ ultracold molecules in applications in ultracold chemistry, quantum information and quantum simulation, and precision measurement. Development of direct laser cooling and trapping techniques for molecules has become a particular focus of experimental and theoretical effort, given the success of analogous techniques in producing ultracold atoms and the potential of these techniques to produce ultracold molecules with a variety of structures suitable for use in distinct applications. Here, we review recent advances in the laser cooling and trapping of strontium monofluoride (SrF). We describe methods for increasing the number of molecules available for trapping via modifications to the scheme for producing and slowing molecules from the molecular beam source. We detail changes to the magneto-optical trapping scheme that allow access to trapped samples at higher densities or lower temperatures. We implement and characterize a sub-Doppler optical molasses stage that efficiently cools SrF molecules to substantially lower temperatures than previously realized. We then describe loading of SrF molecules into a conservative magnetic trap, at temperatures comparable to those commonly used as a starting point in atomic sympathetic and evaporative cooling experiments. Finally, we describe ongoing work towards further laser cooling of SrF and towards observing ultracold atom-molecule collisions in a magnetic trap, an important step in extending sympathetic cooling techniques to ultracold polar molecules.
Lucie Tvrznikova's picture
Lucie Tvrznikova
Daniel McKinsey
Postdoctoral Researcher
Lawrence Livermore National Laboratory
Sub-GeV dark matter searches and electric field studies for the LUX and LZ experiments
Abundant evidence from cosmological and astrophysical observations suggests that the Standard Model does not describe 84% of the matter in our universe. The nature of this dark matter (DM) remains a mystery since it has so far eluded detection in the laboratory. To that end, the Large Underground Xenon (LUX) experiment was built to directly observe the interaction of DM with xenon target nuclei. LUX acquired data from April 2013 to May 2016 at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, which led to publications of many world-leading exclusion limits that probe much of the unexplored DM parameter space. This manuscript describes two novel direct detection methods that used the first LUX data set to place limits on sub-GeV DM. The Bremsstrahlung and Migdal effects consider electronic recoils that accompany the standard DM-nucleus scattering, thereby extending the reach of the LUX detector to lower DM masses. The spin-independent DM-nucleon scattering was constrained for four different classes of mediators for DM particles with masses of 0.4-5 GeV/c2. The detector conditions changed significantly before its final 332 live-days of data acquisition. The electric fields varied in a non-trivial non-symmetric manner, which triggered a need for a fully 3D model of the electric fields inside the LUX detector. The successful modeling of these electric fields, described herein, enabled a thorough understanding of the detector throughout its scientific program and strengthened its sensitivity to DM. The LUX-ZEPLIN (LZ) experiment, the successor to LUX, is a next-generation xenon detector soon to start searching for DM. However, increasingly large noble liquid detectors like LZ are facing challenges with applications of high voltage (HV). The Xenon Breakdown Apparatus (XeBrA) at the Lawrence Berkeley National Laboratory was built to characterize the HV behavior of liquid xenon and liquid argon. Results from XeBrA will serve not only to improve our understanding of the physical processes involved in the breakdown but also to inform the future of noble liquid detector engineering.
Degree Date: December, 2018
Andrew Gasbarro's picture
Andrew Gasbarro
George Fleming
Postdoctoral Associate
Studies of Conformal Behavior in Strongly Interacting Quantum Field Theories
In this dissertation, we present work towards characterizing various conformal and nearly-conformal quantum field theories nonperturbatively using a combination of numerical and analytical techniques. A key area of interest is the conformal window of four-dimensional gauge theories with Dirac fermions and its potential application to BSM model building. We advocate a research program in which the generic low energy physics of nearly-conformal gauge theories is characterized by combining lattice computations of specific gauge theories with more general effective field theory (EFT) analyses. We review lattice studies of the spectrum of a particular nearly-conformal gauge theory, eight flavor QCD, and discuss the light flavor-singlet scalar state as a candidate for a composite Higgs boson. New lattice results are presented for the maximal isospin pi-pi scattering length in the eight-flavor model. We compare the scattering length in the eight-flavor model to the scattering length in two- and six-flavor QCD and to the prediction from leading order chiral perturbation theory. Next, we present a new EFT framework based on the linear sigma model for describing the low-lying states of nearly conformal gauge theories. A particular emphasis is placed on the chiral breaking potential and the power counting of the spurion field. Explicit fits of the model to lattice data are shown. In the second half of the talk, we report on a new formulation of lattice quantum field theory suited for studying conformal field theories (CFTs) nonperturbatively in radial quantization. We demonstrate that this method is not only applicable to CFTs, but more generally for formulating a lattice regularization of quantum field theory on an arbitrary smooth Riemann manifold. The general procedure, which we refer to as quantum finite elements (QFE), is reviewed for scalar fields. Explicit numerical studies are presented for scalar phi-4 theory on the 2-sphere. We demonstrate that it is in close statistical agreement with the c=1/2 minimal 2D Ising CFT. We also investigate scalar phi-4 theory on R X S^2, and we report on progress towards studying the 3D Ising CFT in radial quantization on the lattice. Future directions for the method are discussed.
Elena Gramellini's picture
Elena Gramellini
Bonnie Fleming
Postdoctoral Associate
Fermilab
Measurement of the negative pion and positive kaon total hadronic cross sections on argon at the LArIAT experiment
The Liquid Argon Time Projection Chamber (LArTPC) represents one of the most advanced experimental technologies for physics at the Intensity Frontier due to its full 3D-imaging, excellent particle identification and precise calorimetric energy reconstruction. By deploying a LArTPC in a dedicated calibration test beam line at Fermilab, LArIAT (Liquid Argon In A Testbeam) aims to experimentally calibrate this technology in a controlled environment and to provide physics results key to the neutrino oscillation physics and proton decay searches of the Short Baseline Neutrino (SBND, MicroBooNE, ICARUS) and Long Baseline Neutrino programs (DUNE). LArIAT's physics program entails a vast set of topics with a particular focus on the study of nuclear effects such as pion and kaon characteristic interaction modes. This thesis presents two world's first measurements: the measurement of (π--Ar) total hadronic cross section in the 100-1050 MeV kinetic energy range and the measurement of the (K+-Ar) total hadronic cross section in the 100-650 MeV kinetic energy range. The analyses devised for these measurements use both the core elements of LArIAT: beamline and LArTPC. The first step in each analysis is the development of an event selection based on beamline and TPC information geared towards the identification of the hadron of interest. We then proceed to match the beamline candidate to a suitable TPC track. Finally, we apply the "thin slice method" technique and measure the cross section, correcting for background contamination and detector effects. The thin slice technique is a new method to measure hadron-argon cross sections possible only due to the combination of the tracking and calorimetry capability of the LArTPC technology. Albeit never on argon, the hadronic cross section of pions has been extensively measured before on lighter and heavier different elements in thin target experiments, leading to solid predictions for measurements on argon. Through the use of a different technique, our measurement of the (π--Ar) total hadronic cross section is in general agreement with the predictions by the Geant4 Bertini Cascade model which are based on data from thin target experiments. On the contrary, cross section measurements for kaons are extremely scarce, thus more difficult to model. Our measurement of the (K+-Ar) total hadronic cross section is mostly in tension with the Geant4 prediction over the explored range of energies and provides new experimental data to properly compare existing interaction models. This thesis also reports two ancillary detector physics measurements necessary for the cross section analyses: the measurements of the LArIAT electric field and calorimetry constants. We developed a technique to measure the LArIAT electric field using cathode-anode piercing tracks with cosmic data. We applied a new technique for the measurement of the calorimetry calibration constants based on the particles' momentum measurement. The (π--Ar) and the (K+-Ar) total hadronic cross measurements are the first physics results of the LArIAT experiment and will be the basis for the future LArIAT measurements of pion and kaon cross sections in the exclusive channels. The outcome of these measurements will ultimately enable to quantify and reduce the systematic associated with the hadronic interaction models in neutrino-argon interactions.
Siddharth Prabhu's picture
Siddharth Prabhu
Walter Goldberger
Postdoctoral fellow
Gluon and Graviton Radiation from the Classical Double Copy
Motivated by the BCJ double copy in quantum field theory, we demonstrate a correspondence between perturbative solutions of classical scalar, gauge and gravity theories. First, we show that classical Yang-Mills radiation from a system of colored particles can be obtained from classical bi-adjoint scalar radiation emitted by a corresponding system of bi-adjoint colored particles, via a simple set of color-kinematic substitution rules. This completes a two-fold double copy that can be used to generate classical gravitational radiation emitted by a collection of dynamical point sources, from the simpler bi-adjoint scalar radiation. Next, we extend the classical double copy to spinning sources, thus bringing it closer to the astrophysically relevant case of compact binaries. Here, we compute Yang-Mills radiation generated by a system of spinning color charges. We show that the color-kinematic replacements can be used to map this onto classical gravitational radiation emitted by a system of interacting spinning masses. If this classical double copy correspondence persists to higher orders in perturbation theory, it suggests the possibility of vastly simplifying the calculation of gravitational radiation, in particular, that from colliding compact objects, recently discovered by LIGO. 
Jared Vasquez's picture
Jared Vasquez
Paul Tipton
Senior Data Scientist
Illuminating the Higgs boson: Measurement of the properties of the Higgs boson in the diphoton channel
This thesis presents results for measurements of the Standard Model Higgs boson properties and production as measured in the diphoton decay channel using 36.1/fb of proton–proton collision data collected at √s = 13 TeV by the ATLAS detector in 2015 and 2016. All the measurements are performed under the assumption that the Higgs boson mass is 125.09 GeV and are compared to Standard Model predictions. No significant deviations of the measurements to the Standard Model predictions are found.
Fangzhou Zhu's picture
Fangzhou Zhu
Nikhil Padmanabhan
Quantitative Researcher
Citadel LLC
Information Mining in the Large Scale Structure of the universe
The Baryon Acoustic Oscillations signal has been an important tool to study the properties of dark energy. Designing efficient and robust data analysis methods that optimize the extraction of information is crucial to realize the immense potential of current and future galaxy surveys. To achieve this goal, this thesis presents the development and implementation of the 'redshift weighting' (Zhu et al 2015, 2016) and 'BAO emulation', two techniques that promise to increase the science return of future BAO experiments. In current and future BAO surveys, the samples cover a wide range of redshifts. Traditional analyses split the sample into multiple redshift bins and analyze the signals in these narrower slices to improve the redshift resolution of the distance-redshift relation. This approach results in lower signal-to-noise ratio in each slice, reducing the robustness of BAO detection. Signals at disjoint bin boundaries are also lost. "Redshift weighting" seeks to tackle this problem by compressing a sample's BAO information in the redshift direction onto a small number of weighted correlation functions that preserve nearly all of the signals (Zhu et al 2015). Building off the work in Zhu et al 2015, we implement the redshift weighting scheme and validate the method on the SDSS-III BOSS DR12 mock galaxy catalogs. We demonstrate the method gives unbiased distance and Hubble parameter estimates. The constraint is also in agreement with a Fisher forecast to within 10%, suggesting the method's efficiency in producing optimized BAO measurements (Zhu et al 2016). We apply the redshift weights technique to the clustering of quasars in the SDSS-IV eBOSS DR14 quasar sample. We produce and report 4.6% distance and Hubble parameter measurements using this sample. This work is presented in Zhu et al 2018 (subumitted to MNRAS and available on ArXiv). BAO emulation is a method we developed to address the computational challenge of BAO modeling. MCMC is one of the most commonly used methods to sample the parameter likelihood surface. To reach a convergent MCMC chain, one typically requires tens of thousands of model evaluations. This high demand puts considerable stress on the speed of BAO model prediction. We built a fast emulator for BAO correlation function predictions. The emulator uses a low discrepancy Sobol sequence to efficiently sample the BAO parameter space, and utilizes Principal Component Analysis (PCA) and Gaussian Process regression (GP) for model prediction. We construct the emulator for two popular BAO models at very modest computational costs. Finally, we implement the emulator and produce BAO measurements from the BOSS DR12 galaxy sample. I conclude by assessing avenues for future work that can build on the methods developed in this thesis. Future redshift surveys present exciting opportunities to enrich the science return from the BAO method. Both 'redshift weighting' and 'BAO emulation' can contribute to unlocking the full potential of upcoming surveys, getting us closer to unveiling the mystery of dark energy. 
Degree Date: May, 2018
Jeremy Cushman's picture
Jeremy Cushman
Karsten Heeger
Software Engineer
A search for neutrinoless double-beta decay in tellurium-130 with CUORE
The Cryogenic Underground Observatory for Rare Events (CUORE) is a ton-scale cryogenic experiment designed to search for neutrinoless double-beta decay in tellurium-130. The experiment consists of 988 ultracold tellurium dioxide bolometric crystals, which act as both the double-beta decay sources and detectors, in a close-packed configuration. This dissertation presents a search for neutrinoless double double-beta decay with the first two months of CUORE data. An observation of this decay would be direct evidence of lepton number violation and unambiguously prove that neutrinos are Majorana particles.   We analyze the first 83.6 kg yr of tellurium dioxide exposure and find no evidence for neutrinoless double-beta decay. We set a half-life limit of 1.5 × 10^25 yr (90% C.L.) by combining this exposure with that from two predecessor experiments, CUORE-0 and Cuoricino. With this data, we set the world-leading limit on the rate of neutrinoless double-beta decay in tellurium-130. The CUORE bolometer array is characterized by an effective energy resolution of 7.7 ± 0.5 keV FWHM and background of 0.014 ± 0.002 counts/(keV kg yr) at the double-beta decay Q-value. This is the lowest background level achieved to date in such a large-scale cryogenic experiment, meeting our expectations and requirements for this search.
Stefan Elrington's picture
Stefan Elrington
Sean Barrett
Consultant
Developing new approaches to faster, high-spatial resolution Phosphorus-31 MR imaging of Bone
Magnetic resonance imaging (MRI) is the leading non-invasive imaging technique of soft tissues on anatomical and millimeter scales. In conventional MRI, hydrogen-1 in water and liquid fats are detected; the relatively narrow spectra of these signals are the key feature that enables MR imaging with high-spatial resolution (i.e., sub-mm in each dimension). The broad NMR spectra of solids would be much more difficult to use in MRI, and would ordinarily result in a low spatial resolution image. Previously, our lab developed a pulse sequence to effectively narrow the broad spectrum of lines in solids. This sequence, applied to phosphorus-31 spins in bone, was able to achieve high resolution imaging. Despite this tremendous progress, our approach to MR imaging of solid samples is still signal limited and the imaging times are very long. In this thesis work, I propose using a variation on a solid-state, double-resonance NMR technique to increase the signal of phosphorus-31 imaging and to be able to do so in a shorter time. This approach uses cross-polarization to resonantly transfer magnetization from a hydrogen-1 spin bath that is cooler, i.e. more polarized, and more quickly refreshed, to a phosphorus-31 spin bath. As explained in this thesis work, bone is a poor choice of sample for a cross-polarization experiment.  In order to overcome this disadvantage, I developed a new approach, called StepCP, to boost the phosphorus-31 signal in a stepwise fashion. Using this technique, I demonstrate that the signal to noise ratio using double-resonance in solids can be increased, and that the signal can be attained in fraction of the acquisition times previously used to generate phosphorus-31 images in bone.
Yvonne Gao's picture
Yvonne Gao
Robert Schoelkopf

Multi-cavity Operations in Circuit Quantum Electrodynamics
The eventual success of a quantum computer relies on our ability to robustly initialise, manipulate, and measure quantum bits in presence of the inevitable occurrence of errors. This requires us to encode quantum information redundantly in systems that are suitable for Quantum Error Correction (QEC). One promising implementation is to use three dimensional (3D) superconducting microwave cavities coupled to one or more non-linear ancillae in the circuit quantum electrodynamics (cQED) framework. Such systems have the advantage of good intrinsic coherence properties and large Hilbert space, making them ideal for storing redundantly encoded quantum bits. Recent progress has demonstrated the universal control and realisation of QEC beyond the break-even point on a logical qubit encoded in a mulitphoton state of a single cavity. This thesis presents the first experiments in implementing quantum operations between multiphotons states stored in two separate cavities. We first explore our ability to create complex two-mode entangled states and perform fully characterisation in a novel multi-cavity architecture. Subsequently, we will demonstrate the capability to implement conditional quantum gates between two cavity modes, assisted by a single ancilla. Further, we develop a direct, tunable coupling between two cavities and use it to study complex multiphoton interference between two stationary bosonic states. Combining this with robust single cavity controls, we construct a universal entangling operation between multiphoton states. The results presented in this thesis demonstrate the vast potential of 3D superconducting systems as robust, error-correctable quantum modules and the techniques developed constitute an important toolset towards realising universal quantum computing on error-corrected qubits.
Ariana Hackenburg's picture
Ariana Hackenburg
Bonnie Fleming
Data Scientist
Wayfair
Measurement of a Neutrino-Induced Charged Current Single Neutral Pion Cross Section at MicroBooNE
Micro Booster Neutrino Experiment (MicroBooNE) is a Liquid Argon Time Projection Chamber (LArTPC) operating in the Booster Neutrino Beamline at Fermi National Accelerator Laboratory. MicroBooNE’s physics goals include studying short basline ν oscillation and performing a suite of ν cross section measurements. Of particular interest to MicroBooNE, and the broader LArTPC community, are electromagnetic showers; these showers are at the heart of searches for νe interactions, including MicroBooNE’s flagship search for a MiniBooNE-like low energy excess (LEE). Neutral current π0’s, which decay into 2 electromagnetic showers (γ’s), are the dominant source of non-νe backgrounds in searches for νμ → νe oscillations in LArTPCs, such as the LEE. While precise measurements of this neutral current channel will provide a tight constraint on our modeling uncertainties, such events are particularly difficult to identify in data with our current tools, as there is often little or no activity at the neutrino interaction point. Charged current interactions, on the other hand, have simpler topologies with a long μ track that anchors to the interaction vertex. With a vertex in hand, we can develop automated reconstruction tools for neutrino-induced shower topologies (like the γ’s from π0 decay). Thus, in studying charged current π0 interactions, we are developing tools that can potentially be used to reconstruct an important LEE background, while also studying the physics of neutrino interactions, of which data is sparse for argon. This thesis reports the world’s first measurement of the absolute, flux-averaged cross section of νμ-charged current single π0 production on argon. The analysis chain begins with the selection of inclusive νμ charged current events, where a candidate μ and neutrino interaction vertex are identified. These events are then passed to a reconstruction framework where electromagnetic shower candidates are reconstructed using computer visualization tools. Finally, the cross section is calculated on two reconstructed topologies: those with at least two reconstructed showers and those with at least one. Additionally, this work describes the first fully-automated electromagnetic shower reconstruction process employed by a LArTPC to perform a cross section analysis. We measure the flux averaged cross section on argon at 824 MeV via the two and one shower selections respectively to be σ≥2Shower = (2.56 ± 0.50stat ± 0.31genie ± 0.37flux ± 0.31det) × 10−38 cm2/Ar, σ≥1Shower = (2.64 ± 0.33stat ± 0.36genie ± 0.38flux ± 0.35det) × 10−38 cm2/Ar.
Xin Li's picture
Xin Li
Mark Reed
Postdoctoral Associate
Nano-confined interfaces: from artificial ion channels to nanofluidic battery
Nano-confined interfaces have received growing attention in recent years, with a wide range of applications including artificial ion channels, water desalination, osmotic energy conversion, and electrical energy storage. New phenomena occur and new physics is expected when solid-liquid interfaces are under nanoscale confinement. My research projects include divalent ion transport in artificial ion channels and nanofluidic battery for lithium storage. By taking advantage of precision definition of geometry from top-down nanofabrication, I have designed and built nanofluidic devices with well-defined structures and highly controllable materials. Utilizing these nano devices, I explored the fundamental physics of ion transport in aqueous system with potential applications in artificial protein channels and water desalination. I also investigated electrochemical lithium insertion under nano-confinement for electrical energy storage with electrolytes dissolved in organic solvents.
Anthony Lollo's picture
Anthony Lollo
Jack Harris
Data Science Manager
Yale School of Public Health
Phase slips in isolated mesoscopic superconducting rings
Phase slips result in many interesting properties of superconducting materials, such as the finite resistance of thin superconducting nanowires, the decay of current in superconducting rings, and the flux periodicity of the critical temperature, Tc, in hollow superconducting cylinders or rings. Though the first experiments were performed in 1961, the goal of observing coherent macroscopic quantum tunneling in uniform superconductors has sparked recent interest in the field.   We present measurements of the supercurrent in arrays of uniform isolated mesoscopic aluminum rings as a function of applied magnetic field, measured through cantilever torque magnetometry. These measurements are taken over the full range of applied magnetic fields for which the rings are superconducting and for 400 mK < T < 1300 mK. We fit the supercurrent data to Ginzburg-Landau (GL) theory for a 1-dimensional superconducting ring with the inclusion of finite width. This detailed analysis extends the range of both temperature and applied magnetic fields over which prior measurements were quantitatively analyzed. Further, we show that phase slips occur deterministically as the free energy barrier separating two metastable states vanishes. We also present measurements of the distribution of applied magnetic fields at which a phase slip occurs for two individual isolated superconducting rings each of different radius. We find that as temperature is increased the mean and standard deviation decrease, while the skewness is always close to −1.
David Meltzer's picture
David Meltzer
David Poland
Postdoctoral Fellow
Topics in the Analytic Bootstrap
In this thesis we explore analytical methods to study conformal field theories (CFTs) in a general number of spacetime dimensions. We first use the lightcone bootstrap to systematically study correlation functions of scalar operators charged under global symmetries. We then generalize existing techniques in the lightcone bootstrap to study four-point functions containing operators with spin. As an application, we observe a close connection between anomalous dimensions of large spin, double-twist operators and the conformal collider bounds. Through further refinement of these techniques and the application of known analyticity properties for four-point functions, we also present a proof for these bounds that relies on basic physical consistency conditions. We then generalize these techniques further to study large N CFTs in the Regge limit and the implications of crossing symmetry in this limit. By studying the Regge limit, we can make new predictions for the large twist, large spin spectrum of CFTs and derive new bounds on CFT data. In the final part of this thesis, we use the Regge limit and constraints from unitarity to derive new bounds for both large N and generic CFTs. For large N CFTs, we derive new constraints on theories dual to a weakly-coupled, gravitational theory in an Anti-deSitter (AdS) spacetime, and for generic CFTs we derive generalizations of the conformal collider bounds.
Aleksander Rebane's picture
Aleksander Rebane
Yongli Zhang and James E. Rothman
Postdoctoral Associate
Exploring free energy landscapes of SNARE assembly using optical tweezers
Scientists have long sought to understand the working principles of protein machinery. A decisive step towards this goal has been the development of the Gibbs free energy landscape of protein folding. However, measurement of energy landscapes has remained challenging, particularly when folding occurs over one or more intermediates. An important example is soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex assembly, in which the energetics and kinetics of multiple assembly steps are coupled to distinct stages of vesicle maturation and membrane fusion in synaptic exocytosis. As a result, a quantitative test of this fundamental biophysical mechanism remains outstanding. In recent years it has become possible to measure energy landscapes of proteins in the presence of force using a single-molecule manipulation technique called optical tweezers (OT). However, derivation of energy landscapes in the absence of force from OT data has remained difficult. Here, we present a comprehensive OT data analysis method that uses information from high-resolution protein structures to derive a simplified energy landscape of protein folding at zero force by model fitting of the experimental measurements. We apply our method to derive the energetics, kinetics, and intermediate conformations of SNARE assembly for the wild-type complex and a number of mutants with known phenotypes. We characterize how the steps in SNARE assembly function in the respective stages of synaptic exocytosis and provide quantitative verification of the coupling mechanism. Finally, we investigate the mechanism by which two SNARE mutations cause severe neurological disease. In sum, our work provides a complete methodology to measure energy landscapes to reveal the underlying mechanisms of protein function.
Degree Date: December, 2017
Emine Altuntas's picture
Emine Altuntas
David DeMille
Postdoctoral Researcher
National Institute of Standards and Technology & Joint Quantum Institute
Measurement of Nuclear Spin Dependent Parity Violation in 138Ba19F
Parity, one of the three discrete spacetime symmetries of nature is broken by weak interactions. In atomic systems, parity violation is manifested in two ways: nuclear spin independent and spin dependent effects. The former is a relatively large effect that has been measured to better than 1%, whereas the nuclear spin dependent parity violation (NSD-PV) effect is small and remains poorly understood. To date the only nonzero measurement of NSD-PV effects in atoms was made in Cs, but the uncertainty in this measurement is substantial (~14%) and the result is in disagreement with other data from nuclear physics measurements. Therefore, precise measurements of anapole moments will yield crucial information on the parameters that determine the strength of purely hadronic weak interactions. We study NSD-PV effects using diatomic molecules. In this thesis, I discuss measurements that demonstrate a statistical sensitivity to NSD-PV surpassing that of any previous atomic PV measurement, using the test system 138Ba19F. With total ~168 hours of data, our statistical uncertainty for the NSD-PV weak matrix element, W is ≤0.5 Hz. The sensitivity we demonstrate would be sufficient to measure NSD-PV effects of the size anticipated across a wide range of nuclei. We describe the details of data analysis and also an extensive study of systematic errors that can affect measurements using our technique, and show that these can be controlled at least at the level of the present statistical sensitivity.
Jacob Blumoff's picture
Jacob Blumoff
Rob Schoelkopf
Research Scientist
HRL Laboratories
Multiqubit experiments in 3D circuit quantum electrodynamics
Circuit quantum electrodynamics (cQED) is the field of manipulating and measuring quantum electrical circuits. These circuits operate in the microwave regime, allowing use of sophisticated experimental equipment and techniques developed for industry. The nature of these devices allows for very strong interactions, providing interesting and accessible physics in the single-quantum regime. Recently, part of the field has branched from strictly lithographically designed circuits to exploit the distinctly three-dimensional electromagnetic environment. 3D components can be very high-Q compared to their 2D counterparts. This thesis explores some of the first experiments in 3D cQED to use multiple qubits, both with transmon qubits and qubits encoded in the states of harmonic oscillators. One experiment demonstrates a novel method to use a high-Q resonator to measure a register of transmon qubits in nontrivial ways. We go on to rigorously characterize these measurements. A second experiment realizes the first deterministically teleported two-qubit gate. The qubits in this experiment are encoded in harmonic oscillators. We use an entangled pair of transmons as a resource, exploiting high-fidelity measurement and real-time feedback. The tools employed in these experiments constitute a suite of capabilities necessary for increasingly complex cQED experiments.
Benjamin Brubaker's picture
Benjamin Brubaker
Steve Lamoreaux
NIST NRC Postdoc
JILA (University of Colorado/NIST Boulder)
First Results from the HAYSTAC axion search
The axion is a well-motivated cold dark matter (CDM) candidate first postulated to explain the absence of CP violation in the strong interactions. CDM axions may be detected via their resonant conversion into photons in a “haloscope” detector: a tunable high-Q microwave cavity maintained at cryogenic temperature, immersed a strong magnetic field, and coupled to a low-noise receiver.    This dissertation reports on the design, commissioning, and first operation of the Haloscope at Yale Sensitive to Axion CDM (HAYSTAC), a new detector designed to search for CDM axions with masses above 20 µeV. I also describe the analysis procedure used to derive limits on axion CDM from the first HAYSTAC data run, which excluded axion models with two-photon coupling g_{aγγ} > 2×10^{-14} GeV^{-1}, a factor of 2.3 above the benchmark KSVZ model, over the mass range 23.55 < m_a < 24.0 µeV.    This result represents two important achievements. First, it demonstrates cosmologically relevant sensitivity an order of magnitude higher in mass than any existing direct limits. Second, by incorporating a dilution refrigerator and Josephson parametric amplifier, HAYSTAC has demonstrated total noise approaching the standard quantum limit for the first time in an axion search.
Stephen Horvat's picture
Stephen Horvat
Helen Caines

Measurement of the collision energy dependence of jet-quenching signatures of de-confinement at STAR
Confinement is a phenomenon where quarks and gluons are only found in bound color-neutral states, or hadrons. Experiments at the Brookhaven National Laboratory (BNL) and the European Organization for Nuclear Research (CERN) have measured and published key signatures for the formation of a state of nuclear matter where quarks are temporarily de-confined in the hot, dense aftermath of heavy-ion nuclear collisions at \sqrts\ = 200\,GeV. This de-confined state corresponds to the theoretically predicted quark gluon plasma (QGP). One reported QGP signature was the suppression of high momentum particles using nuclear modification factors. STAR is now analyzing data produced in the RHIC Beam Energy Scan (BES) which spans \sqrts\ = 7.7 - 62.4\,GeV. This dissertation reports the collision energy dependence of nuclear modification factors. The high-transverse-momentum (high-\pt) suppression reported at higher \sqrts\ is seen to turn off and be replaced by an enhancement of high-\pt\ particle production as the collision energy is reduced. The physics that leads to this strong enhancement competes against and obscures the physics leading toward suppression. Only when the suppression is stronger than the enhancement do the nuclear modification factors provide a clean signature of QGP formation. This dissertation also outlines a new procedure to better disentangle suppression effects from enhancement effects. In this procedure, the centrality dependence of enhancement and suppression effects are exploited with the goal of establishing a limit for the minimum collision energy needed to produce a QGP in central heavy-ion collisions.
Anna Kashkanova's picture
Anna Kashkanova
Jack Harris
Postdoctoral fellow
Max Planck institute for the science of light
Optomechanics with Superfluid Helium
The field of optomechanics studies the interaction between electromagnetic and mechanical degrees of freedom via radiation pressure. This interaction is usually enhanced when both electromagnetic and mechanical degrees of freedom are normal modes of resonators, with the canonical optomechanical system being a cavity in which one mirror is mounted on a spring (thereby constituting a mechanical element). The majority of mechanical elements used in optomechanics to date are solid objects (mirrors, membranes, nanowires, etc); however fluids can also be used to form the mechanical element. In this thesis, I describe the use of density waves in superfluid helium as a mechanical element in an optomechanical system. The reasons for using superfluid helium are the following: superfluid helium has high thermal conductivity, allowing it to be easily thermalized to cryogenic temperatures; it has negligible optical loss at IR wavelengths, which means it does not diminish the optical finesse of the cavity; finally its acoustic loss vanishes at low temperatures, allowing it to have a high mechanical quality factor. In this system, we drive a normal mode of the density waves by modulating the optical intensity incident on the cavity. We also observe the mode’s undriven thermal motion and from that extract its phonon number. The optomechanical effects of optical spring and optical damping were observed, as were signatures of the mechanical mode’s quantum motion. These quantum signatures were the asymmetry between the Stokes and anti-Stokes sidebands, which arise from a combination of the mode’s zero point motion and the quantum backaction of the optical readout. We found agreement between these measurements and theoretical predictions (to within 20%) over a large range of mode temperatures.
David Mason's picture
David Mason
Jack Harris
Postdoctoral Associate
Dynamical Behavior near Exceptional Points in an Optomechanical System
Coupled mechanical oscillators have long been an archetypical system for understanding eigenmodes and coupled dynamics. But in the last few decades, the study of open systems (i.e. those open to loss or gain) has brought a fresh interest and perspective to such simple systems, revealing a surprisingly rich set of physical phenomena. Specically, it was realized that degeneracies in open systems ('exceptional points', or EPs) possess a non-trivial topology, with interesting implications for closed adiabatic cycles. The theoretical properties of EPs have been made increasingly clear over the last 20 years, but experimental progress has generally been limited to spectroscopy, with no demonstrations of the predicted dynamical behavior. Here, I'll present work in which we use a cavity optomechanical system as a convenient, highly tunable platform for studying this multimode physics.    I'll begin with a pedagogical introduction to cavity optomechanics, including our particular experimental realization: a Si3N4 membrane coupled to a high-nesse optical cavity. Then, the physics of exceptional points will be reviewed using a toy model, before seeing how these features are accessible in our optomechanical system. I'll then present our study of time-dependent perturbations of this system, which provided the rst experimental demonstration of adiabatic (and non-adiabatic) behavior near an EP. These perturbations can be used to affect energy transfer which is both topology-dependent and non-reciprocal. This demonstration relies on a somewhat fortunate symmetry in our system, but I'll then show that through a modified optical coupling scheme, this restriction can be lifted, to enable this energy transfer in a broad class of systems.
Evan Pease's picture
Evan Pease
Dan McKinsey
Senior Data Analyst
Cisco Meraki
Rare-event searches in liquid xenon with the LUX and LUX-ZEPLIN detectors
Liquid xenon has been used with great success in recent attempts to directly detect dark matter particles. The Large Underground Xenon (LUX) experiment recently concluded nearly four years of underground operations at the Sanford Underground Research Facility in Lead, South Dakota. The final 332 live-days of data were acquired with time-varying detector conditions, which required frequent calibrations and new data quality studies beyond those used for the results from the initial 95-live-day exposure. In addition to its world-leading dark matter search at low recoil energies approaching 1 keV, the LUX experiment has been used for the study of liquid xenon properties out to near 1 MeV. New measurements of scintillation and ionization yields and electron-ion recombination in liquid xenon were made, and a treatment of high-energy detector effects, including saturation and multiple-scatter rejection, was implemented for improved energy resolution. These improved analysis techniques have been used to set a new limit on the half-life for the neutrinoless double-beta decay candidate, Xe-134. The sensitivity of searches for rare events, like dark matter interactions and neutrinoless double-beta decay, scale with target mass, and so the design and successful construction of a 200-kV cathode high voltage system, applicable for future ton-scale noble liquid detectors like LUX-ZEPLIN (LZ), is also described.
Alexey Shkarin's picture
Alexey Shkarin
Jack Harris
Postdoctoral Fellow
Max Planck Institute for the Science of Light
Quantum Optomechanics with Superfluid Helium
The field of optomechanics deals with the interaction between light and mechanical objects. One of the goals in this field is to gain ability to coherently manipulate mechanical states with single-quantum precision and to interface these states with electromagnetic radiation without loss. Recent achievements enabled by this power include cooling of the mechanical oscillator to its quantum ground state, generating optical or mechanical squeezing, or entangling mechanical and optical degrees of freedom. To accomplish these goals, one generally aims to create a system with strong optomechanical coupling, while maintaining low optical and mechanical losses and low temperature. Superfluid helium is a liquid which is uniquely well-suited to meet these requirements.   In this talk I will describe the cavity optomechanics systems in which we couple infrared light to a standing acoustic wave in superfluid helium. With this system, we used light to coherently excite acoustic vibrations and manipulate their frequency and damping rate using the dynamic back-action effect. In addition, we measured thermal fluctuations of the mechanical mode corresponding to mean phonon number of five. These measurements had sufficient precision to reveal quantum signatures in the motion of the acoustic waves and in their interaction with light. Specifically, we observed the expected one-phonon difference between the Stokes and anti-Stokes mechanical sidebands, and indirectly measured the action of the optical shot noise on the mechanical object by investigating the correlations between these sidebands."
Brian Tennyson's picture
Brian Tennyson
Daniel McKinsey
Sensor Systems Engineer
Two Phase Liquid-Gas Xenon Time Projection Chambers: Theory, Applications, and Analysis
Two phase liquid-gas xenon-based detectors employ liquified xenon as the primary detector medium and are able to reconstruct the position and energy of interactions within the detector. These detectors are sensitive to a wide variety of particles, including gamma and beta emissions and neutrinos. They are also hypothetically sensitive to WIMP (Weakly Interacting Massive Particle) dark matter. This dissertation presents the contributions made by the author to three projects using this type of detector. The Large Underground Xenon (LUX) experiment employed this type of detector in a search for WIMPs. The author developed the background model used in the reanalysis of the first WIMP search and the analysis of the second WIMP search with LUX. The author performed a search for a solar neutrino magnetic moment using LUX data. The author also performed a study of the LUX-ZEPLIN (LZ) dark matter detector's sensitive to $^{136}$Xe neutrinoless double beta decay. The author built, operated, and analyzed data from the CoDeX (Compton Detection in Xenon) detector. The CoDeX detector is a  research and development prototype intended to demonstrate the feasibility of using two phase, liquid-gas, xenon-based detectors to passively detect and image special nuclear materials (SNM) over a wide field of view. The author's analysis and results for each of these project is presented. 
Uri Vool's picture
Uri Vool
Michel Devoret
Postdoc fellow (JHDSF)
Harvard University
Engineering synthetic quantum operations
Coherent quantum effects are the hallmark of atomic systems. The field of circuit quantum electrodynamics (cQED) also allows for the control of coherent quantum systems. However, these quantum states do no correspond to atomic degrees of freedom, but to the quantized behavior of the electromagnetic field in a marcoscopic superconducting circuit. These "artificial atoms" simulate many of the effects in atomic systems, with the added benefits of tunability and fast control and measurement. This thesis explores the different artificial atoms and quantum operations accessible to us using superconducting circuits, and the techniques we can use to create more interesting and complex atoms. One experiment focuses on selection rules in superconducting circuits. Using non-linear coupling, we are able to break the selection rules of a fluxonium artificial atom and drive forbidden transitions. We use this technique to construct a $\Lambda$ system from the fluxonium coupled to a resonator at the fluxonium sweet spot. Another experiment focuses on the new artificial atoms and operations accesible by adding continuous external drives to the circuit. By taking the Jaynes-Cummings (JC) Hamiltonian of a qubit coupled to a cavity and adding two continuous tones, we are able to simulate an effective JC Hamiltonian in the transverse ($\bm{\sigma_x}$) basis. The energies and interaction terms are completely governed by the drives, and the system can be tuned to any interaction regime in situ. This scheme also allows us to cool the qubit to the eigenstates of the transverse basis, and perform a continuous quantum non-demolition (QND) measurement of the transverse component of a qubit.
Degree Date: May, 2017
Corey Adams's picture
Corey Adams
Bonnie Fleming

First Detection of Low Energy Electron Neutrinos in Liquid Argon Time Projection Chambers
Electron neutrino appearance is the signature channel to address the most pressing questions in neutrino oscillations physics, at both long and short baselines. This includes the search for CP violation in the neutrino sector, which the U.S. flagship neutrino experiment DUNE will address. In addition, the Short Baseline Neutrino Program at Fermilab (MicroBooNE, SBND, ICARUS-T600) searches for new physics, such as sterile neutrinos, through electron neutrino appearance. Liquid argon time projection chambers are the fore-front of neutrino detection technology, and the detector of choice for both short and long baseline neutrino oscillation experiments. This work presents the first experimental observation and study of electron neutrinos in the 1-10 GeV range, the essential oscillation energy regime for the above experiments. The systematic uncertainties for an electron neutrino appearance search for the Fermilab Short Baseline Neutrino Program are carefully quantified, and the characterization of separation between electrons and high energy photons is examined.
Victor Albert's picture
Victor Albert
Liang Jiang
Physicist
National Institute of Standards and Technology
Research Website
Lindbladians with multiple steady states: theory and applications
Lindbladians, one of the simplest extensions of Hamiltonian-based quantum mechanics, are used to describe decay and decoherence of a quantum system induced by that system's environment. Traditionally, an environment is viewed as detrimental to fragile quantum properties. Nevertheless, it offers the ability to drive the system toward exotic phases of matter, which may be difficult to stabilize in nature, or toward protected subspaces, which can be used to store and process quantum information. The latter case (and sometimes the former case) requires the Lindbladian to have more than one steady state. Such Lindbladians, while not generic, are dissipative analogues of Hamiltonian systems with degenerate ground states. However, while Hamiltonians with degenerate ground states have been well-understood for some time, a corresponding treatment of Lindbladians is still underway. This dissertation aims to provide a snapshot of such a general treatment, reviewing Lindbladian extensions of topics commonplace to Hamiltonian systems as well as presenting some new results in this direction. An important property of Lindbladians is their behavior in the limit of infinite time, and the first part of this work focuses on deriving a formula for the asymptotic projection — the map corresponding to infinite-time Lindbladian evolution. This formula is applied to determine the dependence of a system's steady state on its initial state, to determine the extent to which decay affects a system's linear or adiabatic response, and to determine geometrical structures (holonomy, curvature, and metric) associated with adiabatically deformed steady-state subspaces. Using the asymptotic projection to partition the physical system into a subspace free from nonunitary effects and that subspace's complement (and making a few other minor assumptions), a novel Dyson series is derived to all orders in an arbitrary perturbation. The number of terms in the series which are the same order in the perturbation is shown to be a Catalan number. In process of the above derivations and in a series of previously published and novel results, we make contact (to various degree) with the following previously studied topics: frustration-free Hamiltonians, dark states, geometric/quantized response, quantum Zeno dynamics, the effective operator formalism, Noether's theorem, quantum channel simulation, non-Hermitian Hamiltonian systems, and cat codes. In the last few chapters, we apply the developed general techniques to thoroughly investigate the Lindbladian admitting the simplest cat code and introduce new extensions of cat codes to multiple bosonic modes. Click here for copy of full thesis.
Marco Bonett-Matiz's picture
Marco Bonett-Matiz
Yoram Alhassid
Teacher Summer Programs
Yale University
Statistical and Spectroscopic Properties of Nuclei in the Shell Model Monte Carlo Method
The predictive power of the interacting shell model in describing properties of nuclei is restricted by the limitations of conventional diagonalization techniques. The shell model Monte Carlo (SMMC) method allows the calculation of thermal properties in very large model spaces, much beyond what is possible with exact diagonalization. In particular, the SMMC has become the state-of-the-art method for the calculation of statistical properties of nuclei. The total state density that is calculated in SMMC includes the magnetic degeneracy of nuclear levels. On the other hand, the quantity that is usually measured experimentally is the level density, in which each level is counted once irrespective of its spin. We present a method to calculate level densities in SMMC and apply it to mid-mass and heavy nuclei. In particular, we present the first calculation of densities for odd-mass nuclei, circumventing a sign problem that originates in the projection on odd number of particles. We find good agreement with various experimental results. We also introduce and validate a method to calculate low-lying excitation energies, for the first systematic calculation of spectroscopic properties within SMMC. The method is based on the imaginary time-dependent correlation matrix (ITCM) of the one-body densities. We successfully reproduce the first few low-lying excitation energies for each spin and parity in a light nucleus, for which exact diagonalization is possible.
Mehmet Dogan's picture
Mehmet Dogan
Sohrab Ismail-Beigi
Postdoctoral researcher
University of California, Berkeley
Ab initio studies of ferroelectric thin films
Epitaxial interfaces between metal oxides and semiconductors have been of significant research interest due to their potential use in electronic device applications. Thin films of metal oxides can display many functional physical properties, an important example of which is ferroelectricity. Ferroelectric thin metal oxide films grown on semiconductors can enable non-volatile transistors, where the state of the device is encoded in the polarization state of the oxide which determines the electronic transport properties of the semiconductor. This thesis presents theoretical studies of a number of metal oxide on semiconductor systems using first principles electronic structure methods. We have studied the BaTiO_{3}/Ge interface as a candidate of a ferroelectric oxide/semiconductor system. In one set of studies of this interface, we have shown how cross-interfacial structural couplings can create atomic-scale structural motifs in the metal oxide that do not exist in any of its bulk phases. Separately, we have found that multiple polarization states in the BaTiO_{3} film are possible and, in principle, that one can switch between them by the application of an external electric field. Unfortunately, the overall direction of the polarization is pinned by the interface chemistry in this system. In order to modify the interface chemistry to promote ferroelectricity, we have proposed the usage of a buffer layer between the oxide and the semiconductor, such as a monolayer of zirconia. We have explored the possible stable configurations of single monolayers of ZrO_{2} on Si and found that multiple polarization states are indeed stabilized. We have found that ferroelectric switching between these two structures would lead to modifications of the Si electronic band properties in a manner comparable to available experimental results. We have developed a discrete lattice model to predict domain behavior in these monolayer films at finite temperatures. In a final set of works, we have conducted a study of thin films of doped hafnia which have recently shown ferroelectric behavior. We have focused on strain effects in doped HfO_{2} to explain some of the experimental observations from a structural point of view. Our findings provide an understanding for the stabilization of ferroelectricity in hafnia based thin films.
Alexandru Bogdan Georgescu's picture
Alexandru Bogdan Georgescu
Sohrab Ismail-Beigi

New Methods and Phenomena in The Study of Correlated Complex Oxides
Transition metal oxides have long been an important subject of study, both theoretically and experimentally. The wide array of phases possible in their bulk forms (high T$_c$ superconductivity, colossal magnetoresistance, ferroelectricity, etc.) makes them of scientific and technological significance, while relatively recent materials deposition techniques have allowed researchers to grow new, 'artificial' materials in the form of heterostructures and thin films. These structures offer a rich array of parameters to explore, as interfaces and thin films often show patterns of behavior that are quite different from their parent bulk compounds. From the point of view of electronic structure theory, this offers a rich playground where one can search for new physical phenomena. What makes transition metal oxides physically interesting is also what makes them difficult to study theoretically: the transition metal d-orbitals that dictate the wide array of phases in this class of materials cannot always be treated appropriately within band theory due to strong local electron-electron interactions. The local interactions are most often treated with a multi-band Hubbard model 'glued' on top of the first principles calculation. In this thesis, we have explored both a variety of complex oxide heterostructures and phenomena as well as advanced the computational framework used to describe them. We have analyzed the effect of local electrostatic fields at a ferroelectric-manganite interface as seen by electron energy loss spectroscopy, found a dimer-Mott state in a cobaltate-titanate interface, and identified new sources of orbital polarization at a nickelate-aluminate interface. We have also developed a generalized slave-boson formalism for multi-band Hubbard models that can be applied in large scale calculations involving complex oxide heterostructures and thin films.
Peiyuan Mao's picture
Peiyuan Mao
Meg Urry
Quantitative Researcher
Akuna Capital
Blazar Demographics: Intrinsic Properties of Jet-Dominated Active Galactic Nuclei
Blazars with their Doppler-boosted relativistic jets are perfect laboratories to study jet physics and provide crucial insights into jet mechanism, black hole spin, and growth history of the host galaxy. However, the blazar samples we observe are highly biased subsets of the true population because of the complicated shape of their spectral energy distributions. Thus to infer the intrinsic properties of blazars we have to extrapolate from the biased samples — and there are two opposing theories about the correct way to do this extrapolation: one in which the synchrotron-radiating electron energies are linked to the total jet power, the other in which they are independent. My dissertation is a thorough investigation of the intrinsic properties of blazars, from their spectral energy distributions to their luminosity functions to their cosmic evolution. In particular, I collected data for the largest collection of blazars to date, i.e., every single BL Lac object and flat-spectrum radio quasar in the literature; reduced new X-ray data for the majority; and developed a statistical description of blazar SEDs. Using the deepest and most complete radio-selected sample of flat-spectrum radio quasars, I constructed a new luminosity function of this population, based on a sample more than twice as large as previous work, using a maximum likelihood estimation method rather than fitting to binned data. Combining the SED information with the luminosity function, I carried out Monte Carlo simulations populating the universe with blazars and sampling them at different wavelengths and flux limits (corresponding to real surveys). Our analysis conclusively ruled out any model that assumes an independence between blazar SED and luminosity, and showed that SEDs and radio luminosities must be related in order to obtain the observed blazar distribution. We also explain the observed difference in the cosmic evolution of some BL Lac objects from other blazars as a simple selection effect that follows naturally from the SED shape. The most important implication of this work concerns the true demographics of the blazar population, namely, that low-power jets are far more common than high-power jets. This is a critical constraint on models for jet production and propagation.
Brendon O&#039;Leary's picture
Brendon O'Leary
David DeMille

In search of the electron's electric dipole moment in thorium monoxide: an improved upper limit, systematic error models, and apparatus upgrades
Searches for violations of discrete symmetries can be sensitive probes of physics beyond the Standard Model. Many models, such as supersymmetric theories, introduce new particles at higher masses that include new CP-violating phases which are thought to be of order unity. Such phases could generate measurable permanant electric dipole moments (EDMs) of particles. The ACME collaboration has measured the electron’s EDM to be consistent with zero with an order of magnitude improvement in precision compared to the previous best precision (J. Baron et al., ACME collaboration, Science 343 (2014), 269--272) with a spin precession measurement performed in the $H$ state of a beam of thorium monoxide (ThO). This limit constrains time-reversal violating physics for particles with masses well into the TeV scale. In this thesis I discuss the details of this measurement with an emphasis on the data analysis, search for systematic errors, and systematic error models that contributed to this result. I also discuss implemented and planned upgrades to the experimental apparatus intended to both improve the statistical sensitivity and reduce its susceptibility to systematic errors. At this time, the upgraded apparatus has been demonstrated to have a statistical sensitivity to the electron EDM that is more than a factor of 10X better than that our previous measurement.
Saehanseul Oh's picture
Saehanseul Oh
John Harris
Postdoctoral Associate
Brookhaven National Lab
Correlations in particle production in proton-lead and lead-lead collisions at the LHC
In high-energy heavy-ion collisions at the Large Hadron Collider (LHC), a hot and dense state of matter called the Quark-Gluon Plasma (QGP) is formed. The initial collision geometry and the subsequent expansion during the QGP stage result in the correlations of produced particles, through which the properties of the QGP can be investigated. Two analyses based on the geometrical correlations of produced particles, one in proton-lead (p–Pb) collisions and the other in lead-lead (Pb–Pb) collisions, are presented in this thesis. The data analyzed in this thesis were collected with the ALICE detector at the LHC in p–Pb collisions at a nucleon–nucleon center-of-mass energy of 5.02 TeV, and Pb–Pb collisions at a nucleon–nucleon center-of-mass energy of 2.76 TeV. In the forward-central two-particle correlation analysis in p–Pb collisions, two-particle angular correlations between trigger particles in the forward pseudorapidity range (2.5 < |η| < 4.0) and associated particles in the central range (|η| < 1.0) are studied. The trigger particles are muon tracks reconstructed in the Forward Muon Spectrometer, and the associated particles are charged tracks reconstructed in the central barrel tracking detectors. In high-multiplicity events, the double-ridge structure, previously observed in two-particle angular correlations at midrapidity (|η| < 1.2), is also found in the pseudorapidity ranges studied in this analysis. The azimuthal distribution is quantified using the Fourier decomposition, and the second-order Fourier coefficients for muons in the forward pseudorapidity range in high-multiplicity events are extracted after the contributions from jet-like correlations are removed. The coefficients are measured as a function of transverse momentum (pT) in the p-going direction and in the Pb-going direction, separately. Similar pT dependence of the coefficients in both directions are observed, with the Pb-going coefficients larger by 16±6% independent of pT. These observations further characterize the collective features in a small collision system (p–Pb). The results are compared with calculations using the AMPT model, which produces qualitatively the different pT and η dependence of the observables. In the analysis of the azimuthal collectivity of longitudinal structures in Pb–Pb collisions, the newly developed method is applied to investigate correlations among the longitudinal structures of produced particles in different azimuthal regions. In addition to the expansion of the QGP in the transverse direction, commonly quantified using Fourier coefficients, the initial geometry and resulting longitudinal expansion as a function of azimuthal angle enable us to better understand the full 3-dimensional evolution of heavy-ion collisions. Azimuthal angle is divided into regions in-plane and out-of-plane, and coefficients (an) of Legendre polynomials from a decomposition of the longitudinal structure at midrapidity (|η| < 0.8) on an event-by-event basis are estimated in each region for different centralities. Correlations among the longitudinal structures in different azimuthal regions are studied via the correlations among coefficients from the decomposition. The results of conditional an measurements indicate collective features of longitudinal structure in the azimuthal direction in particular with the first and second order coefficients, which represent the forward-backward asymmetry and mid-peripheral asymmetry, respectively. The results are compared with various heavy-ion collision models, including the Ncoll PYTHIA, HIJING and AMPT models. While the AMPT model shows similar centrality dependence in the conditional a1, the distinctive feature in conditional a2 observed in data is absent in all models considered in this thesis.
Andrei Petrenko's picture
Andrei Petrenko
Robert Schoelkopf

Enhancing the Lifetime of Quantum Information with Cat States in Superconducting Cavities
The field of quantum computation faces a central challenge that has thus far impeded the full-scale realization of quantum computing machines: decoherence.  Remarkably, however, protocols in Quantum Error Correction (QEC) exist to correct qubit errors and thus extend the lifetime of quantum information.  Reaching the "break-even" point of QEC, at which a qubit's lifetime exceeds the lifetime of the system's constituents, has thus far remained an outstanding goal.  In this work, we implement a QEC code within a superconducting cavity Quantum Electrodynamics (cQED) architecture that exploits the advantages of encoding quantum information in superpositions of coherent states, or cat states, in highly coherent superconducting cavities.  This hardware-efficient approach, termed the cat code, simplifies the encoding scheme and requires the extraction of just one error syndrome via single-shot photon number parity measurements.  By implementing the cat code within a full QEC system, we demonstrate for the first time quantum computing that reaches the break-even point.  Beyond applications to error correction, logical qubit encodings based on the cat code paradigm can be used to probe more fundamental questions of quantum entanglement between physical qubits and coherent states.  We demonstrate the violation of a Bell inequality in such a setup, which not only exhibits our ability to efficiently extract information from continuous variables encodings, but moreover illustrates a striking example of a system straddling the quantum-to-classical interface.  These results highlight the power of novel, hardware-efficient qubit encodings over traditional QEC schemes. Furthermore, they advance the field of experimental error correction from confirming the basic concepts to exploring the metrics that drive system performance and the challenges in implementing a fault-tolerant system.
Toshihiko Shimasaki's picture
Toshihiko Shimasaki
David DeMille

Continuous Production of 85Rb133Cs Molecules in the Rovibronic Ground State via Short-Range Photoassociation
We present our results on continuous production of ultracold ^{85}Rb^{133}Cs molecules in the rovibronic ground state via short-range photoassociation (PA). Starting with ultracold Rb and Cs atoms trapped in dual-species dark-SPOT MOT, we photoassociate a pair of Rb and Cs atoms into an excited molecular state, which decays into the electronic ground state by spontaneous emission. We apply depletion spectroscopy to the RbCs system and establish a rotationally-resolved, state-selective detection method for molecules in the vibronic ground state X^1S+(v = 0). With this technique, we verified molecule production in the rovibronic ground state X^1S+(v = J = 0). We explored a variety of short-range PA pathways, namely, via excitation to the 2^3P_0+ and 2^3P_0- states, the 2^1P_1-2^3P_1-3^3S^+_1 complex, and the b^3P_1^c^3S^+_1-B^1P_1 complex. One of the b^3P_1^c^3S^+_1-B^1P_1 complex of states, with PA frequency 10151.24 cm^-1, was found to be the most efficient pathway so far. Using PA through this state, we achieved a molecule production rate of up to 8000 molecules /s in X1^S+(v = J = 0). This opens up the possibility of utilizing short-range PA for preparing a large sample of ultracold, rovibronic ground state molecules via continuous accumulation.
Jukka Vayrynen's picture
Jukka Vayrynen
Leonid Glazman
Assistant Professor of Physics and Astronomy
Purdue University
Electron transport along the edge of a topological insulator
A two-dimensional topological insulator has a gap for bulk excitations, but conducts on its boundaries via gapless edge modes. Time-reversal symmetry prohibits elastic backscattering of electrons propagating within the edge, leading to quantized conductance at zero temperature. Inelastic backscattering, present at finite temperature, breaks the quantization and  increases the edge resistance; the resistance of a long edge acquires a linear dependence on its length. A phenomenological theory that  introduces  the least irrelevant operators in the low-energy description  predicts a strong temperature-dependence for the edge resistivity. Such a prediction is at odds with the experimentally observed weak  temperature dependence of the resistance. In the first part of this thesis, we attempt to resolve the issue by studying a realistic microscopic mechanism for  inelastic backscattering. The small band gaps in the existing putative two-dimensional topological insulators make them sensitive to static charge fluctuations created by randomness in the density of dopant. Charge fluctuations may lead to the formation of electron and hole puddles. Such a puddle -- a quantum dot -- tunnel-coupled to the edge may significantly enhance the inelastic backscattering rate. The added resistance is especially strong for dots carrying an odd number of electrons, due to the Kondo effect. For the same reason, the temperature dependence of the added resistance becomes rather weak. We present a detailed theory of the puddles' effect on the helical edge properties, including (i) linear conductance, (ii) non-linear $I$--$V$ characteristic, and (iii) current noise. In the second part of this thesis, we study a superconducting setup, namely a topological Josephson junction. Such a junction can be formed by bringing an s-wave superconductor in contact with a topological insulator. The junction is expected to host exotic Majorana zero modes whose observation is still being contested. Most of the present schemes to detect Majorana modes rely on the fractional ac Josephson effect. However, extracting a robust signature from this complicated non-equilibrium effect is difficult. Instead, we propose a linear-response method -- microwave spectroscopy -- to probe the Majorana modes. These modes lend a signature low-frequency behavior to the junction's microwave admittance $Y(\omega)$, which we calculate. 
Degree Date: December, 2016
Filip Kos's picture
Filip Kos
David Poland
Postdoc
UC Berkeley
Bootstrapping 3D CFTs
We use the method of conformal bootstrap to systematically study the space of allowed conformal field theories (CFT) in three spacetime dimensions. We consider the crossing symmetry equations coming from the correlators of several lowest dimension operators in a given CFT and show how to setup the semidefinite program to explore the constraints implied by the equations. Constraints lead to general bounds on dimensions and 3-point functions of the operators in CFT. Three classes of CFTs considered in this work are theories containing scalars with Z_2 symmetry, theories containing scalars with O(N) symmetry and theories containing fermions with parity symmetry. While studying the general constraints on such theories, we rediscover several previously known CFTs – Ising model, O(N) vector models and Gross-Neveu-Yukawa models. We determine the properties of the spectrum of low-lying operators in all of these cases. We show that conformal bootstrap can be a very powerful method for precise computation of CFT spectrum. In particular, our result for the dimensions of two lowest lying operators in the Ising model leads to the most precise determination of critical exponents in the model while our results for the 3-point functions in O(N) models lead to most precise determination of the high-frequency conductance in O(N) vector models.
Tomomi Sunayama's picture
Tomomi Sunayama
Nikhil Padamanabhan
Postdoctoral Fellow
Kavli Institute of Physics and Mathematics of the Universe
Using galaxy surveys as a precision tool to measure dark energy
Future surveys will provide a deeper understanding of dark energy, dark matter, and early universe physics through the measurements of large scale structure. In particular, the baryon acoustic oscillation (BAO) method and the redshift-space distortion (RSD) method aim to achieve sub-percent precision on cosmological parameters. Understanding and reducing the systematics caused by the non-linear evolution of gravitational structures and galaxy formation and evolution is crucial for future galaxy spectroscopic surveys. Over the course of this thesis, I develop the tools necessary to understand and quantify these systematic effects, and present a series of applications relevant to future surveys. I first constructed mock catalogs using the N-body simulations with a reduced number of time steps for future galaxy surveys. Then, I investigated the effects of galaxy bias on the BAO peak and the robustness of the reconstruction method, which is a standard technique used in the BAO measurements to reduce the error. We tested whether the reconstruction method could be applicable to the RSD method to test gravity models. In addition to these studies on large scales, I also explored how assembly history of dark matter halos affects its internal structure and clustering on small scales, including a first measurement of the scale dependence of this effect. These effects of the assembly history on small scales can be a potential source of systematics for future galaxy surveys.
Mitchell Underwood's picture
Mitchell Underwood
Jack Harris

Cryogenic Optomechanics with a Silicon Nitride Membrane
The field of optomechanics involves the study of the interaction between light and matter via the radiation pressure force. Though the radiation pressure force is quite weak compared with forces we normally experience in the macroscopic world, modern optical and microwave resonators are able to enhance the radiation pressure force so that it can be used to both measure and control the motion of macroscopic mechanical oscillators. Recently, optomechanical systems have reached a regime where the sensitivity to mechanical motion is limited only by quantum effects. Together with optical cooling techniques such as sideband cooling, this sensitivity has allowed experiments to probe the quantum behaviors of macroscopic objects, and also the quantum limits of measurement itself. In this dissertation I describe the physics underlying the modern field of optomechanics and provide an overview of experimental accomplishments of the field such as ground state cooling of mechanical oscillators, detection of radiation pressure shot noise, and preparation, storage, and transfer of quantum states between macroscopic objects and the electromagnetic field. I then describe the specific experimental work done in pursuit of my degree involving the ground state cooling of a silicon nitride membrane in a high finesse Fabry-Perot cavity, and a systematic characterization of the dynamics that occur when the membrane is coupled to two nearly degenerate cavity modes at an avoided crossing in the cavity spectrum. In the section on ground state cooling, particular attention is given to the influence of classical laser noise on the measurement of the membrane’s motion at low phonon occupancies, and techniques for laser noise measurement and reduction are discussed.
Degree Date: May, 2016
Rostislav Boltyanskiy's picture
Rostislav Boltyanskiy
Eric Dufresne
Senior Scientist
Spheryx
Mechanical Response of Single Cells to Stretch
A living cell is a complex soft matter system far from equilibrium. While it consists of components with definite mechanical properties such as stiffness, viscosity, and surface tension, the mechanics of a cell as a whole are more elusive. We explore cell mechanics by stretching single fibroblast cells and simultaneously measuring their traction stresses. Upon stretch there is a sudden, drastic increase in traction stresses, often followed by a relaxation over a time scale of ~1min. Upon release of stretch, traction stresses initially drop and often recover on a similar time scale of ~1min. We propose a minimally active linear viscoelastic model that captures the main features of cell response to stretch. The response we observe is also consistent with a biological model of tensional buffering. We propose that cell behavior in response to a mechanical perturbation, such as stretch, consists of crosstalk between these two models.
Diego Caballero Orduna's picture
Diego Caballero Orduna
Corey O'Hern
Counterparty Credit Risk Model Manager
BNP Paribas
Computational Studies of Protein Structure
Despite the abundance of crystallographic and structural data and many recent advances in computational methods for protein design, we still lack a quantitative and predictive understanding of the driving forces that control protein folding and stability.   For example,  we do not know the  relative  magnitudes  of the  side-chain entropy, van der Waals contact interactions, and other enthalpic contributions to the free energy of folded proteins. The inadequacy of current computational approaches to the analysis and design of protein structures  has hampered  the  development  of many novel  therapeutic  and diagnostic  agents, and is arguably one of the  biggest open challenges in biophysics and biochemistry. My work will build on the pioneering work of Ponder and Richards in the 1980s, who while at Yale demonstrated that steric constraints and packing criteria in protein interiors were the most stringent criteria in determining protein conformations. I will present my work developing and using a sterically centered molecular dynamics force field to study amino acid conformations.  I specifically use this force field to model hydrophobic amino acids and study the fundamental driving forces that determine amino acid side chain conformations. In my research,  I have employed  a hard-sphere plus  stereo-chemical  constraint molecular dynamics model. I have complemented this approach with other numerical and computational techniques such as approximate Markov state models to predict and rank amino acid conformations in different environments. This dissertation presents three separate but related computational studies.  In the first one, I present an analysis of the equilibrium backbone conformations that the amino acid Alanine can take.  I also study the inter-conversion mechanisms between these equilibrium states and compare my predictions with crystallographic data. I then generalize the method to study side-chain dihedral angle equilibrium con- formation states and the different transition mechanisms between them in the amino acids Leucine and Isoleucine. I analyze bond and dihedral angle correlations  and predict  a novel  transition  method, which is  consistent  with  crystallographic data, between different side-chain conformations. I then employ my hard-sphere plus stereo-chemical constraint model to comprehensively study and predict the side-chain dihedral angle distributions of eight different hydrophobic residues in the context of high resolution protein crystal structures. I  distinguish  between  local and environment  effects  and quantify how well  we can predict the conformations that these amino acids can take in protein environments. This thesis work will not only lead to a more fundamental understanding of the underlying physical forces behind amino acid conformations.  Future progress built on this  work will  also enable  the  reliable  design of specific protein-ligand motifs, the development of efficient computational methods to rationally re-design protein cores and interfaces with tunable stabilities, specificities and affinities, and numerous applications in biomedicine.  
Jane Cummings's picture
Jane Cummings
Sarah Demers
Chief Technology Officer
Cainthus
Tau Polarization at a Hadron Collider: W to tau,nu and Z to tau,tau decays at ATLAS
In this thesis, the first measurement of tau polarization at a hadron collider, and the first measurement of tau polarization in W boson decays to a tau and neutrino altogether, is presented.  The measurement of tau polarization is a test of the structure of the vector (V) and axial vector (A) couplings of the W boson to the third generation leptons.  Such a test is not possible with first and second generation leptons for which the helicity state is not accessible in a collider experiment.  The V-A structure of the charged current weak interaction at small W virtuality (Q^2) is well established from the kinematics of tau decays while the structure at Q^2 = m_W^2 had not been measured explicitly. In addition, an advanced analysis of the tau polarization in Z boson decays to tau pairs is presented.
Arvin Kakekhani's picture
Arvin Kakekhani
Sohrab Ismail-Beigi
Postdoctoral Researcher
University of Pennsylvania
Research Website
Ferroelectrics to Tackle Fundamental Challenges in Catalysis
Surface catalysis based on transition metals and their alloys has been one of the most important research fields in theoretical and experimental catalysis and chemistry. Recently, the development of a microscopic theoretical framework combined with the computational capability and accuracy of first principles calculations has changed the nature of this field from a largely trial and error approach to a predictive and controlled design process. In addition to deepening our knowledge of catalysis at the microscopic scale and helping design improved catalysts for important chemical reactions, this framework for heterogeneous catalysis has helped us understand some of the fundamental limitations of the catalytic activities of current materials and processes. In this thesis, we briefly introduce these limitations and their root causes in terms of electronic and structural properties. Next, we describe how using ferroelectric surfaces and exploiting their polarization dependent switchable surface chemistry can tackle some of these fundamental challenges. We then design processes and materials that can catalyze reactions that are considered challenging (if not near impossible) using current catalytic materials and processes. As specific examples, we discuss the possibility of using ferroelectric-based materials to catalyze industrially and environmentally important reactions including water splitting, direct NO$_{x}$ decomposition, and partial oxidation of methane to methanol.
Nicole Larsen's picture
Nicole Larsen
Daniel McKinsey
AAAS Science & Technology Policy Fellow
United States Department of Defense
An Effective Field Theory Analysis of The First LUX Dark Matter Search
A wealth of astrophysical research supports the existence of dark matter in the universe, yet the exact nature of this unknown particle remains elusive. The Large Underground Xenon (LUX) experiment is a 370-kg dual-phase xenon-based time projection chamber (TPC) that seeks to detect dark matter candidates such as Weakly Interacting Massive Particles (WIMPs) through the light and ionization signals generated by their collisions with xenon nuclei. The first part of this talk details the design of the LUX experiment and describes several novel hardware developments that enable LUX to search for these rare events with extremely high precision. In 2013, with the release of the world's first sub-zeptobarn spin-independent WIMP-nucleon cross-section limit, the LUX (Large Underground Xenon) experiment emerged as a frontrunner in the field of dark matter direct detection. However, tension between experiments and the absence of a definitive positive detection suggest a search for answers outside the standard spin-independent/spin-dependent analyses. In particular, the standard analyses neglect momentum- and velocity-dependent interactions on the grounds that WIMP-nucleus collisions are nonrelativistic. At the parton level, this is not always the case, and moreover, models exist in which the standard spin-independent and spin-dependent interactions are subdominant to momentum- and velocity-dependent interactions. Recent theoretical work has identified a complete set of 14 possible independent WIMP-nucleon interactions using basic symmetries and an effective field theory formulation. In the second part of this talk we report on the extension of the LUX analysis to search for all 14 of these new interactions, we comment on the possible suppression of event rates due to operator interference, and we show that under this new framework LUX again exhibits world-leading sensitivity.
Manuel Mai's picture
Manuel Mai
Corey O'Hern
Machine Learning Engineer
Spotify
Outcome Prediction and Reconstruction for Systems of Ordinary Differential Equations
I will present two related analyses of systems of ordinary differential equations (ODEs). The first one investigates outcome prediction in several systems of ODEs for the immune response to infection. We show that patient-to-patient variability sets a fundamental limit on the outcome prediction accuracy. However, accuracy can be increased at the expense of delayed prognosis. In the second study, I develop a method to build, general nonlinear ODE models from time series data using machine learning techniques related to sparse coding. I employ sparse basis learning to identify a basis set of functions that accurately represents the system of ODEs. We then employ L1-regularized regression, which finds sparse solutions to underdetermined systems of equations. We use this method to recover a full function (i.e. the right-hand side of an ODE) from a small number of measurements. Results are presented for one, two and three dimensional non-linear ODEs.
Eric Norrgard's picture
Eric Norrgard
David DeMille
Research Scientist
NIST
Magneto-optical trapping of diatomic molecules
Laser cooling in a magneto-optical trap (MOT) is the workhorse technique for atomic physics in the ultracold regime, serving as the starting point in applications from optical clocks to quantum-degenerate gases. It was recently shown that optical cycling, and thus laser cooling, should be possible for a class of at least 40 molecular species, using just three (or fewer) lasers. In this work, we demonstrate the first laser slowing and first magneto-optical trapping of a molecule, strontium monofluoride (SrF). In our experiments, a laser-slowed molecular beam is used to load a MOT. The rotational structure in molecules prevents cycling on a two-level system (as is typical with atoms) and leads to populating states which are dark to the confining laser. In the first molecular MOT demonstration, the confining forces were very weak compared to typical atomic MOTs. In a second iteration, with the aid of recent theoretical insights into the origin of the restoring force in a MOT, we optimized the confining force for static fields in the MOT in lieu of the dark states. We then demonstrated a trap which eliminates dark states by trapping in time-varying fields. This method allows for capture of a large sample of molecules at ultracold temperatures and high phase space densities with long trap lifetimes. The trapped population is sufficiently cold to proceed with many proposed experiments posed to expand the frontiers of knowledge, such as operation of molecular fountains and study of ultracold molecule-atom collisions.
Katrina Sliwa's picture
Katrina Sliwa
Michel Devoret
Physicist
Quantum Circuits, Inc.
Minimizing Effects Detrimental to the Heisenberg Back-Action of Qubit Measurements with Parametric Amplifiers
The quantum back-action of the measurement apparatus arising from the Heisenberg uncertainty principle is both a fascinating phenomenon and a powerful manipulation tool. Unfortunately, there are other effects which may overwhelm the Heisenberg back-action. This thesis focuses on two effects arising in the dispersive measurement of superconducting qubits made with two commonly used ultra-low-noise parametric amplifiers, the Josephson bifurcation amplifier (JBA) and the Josephson parametric converter (JPC). The first effect is qubit dephasing due to excess photons in the cavity coming from rogue radiation retro-emitted by the first amplifier stage toward the system under study. This problem arises primarily in measurements made with the JBA, where a strong resonant pump tone is traditionally used to provide the energy for amplification. Replacing the single strong pump tone with two detuned pump tones minimized this dephasing to the point where the Heisenberg back-action of measurements made with the JBA could be observed. The second effect is reduced measurement efficiency arising from losses between the qubit and the parametric amplifier. Most commonly used parametric amplifiers operate in reflection, requiring additional lossy, magnetic elements known and circulators both to separate input from output, and to protect the qubits from dephasing due to the amplified reflected signal. This work presents two alternative directional elements, the Josephson circulator, which is both theoretically loss-less and does not rely upon the strong magnetic fields needed for traditional circulators, and the Josephson directional amplifier which does not send any amplified signal back toward the qubit. Both of these elements achieve directionality by interfering multiple parametric processes inside a single JPC, allowing for in-situ switching between the two. This brings incredible experimental flexibility, and also makes these devices strong candidates for `on-chip' integration, which would in turn eliminate loss between the qubit and parametric amplifier as a dominant source of reduced measurement efficiency.
William Smith's picture
William Smith
Corey O'Hern
Senior Software Engineer
Datadog
Modeling Diffusion and Motion in Cells at the Molecular Level
Due to the complexity inherent in biological systems, many particles involved exhibit complicated spatiotemporal dynamics that go beyond the standard models of diffusion of molecules and dynamics of polymers. Here, we investigate two examples of this: the dynamics of intrinsically disordered proteins, and the diffusion of a probe particle in a bacterial cell. Intrinsically  disordered proteins (IDPs) are a class of proteins that do not possess well-defined three-dimensional  structures in solution under physiological conditions. We have developed all-atom, united-atom, and coarse-grained Langevin dynamics sim- ulations for the IDP α-synuclein that include geometric, attractive hydrophobic, and screened electrostatic interactions and are calibrated to the inter-residue separations measured in recent single-molecule fluorescence energy transfer (smFRET) experiments. We find that α-synuclein is disordered, with conformational statistics that are inter- mediate between random walk and collapsed globule behavior.  We  then show the equivalence of these three models, and apply the coarse-grained model to a set of five IDPs for which smFRET data is available, and identify a strong correlation between the distance to the dividing line between folded proteins and IDPs in the mean charge and hydrophobicity space and the scaling exponent of the radius of gyration with chemical distance along the protein. Recent  experiments  have also shown that  probe particles in the cytoplasm of E. coli can exhibit sub-diffusive behavior. These cells contain molecules of sizes differing by orders of magnitude, and so we first investigate the effects of this polydispersity on crowding behavior. we show that for bidisperse packings of particle species significantly different in size (diameter ratio r 2: 3), the participation of the smaller species in the packing is dependent on the total volume ratio of the two species, and that the dividing line can be predicted by geometric arguments.  Previous studies have hypothesized that this sub-diffusive behavior could arise from either crowding in the cellular cytoplasm or the metabolic activity  of the living cell. we employ simulations on active matter in solution to demonstrate that while active matter can lead to sub-diffusive behavior, the energy scales required  to do so lead to a significantly raised temperature that would be noticeable in any experimental configuration, and therefore that this is not a significant effect in cells. Similarly, we show that confinement in a colloidal suspension such as the cytoplasm can lead to sub-diffusive and highly non-Gaussian behavior, and lead to a bimodal distribution of step sizes that provide evidence for the cooperative relaxation hypothesis commonly  discussed in the colloidal community.
Degree Date: December, 2015
Jeffrey Ammon's picture
Jeffrey Ammon
David DeMille

Progress towards a measurement of nuclear-spin-dependent parity violation in diatomic molecules
Nuclear-spin-dependent parity violation (NSD-PV) effects arise from exchange of the Z boson between electrons and the nucleus, and from interaction of electrons with the nuclear anapole moment (a parity-odd magnetic moment induced by electroweak interactions within the nucleus).  These effects cause a mixing of opposite-parity levels in atoms and molecules, where the size of the mixing is inversely proportional to the energy difference of the mixed levels.  We study NSD-PV effects using the rotational levels of diatomic molecules, which can have energy splittings that are about four orders of magnitude smaller than the typical 1 eV atomic energy scale.  We amplify observable signals from NSD-PV by about seven additional orders of magnitude by bringing two rotational levels of opposite parity close to degeneracy with a strong magnetic field.  In this talk, the experimental strategy and expected signal will be discussed, followed by a description of the experimental apparatus. In particular, two new components of the experiment will be discussed in detail: the "interaction region", consisting of electrodes and optical delivery components, and our data acquisition system.  We will then discuss our methods for measuring and nullifying nonuniformities in the magnetic and electric fields that we apply, since such nonuniformites can cause systematic errors in our measurements of NSD-PV.  Finally, we will present preliminary results using the test system 138^Ba19^F.  Over the long term, our technique is sufficiently general and sensitive that it should apply to measurements of the NSD-PV couplings for a wide range of nuclei.
Camille Avestruz's picture
Camille Avestruz
Daisuke Nagai
KICP Fellow
Kavli Institute for Cosmological Physics at the University of Chicago
Modeling Galaxy Cluster Outskirts with Cosmological Simulations
The observational study of galaxy cluster outskirts is a new territory to probe the thermodynamic and chemical structure of the X-ray emitting intracluster medium (ICM) and the intergalactic medium (IGM). Cluster outskirts are particularly important for modeling the Sunyaev-Zel’dovich effect, which is sensitive to hot electrons at all radii and has been used to detect hundreds of galaxy clusters to high-redshift (z<1) with recent microwave cluster surveys such as ACT, Planck, and SPT. In cluster-based cosmology, measurements of cluster outskirts are an important avenue for estimating the cluster mass, as the outskirts are less sensitive to astrophysical uncertainties associated with gas cooling, star formation, and energy injection from supermassive black holes. However, recent observations of cluster outskirts deviate from theoretical expectations, indicating that cluster outskirts are more complicated than previously thought. For instance, recent observations from Suzaku X-ray satellite showed clusters with flat entropy profiles and gas fractions exceeding the cosmic baryon fraction at large radii. Computational modeling of cluster outskirts is necessary to interpret these observations. In my dissertation, I present hydrodynamical cosmological simulations of galaxy cluster formation that follow the thermodynamic and chemical structures in the virialization regions of the ICM and transition to the IGM. Specifically, I show how observational signatures of galaxy clusters are affected by (a) gas density and temperature inhomogeneities in the ICM due to infalling gas clumps and large-scale filaments, and (b) non-equilibrium electrons generated by accretion shocks at the outer boundary of clusters. As an example of how this work is directly relevant for observations, I will discuss implications for recent ultra-deep Chandra XVP observations of Abell 133.
Barry Bradlyn's picture
Barry Bradlyn
Nicholas Read
Assistant Professor
University of Illinois at Urbana-Champaign
Linear response and Berry curvature in two-dimensional topological phases
              In this thesis we examine the viscous and thermal transport properties of chiral topological phases, and their relationship to topological invariants. We start by developing a Kubo formalism for calculating the frequency dependent viscosity tensor of a general quantum system, both with and without a uniform external magnetic field. The importance of contact terms is emphasized. We apply this formalism to the study of integer and fractional quantum Hall states, as well as p+ip paired superfluids, and verify the relationship between the Hall viscosity and the mean orbital spin density. We also elucidate the connection between our Kubo formulas and prior adiabatic transport calculations of the Hall viscosity. Additionally, we derive a general relationship between the frequency dependent viscosity and conductivity tensors for Galilean-invariant systems. We comment on the implications of this relationship towards the measurement of Hall viscosity in solid-state systems.               To address the question of thermal transport, we first review the standard Kubo formalism of Luttinger for computing thermoelectric coefficients. We apply this to the specific case of non-interacting electrons in the integer quantum Hall regime, paying careful attention to the roles of bulk and edge effects. In order to generalize our discussion to interacting systems, we construct a low-energy effective action for a two-dimensional non-relativistic topological phase of matter in a continuum, which completely describes all of its bulk thermoelectric and visco-elastic properties in the limit of low frequencies, long distances, and zero temperature, without assuming either Lorentz or Galilean invariance, by coupling the microscopic degrees of freedom to the background spacetime geometry. We derive the most general form of a local bulk induced action to first order in derivatives of the background fields, from which thermodynamic and transport properties can be obtained. We show that the gapped bulk cannot contribute to low-temperature thermoelectric transport other than the ordinary Hall conductivity; the other thermoelectric effects (if they occur) are thus purely edge effects. The stress response to time-dependent strains is given by the Hall viscosity, which is robust against perturbations and related to the spin current.               Finally, we address the issue of calculating the topological central charge from bulk wavefunctions for a topological phase. Using the form of the topological terms in the induced action, we show that we can calculate the various coefficients of these terms as Berry curvatures associated to certain metric and electromagnetic vector potential perturbations. We carry out this computation explicitly for quantum Hall trial wavefunctions that can be represented as conformal blocks in a chiral conformal field theory (CFT). These calculations make use of the gauge and gravitational anomalies in the underlying chiral CFT.
Faustin Carter's picture
Faustin Carter
Daniel Prober
Physicist
HRL Laboratories
A transition-edge-sensor-based instrument for the measurement of individual He2* excimers in a superfluid 4He bath at 100 mK
This dissertation is an account of the first calorimetric detection of individual He2* excimers within a bath of superfluid 4He. When superfluid helium is subject to ionizing radiation, diatomic He molecules are created in both the singlet and triplet states. The singlet He2* molecules decay within nanoseconds, but due to a forbidden spin-flip the triplet molecules have a relatively long lifetime of 13 seconds in superfluid He. When He2* molecules decay, they emit a ~15 eV photon. Nearly all matter is opaque to these vacuum-UV photons, although they do propagate through liquid helium. The triplet state excimers propagate ballistically through the superfluid until they quench upon a surface; this process deposits a large amount of energy into the surface. The prospect of detecting both excimer states is the motivation for building a detector immersed directly in the superfluid bath. The detector used in this work is a single superconducting titanium transition edge sensor (TES). The TES is mounted inside a hermetically sealed chamber at the baseplate of a dilution refrigerator. The chamber contains superfluid helium at 100 mK. Excimers are created during the relaxation of high-energy electrons, which are introduced into the superfluid bath either in situ via a sharp tungsten tip held above the field-emission voltage, or by using an external gamma-ray source to ionize He atoms. These excimers either propagate through the LHe bath and quench on a surface, or decay and emit vacuum-ultraviolet photons that can be collected by the detector. This dissertation discusses the design, construction, and calibration of the TES-based excimer detecting instrument. It also presents the first spectra resulting from the direct detection of individual singlet and triplet helium excimers.
Nathan Cooper's picture
Nathan Cooper
Volker Werner
Deceased
Structure of A = 76 Nuclei and Fast-Timing Studies of the Rare-Earth Region
Photon strength of nuclei has been a topic of recent intrigue due to postulated exotic modes of excitation, such as a neutron skin resonance, as well as the difficulty of its measurement near the neutron separation energy. The large number of levels up to the neutron separation energy, a region of particular interest in the calculation of nuclear reaction probabilities, causes detailed and accurate measurements to be close to the threshold of current experimental limits. This talk will begin with the history of photon strength measurement and theory. The results of six nuclear resonance fluorescence experiments on stable A = 76 nuclei will presented along with a comparison between the results of these experiments and many others with the combined theories of Bethe, Brink, and Dyson. The significance of the analysis in relation to the measurements of photon strength below the neutron separation energy will be discussed in light of the current efforts to improve the amount of information extracted from experiment. The predictions of observables such as total gamma-ray spectra from hot nuclei will also be discussed. The second part of the talk will consist of results from a gamma-gamma angular correlations experiment performed on 76As at WNSL, and how the results of this experiment may influence nuclear structure calculations of the matrix element of the 76Ge neutrinoless double-beta decay mode. Finally, the results from fast-timing experiments on rare-earth nuclei will be presented.
Daniel Guest's picture
Daniel Guest
Paul Tipton/Tobias Golling

A Search for Scalar Charm Quarks with the ATLAS Detector at the LHC
This thesis presents the results of a search for pair-produced scalar charm quarks with the ATLAS detector at the LHC. The search uses 20.3 fb^-1 of data collected during the sqrt(s) = 8 TeV 2012 run. Each charm quark decays to neutrilinos and charm quarks, resulting in a final state consisting of two charm jets and missing transverse energy. A novel `charm tagging' algorithm was developed to separate this signature from backgrounds, and is discussed in detail. As no evidence of physics beyond the standard model is found, the search is used to set exclusions with 95% confidence in the scalar charm vs neutrilino mass plane.
Eric Holland's picture
Eric Holland
Robert Schoelkopf
Staff Scientist
Lawrence Livermore National Laboratory
Cavity State Reservoir Engineering in Circuit Quantum Electrodynamics
Engineered quantum systems are poised to revolutionize information science in the near future. A persistent challenge in applied quantum technology is creating controllable, quantum interactions while preventing information loss to the environment, decoherence. In this thesis, we realize mesoscopic superconducting circuits whose macroscopic collective degrees of freedom, such as voltages and currents, behave quantum mechanically. We couple these mesoscopic devices to microwave cavities forming a cavity quantum electrodynamics (QED) architecture comprising entirely of circuit elements. This application of cavity QED is dubbed Circuit QED and is an interdisciplinary field seated at the intersection of electrical engineering, superconductivity, quantum optics, and quantum information science. Two popular methods for taming active quantum systems in the presence of decoherence are discrete feedback conditioned on an ancillary system or quantum reservoir engineering. Quantum reservoir engineering maintains a desired quantum state through a combination of drives and designed entropy evacuation. Circuit QED provides a favorable platform for investigating quantum reservoir engineering proposals. A major advancement of this thesis is the development of a quantum reservoir engineering protocol which maintains the quantum state of a microwave cavity in the presence of decoherence. This thesis synthesizes of strongly coupled, coherent devices whose solutions to its driven, dissipative Hamiltonian are predicted a priori. This work lays the foundation for future advancements in cavity centered quantum reservoir engineering protocols that have potential to realize hardware efficient protocols to protect more elaborate quantum states.
Emma Ideal's picture
Emma Ideal
Sarah Demers
Research Scientist, Core Data Science
Facebook
A Search for the Standard Model Higgs Boson Produced in Association with a Vector Boson and Decaying to a Hadronically-Decaying Tau Pair at ATLAS
On July 4, 2012, the discovery of the Higgs boson was simultaneously announced by the ATLAS and CMS collaborations. Since then, evidence for its decay to tau leptons has been claimed. As of now, there have been no Higgs discoveries in any of its associated production modes. For this thesis, a search for the Higgs boson produced in association with a vector boson V = W^(+-), Z and decaying to a tau lepton pair was conducted using 2012 ATLAS data. The data corresponds to 20.3 fb^(-1) of 8 TeV center-of-mass energy proton-proton collisions delivered by the LHC. This analysis focuses on the search in the WH and ZH categories in which the Higgs boson decays to a pair of hadronically decaying taus. In order to better identify this signature in the midst of overwhelming hadronic activity, the vector bosons are required to decay leptonically. A data-driven multi-lepton fake factor method is used as the primary background estimation technique, modeling all reducible backgrounds where jets are misidentified as hadronic taus; Monte Carlo simulation is used to model the irreducible diboson backgrounds. There are insufficient statistics in the 2012 dataset to discover the Standard Model (SM) Higgs boson in these categories, so 95% confidence-level upper limits on the SM Higgs associated production cross section times the ditau branching ratio are calculated. For a Higgs boson mass m_H = 125 GeV, an upper limit of 5.3 times the SM Higgs cross section is expected.
Peter Koo's picture
Peter Koo
Simon Mochrie
Assistant Professor
Simons Center for Quantitative Biology at cold Spring Harbor Laboratory
Novel optical-based methods and analyses for elucidating cellular mechanics and dynamics
Resolving distinct biochemical interaction states by analyzing the diffusive behaviors of individual protein trajectories is challenging due to the limited statistics provided by short trajectories and experimental noise sources, which are intimately coupled into each protein’s localization. In the first part of this thesis defense, I will describe a novel, machine-learning based classification methodology, called perturbation expectation-maximization (pEM), which simultaneously analyzes a population of protein trajectories to uncover the system of diffusive behaviors which collectively result from distinct biochemical interactions. I will then discuss an experimental application of pEM to Rho GTPase, an integral regulator of cytoskeletal dynamics and cellular homeostasis, inside live cells. In the second part of the defense, I will describe optical tweezers measurements of isolated yeast nuclei, together with a novel imagining of yeast nuclei in living cells. These measurements, which were carried out on a suite of different yeast strains, reveal that the tethering of chromatin to the nuclear envelope is an important determinant of eukaryotic nuclear mechanical properties.
Daliang Li's picture
Daliang Li
Witold Skiba
Postdoctoral Associate
Harvard University
Covariant Methods for Superconformal Field Theories
In this thesis, we develop manifestly covariant methods for 4 dimensional, N = 1 superconformal field theories. First, we generalize the embedding formalism in conformal field theories (CFTs) to N = 1 superconformal field theories (SCFTs). As applications we construct manifestly superconformally covariant expressions for 2- and 3-point correlation functions involving the supercurrent multiplet or the global symmetry current superfield. Next, we combine this superembedding formalism with the shadow formalism in CFTs into a new method for computing superconformal blocks appearing in 4-point functions of SCFTs. This new method, called the supershadow formalism, expresses a superconformal block as manifestly covariant integrations over a product of 3-point functions. The supershadow formalism is much more efficient computationally than the brute force methods used previously in the literature. We obtain the superconformal blocks appearing in the 4-point functions of general scalar operator and then specialize to the 4-point functions involving chiral and global symmetry current multiplets. The results in the chiral case can be further generalized to N = 2 SCFTs. Finally, we present a systematic algorithm to extract the correlation functions of conformal primary component operators from the superfield correlation functions. We implemented this algorithm in Mathematica and applied it to the superfield 2-point function between general operators, from which we obtain all the component 2-point functions and all possible shortening conditions for an N = 1 superconformal multiplet. We also discuss a few potential directions for future researches.
Tudor Petrescu's picture
Tudor Petrescu
Karyn Le Hur
Postdoctoral Fellow
University of Sherbrooke
Topological phases with ultracold atoms and photons
In the first part of the talk, we introduce two–dimensional lattice tight–binding models that realize the quantum anomalous Hall effect (QAHE). For a Kagome lattice whose degrees of freedom are photons in microwave resonators, we discuss protocols to access the local Berry curvature and the Chern number of Bloch bands from the semiclassical dynamics of wavepackets. We proceed to Haldane’s model for QAHE on the honeycomb lattice, but with repulsively interacting bosons at unit filling, and uncover a rich phase diagram containing a Mott insulator with plaquette currents, as well as chiral and normal superfluids. The elementary excitations in both models have nonzero topological invariants which can be probed with current experiments. In Part II, we turn to quasi one–dimensional lattices in a uniform magnetic field, which have been recently realized with ultracold atoms. For bosons at low magnetic flux, the ground state can sustain the Meissner effect. The Meissner currents persist inside of a Mott insulating phase at half–filling. Whenever flux and density are commensurate, the ground state is a low–dimensional precursor of Laughlin’s trial state for the fractional quantum Hall effect at filling $\nu = 1/(2m)$. We then enumerate two–dimensional generalizations, and a variety of analogous topological phases of spinful fermions. We finally discuss observables that discern such states in current experiments, including local probes such as currents and particle number fluctuations.
Brian Vlastakis's picture
Brian Vlastakis
Robert Schoelkopf
Marie Sktodowska-Curie Fellow
Leek Lab, University of Oxford
Controlling coherent state superpositions with superconducting circuits
Quantum computation requires a large yet controllable Hilbert space. While many implementations use discrete quantum variables such as the energy states of a two-level system to encode quantum information, continuous variables could allow access to a larger computational space while minimizing the amount of required hardware. With a toolset of conditional qubit-photon logic, we encode quantum information into the amplitude and phase of coherent state superpositions in a resonator, also known as Schrödinger cat states. We achieve this using a superconducting transmon qubit with a strong off-resonant coupling to a waveguide cavity. This dispersive interaction is much greater than decoherence rates and higher-order nonlinearites and therefore allows for simultaneous control of over one hundred photons. Furthermore, we combine this experiment with fast, high-fidelity qubit state readout to perform composite qubit-cavity state tomography and detect entanglement between a physical qubit and a cat-state encoded qubit. These results have promising applications for redundant encoding in a cavity state and ultimately quantum error correction with superconducting circuits.
Degree Date: May, 2015
Tomas Aronsson's picture
Tomas Aronsson
John Harris
Researcher
Zurich Financial Services
Cross section of bottom electrons in proton-proton collisions in the ALICE experiment
High-energy heavy-ion collisions at the LHC allow for the study of the properties of the quark-gluon plasma (QGP). Heavy quarks, charm and bottom, produced in the initial hard scattering processes of the collision are excellent probes of the QGP. When heavy quarks traverse the QGP they are expected to lose energy and such energy loss is predicted to be smaller than for gluons and light quarks. On the other hand, recent experimental data indicate larger energy loss than expected. Heavy flavor production can be studied using electrons from semi-leptonic decays of D and B hadrons. The separation of electrons from these two sources (charm and bottom) is of crucial importance to address the expected mass dependence of energy loss. The ALICE EMCal detector possesses outstanding particle identification for electrons at high transverse momentum, pT, and this detector is used to identify electrons with high purity. A two-track algorithm to select secondary vertices involving these electrons and surrounding charged tracks was developed and implemented, so called b-tagging; exploiting the tracking precision provided by the Inner Tracking System the electron and surrounding tracks are used to select displaced decay vertices that are likely to originate from B decay. Bottom electron production in the transverse momentum range 7-13 GeV/c in 7 TeV pp collisions is measured. The invariant cross section for bottom-decay electrons is calculated and compared to FONLL pQCD predictions. This will serve as a reference for studies of bottom suppression in PbPb collisions.
Joseph Hongchul Bae's picture
Joseph Hongchul Bae
Vincent Moncrief
Senior Data Scientist
NatWest
Wormhole Solutions to the Bianchi IX Wheeler-DeWitt Equation using the Euclidean-signature Semi-classical Method
A Euclidean-signature semi-classical method is used to construct both ground and excited state solutions to the canonically quantized Bianchi IX (Mixmaster) cosmological models. Employing a modified form of the semi-classical ansatz, we solve the relevant Wheeler-DeWitt equation asymptotically by integrating a set of linear transport equations along the flow of a suitably chosen solution to the corresponding Euclidean-signature Hamilton-Jacobi equation. For the Moncrief-Ryan (or ‘wormhole’) Hamilton-Jacobi solution, we compute the ground state quantum correction terms associated with operator-ordering ambiguities. We next determine the explicit, leading-order forms of a discrete spectrum of excited states and show how to compute their quantum corrections as smooth, globally-defined functions on the Bianchi IX minisuperspace. These excited states are labeled by a pair of positive integers that can be plausibly interpreted as graviton excitation numbers for the two independent anisotropy degrees of freedom. To help with the interpretation of these solution states, we perturb about the Lorentz-signature Taub solutions using the Jacobi method of second variation. Through a series of canonical transformations, we decouple the final linearized Hamiltonian into a gauge-invariant and gauge-dependent part, further splitting the gauge-invariant part into two Hamiltonians that are of time-dependent harmonic oscillator form. For the perturbation Q+' = p_+ a' + p_a b_+' that stays entirely within Taub models, we solve for the quantum discrete state and squeezed state solutions explicitly, using the method of invariants of Lewis & Riesenfeld. While our analysis is currently limited to spatially homogeneous cosmological models, we anticipate that the techniques under development should eventually be much more generally applicable and thus represent a significant advance in the Wheeler-DeWitt approach to quantum gravity.
Alexander Cerjan's picture
Alexander Cerjan
A. Douglas Stone
Postdoc
Penn State University
Fundamental physics and device design using the steady-state ab initio laser theory
In this thesis we generalize and extend the steady-state ab initio laser theory (SALT), first developed by Tureci and Stone, and apply it problems in laser design. SALT as first formulated modeled the gain medium as identical two-level atoms, leading to the well-known Maxwell Bloch laser equations. The result is a set of coupled non-linear wave equations that treats the openness of the cavity exactly and the non-linear modal interactions to infinite order. Most gain media have more than two atomic levels, and in this thesis we generalize the SALT equations to treat realistic and complex gain media, specifically N-level atomic media with a single lasing transition, N-level atomic media with multiple lasing transitions and semiconductor gain media with particle-hole band excitations. The extension to multiple transitions requires fundamentally enlarging the set of SALT equations, by adding a set of population equations that must be solved self-consistently with the non-linear wave equations for the lasing modes in standard SALT. In addition, the population equation can be generalized to treat gain diffusion, an important problem in a number of laser systems, not treated in SALT or in most earlier laser theories. The semiconductor version (Semi-SALT) includes the continuum of particle-hole transitions and the effect of Pauli blocking of transitions, but is only developed and applied in the free-carrier approximation. The resulting theory is termed complex SALT (C-SALT). We also demonstrate how to incorporate amplification and injected signals naturally within the SALT framework, yielding injection SALT (I-SALT). The generalization to I-SALT leads to a larger set of self-consistent coupled non-linear wave equations, a set for the lasing modes and a set for the injected and amplified fields, coupled through cross gain saturation. It clearly distinguishes the lasing modes, which correspond to poles of the scattering matrix, from the injected fields, which do not; in this limit the locking of a lasing mode corresponds to the injected signal forcing the lasing pole off the real axis, reducing its amplitude to zero. I-SALT is shown to reduce to a version of the standard Adler theory of injection-locked lasing in a certain limit (the single pole approximation). We apply SALT to design a highly multimode cavity for use as a spatially incoherent light source for applications to imaging and microscopy. Laser illumination typically leads to coherent artifacts that degrade optical images; this can be alleviated by having a very large number of modes (~ 500) which are spatially independent and average out such artifacts. We used SALT to model a D-shaped laser cavity with chaotic ray dynamics and showed that a certain shape greatly increases the number of lasing modes for the same cavity size and pump strength, due to a flat distribution of Q-values and reduced mode competition. An on-chip electrically-pumped semiconductor laser was realized using this cavity design and showed negligible coherent artifacts in imaging, as well as much better efficiency and power per mode than traditional incoherent light sources such as LEDs. The thesis also goes beyond semiclassical laser theory to treat quantum noise and the laser linewidth in a SALT-based approach. We demonstrate that SALT solutions can be used in conjunction with a temporal coupled mode theory (TCMT) to derive an analytic formula for the quantum limited laser linewidth in terms of integrals over SALT solutions. This linewidth formula is a substantial generalization of the well-known Schawlow-Townes result and includes all previously known corrections: the Petermann factor, Henry alpha factor, incomplete inversion factor and the ``bad cavity factor''. However, unlike previous theories these corrections are not simply multiplicative and are not separable in general. The predictions of TCMT linewidth theory are tested quantitatively by means of an FDTD algorithm that includes the Langevin noise as a source term.
Andrew Leister's picture
Andrew Leister
Sarah Demers
Data Scientist Senior
Freddie Mac
A Search for Z' Gauge Bosons Decaying to Tau-Antitau Pairs in Proton-Proton Collisions with the ATLAS Detector
Although the Standard Model of Particle Physics has been generally successful in modeling fundamental particles and their interactions, it does not incorporate many observed physical phenomena. Many theoretical models attempting to explain physics beyond the Standard Model have been developed, several of which include one or more additional neutral gauge bosons, or Z'. The potential mass range of a Z' is quite large and for some models includes Z' masses at the TeV scale. A search for a TeV-scale resonance of Z' decaying to tau-antitau pairs is presented. The search is performed using proton-proton collisions with center-of-mass energies of 8 TeV, produced at the Large Hadron Collider. The data was collected by the ATLAS detector in 2012 and corresponds to 19.5-20.3 fb^(-1) of integrated luminosity. The analysis is performed separately across three channels, each corresponding to a particularly decay mode of the tau-antitau pair. The numbers of events in high-mass regions of data are counted and compared to the expectations from Standard Model backgrounds and Z' Signals. Bayesian 95%-credibility upper limits are placed on the Z' cross section times ditau branching ratio for each channel and for a combination of the channels. These are used to set limits on the Z' mass. The Z' models considered include the Sequential Standard Model (SSM), SSM variations with altered decay widths and couplings to fermions, and the Non-Universal G(221) model with enhanced couplings to third-generation fermions.
Christopher McKitterick's picture
Christopher McKitterick
Daniel Prober
Engagement Manager
McKinsey and Company
Prospects for Ultra-sensitive Terahertz Photon Detection with Graphene
This dissertation investigates a new scheme for the detection of terahertz (THz) photons. The vast majority of photons visible to outer space observatories occur in the far-infrared, but there do not yet exist detectors sensitive enough to accurately measure the faintest signals. I propose to use graphene, a single atomic layer of graphite, as the detecting element to observe these weak sources. As a result of its nano-scale dimensions, there are few charge carriers in graphene systems per unit area. Thus it is envisioned that the few charge carriers will be significantly heated by photon absorption. By measuring the increase of emitted blackbody radiation (Johnson noise) as a result of the heated charge carriers, ideally one would be able to measure even very weak THz signals. After outlining the detection requirements that motivate the research, I describe the mechanisms which affect the sensitivity of graphene-based photodetectors. Then I describe measurements which probe the physical parameters of graphene to allow for accurate predictions of device performance. Chiefly, the studies focus on the electron-phonon cooling pathway, which has a substantial role in determining device sensitivity. I find that my results are consistent with theories for electron-phonon cooling in two separate regimes: cooling in the absence of disorder (i.e., infinite mean free path) and disorder-assisted scattering at low temperatures. From the results of my studies on electron-phonon scattering, I make preliminary estimates of the performance of a prospective graphene photodetector using Johnson noise emission as its detection mechanism. I find that the outlook for using graphene as a single photon detector for THz photons is not promising. However, larger areas of graphene could be used to perform power measurements by averaging over the detection of thousands of arriving photons. The performance of the device is competitive with current state-of-the art detectors.
Xiaoxiao Wang's picture
Xiaoxiao Wang
Tobias Golling
Senior Data Scientist
Google
Search for a supersymmetric partner of the top quark using boosted top quark identification with the ATLAS detector
The Higgs discovery at the LHC has brought new attention to the hierarchy problem and the theory of Supersymmetry (SUSY). The supersymmetric partner of the top quark - the top squark (stop), is of paramount importance as it contributes the largest higher-order radiative corrections to the Higgs squared-mass. Naturalness arguments favor a light stop, making it a good candidate for discovery at the LHC. A search of the stop pair production in final states with one isolated lepton, jets, and missing transverse momentum will be presented. The search is performed with proton-proton collision data at \sqrt{s}=8 TeV collected by the ATLAS detector at the LHC in 2012, corresponding to an integrated luminosity of 20 fb^{-1}. The stop decay mode studied is mainly that to a top quark and a lightest neutralino, which is assumed to be the lightest supersymmetric particle (LSP) and evades detection. A technique that reconstructs the decays of the boosted top quark is used to increase the sensitivity at high stop masses. Similar techniques will be crucial for LHC Run2 searches where regimes of higher masses are probed. Results of other stop decays will also be reported, including a mixed decay where the stop can in addition decay into a bottom quark and the lightest chargino.
Degree Date: December, 2014
Ana Malagon's picture
Ana Malagon
O. Keith Baker
Software Developer
Droit Financial Technologies, LLC
Search for 140 microeV Pseudoscalar and Vector Dark Matter Using Microwave Cavities
Dark matter plays an important role in structure formation and composes 26.8% of the total energy density in the universe. There are many postulated particles that are theorized to be the constituents of cold dark matter; however, none have been observed experimentally. One strongly motivated particle that could be cold dark matter is the axion, a pseudoscalar with a two photon vertex. Experimental techniques to detect dark matter axions rely on a multiphoton radiative transition; in the presence of a strong magnetic field axions can convert to photons. If a microwave cavity is placed in the interaction region, when the produced photon's frequency is resonant with a mode of the cavity, the signal power is enhanced further. As the axion mass is not known, the resonant frequency of the cavity must be swept to search for possible converted photons. We present here the first microwave cavity search for dark matter axions with masses near 140 microeV. The experiment measured the power in the Ka-band frequency range from a cryogenically cooled cavity in a 7 Tesla background magnetic field. We took data for six months and swept the microwave cavity resonant frequency from 33.9-34.5 GHz, corresponding to an axion mass range of 140.2-142.7 microeV. We did not observe any statistically significant signals, and thus placed an upper bound on the axion to two photon coupling of $g_{a\gamma\gamma} < 8.75 \times10^{-11}$ 1/GeV. With the same data set we were also able to set new limits on dark matter "hidden photon"-photon interactions of $\chi < 5\times10^{-10}$. Hidden photons are postulated massive vector bosons that would only interact with Standard Model photons, and arise from new gauge extensions to the Standard Model.
Gennady Voronov's picture
Gennady Voronov
George Fleming
Applied Scientist
Amazon
The Extent of the Two-Color Fundamental Conformal Window
The $SU\!\left(2\right)$ gauge theories with $N_{f}$ flavors of massless vector-like Dirac fermions transforming in the pseudo-real fundamental representation have an enhanced global chiral symmetry and a distinct symmetry breaking pattern. These theories are expected to be qualitatively different from quantum chromodynamics (QCD) in the infrared (IR), especially with respect to the properties of the Nambu-Goldstone bosons (NGB). Having the potential to elucidate many features of both known and conjectured strongly-coupled physics, these theories ought to be investigated. Here I study the six-flavor theory, which is expected to lie near the lower edge of the conformal window. A determination of whether this specific theory is inside the window holds the promise of revealing a great deal of the structure of this interesting class of theories.    Toward this end, I compute the running gauge coupling in the non-perturbative Schroedinger functional (SF) scheme. This computation is performed numerically with lattice simulations of the $SU\!\left(2\right)$ lattice gauge theory with six flavors of stout-smeared Wilson fermions. I find no evidence for an infrared fixed point (IRFP) up through renormalized gauge couplings $\bar{g}^{2}$ of order $20$. This implies that the theory either is governed by an IRFP of considerable strength, unseen so far in non-supersymmetric gauge theories, or breaks its global chiral symmetries producing a large number of composite NGBs relative to the number of underlying degrees of freedom. Thus either of these phases exhibits novel behavior.
Liyao Wang's picture
Liyao Wang
Mokshay Madiman

Heat capacity bound, energy fluctuations and convexity
In classical statistical mechanics, the heuristic that in the canonical ensemble the energy distribution is sharply peaked at the mean energy is crucial in justifying the equivalence between the canonical and microcanonical ensembles. It turns out that this is closely related with the fundamental notion of a typical set and the Shannon-McMillan-Breiman theorem in information theory. In this dissertation, we explore the connections between the two and establish some new rigorous results that are of interest to both fields. In the first part of this dissertation, we show that if the heat capacity $C_{v,n,\beta}$ is bounded by $nc$, with $n$ being the number of degrees of freedom and $c$ being a constant independent of $n$ and $\beta$, then the energy distribution is ``sharply peaked'' around the mean energy. Our result is expressed mathematically as a concentration inequality. It is also demonstrated that if the average third moment of $\beta(H_n-\langle H_n\rangle)$ is of order $o(n^{\frac{3}{2}})$, then the energy distribution is approximately a Gaussian distribution centered at the mean energy, with variance being $C_{v,n,\beta}T^2$. In the second part, we first show that if the Hamiltonian $H_n$ is convex, then the heat capacity $C_{v,n,\beta}$ is bounded by $n$, the number of degrees of freedom. The bound $n$ is achieved by some convex Hamiltonian and hence is optimal in this sense. The universal and optimal nature of the bound is perhaps surprising from a physical point of view. We also show that assuming convexity of $H_n$, the absolute value of the average third moment of $\beta(H_n-\langle H_n\rangle)$ is bounded by a (complicated) function $g(n)$, depending only on $n$. $g$ is plotted numerically up to some point and we suspect $g(n)=o(n^{\frac{3}{2}})$, which would imply the closeness to Gaussian of the energy distribution.
Degree Date: May, 2014
Colin Bruzewicz's picture
Colin Bruzewicz
David DeMille
Technical Staff - Lincoln Laboratory
MIT
Continuous Optical Production of Ultracold Vibronic Ground State Polar Molecules
Polar molecules present an exciting new test bed for ultracold physics with applications in numerous fields, such as chemical reaction dynamics, many-body systems, and quantum computation. Creating large samples of these molecules that can be trapped for long times, however, remains an ongoing challenge. We demonstrate the direct formation of vibronic ground state RbCs molecules by photoassociation of ultracold atoms followed by radiative stabilization. From analysis of the relevant free-to-bound and bound-to-bound Franck-Condon factors, we have predicted and experimentally verified a set of photoassociation resonances that lead to efficient creation of molecules in the v=0 level of the lowest electronic ground state. We discuss the prospects for using this method to create and accumulate samples of ultracold polar molecules in their rovibronic ground state.
Bernard Hicks's picture
Bernard Hicks
Helen Caines

Differential Production Cross-Section of Heavy-Flavor Electrons in √s = 2.76 TeV pp collisions at the LHC with the ALICE detector
Recent results at RHIC seem to confirm T.D.Lee’s hypothesis that a new form of matter, the quark- gluon plasma (QGP), could be formed in heavy-ion collisions at high energies. Heavy quarks, being formed in the early stages of heavy-ion collisions, form a good probe for the properties of the QGP. The energy loss of heavy quarks as they traverse the medium is predicted to be less than that of the lighter quarks. However, previous measurements of the nuclear modification factor at RHIC indicate that the energy loss of heavy and light quarks is comparable. Thus measurements of the in-medium energy loss of heavy-quarks are of particular interest. In this thesis, a measurement of the differential production cross-section of electrons from the semi-leptonic decay of heavy-flavor quarks in √s = 2.76 TeV pp is presented. This provides a stringent test of perturbative QCD in a new energy regime, and forms a crucial baseline for Pb-Pb collisions where the in-medium energy loss mechanism can be studied.
Lawrence Lee's picture
Lawrence Lee
Tobias Golling
Postdoctoral Fellow
Harvard University
A Search for B-violating Supersymmetry in Multijet Signatures at the ATLAS Experiment
With supersymmetry (SUSY) increasingly constrained, more attention is placed on alternate flavors of SUSY that allow for an unexcluded natural theory. A search for new physics phenomena in all-hadronic signatures in \sqrt{s}=8 TeV pp collisions using an integrated luminosity of 20.3 fb^-1 collected by the ATLAS detector at the LHC will be presented. Within the context of SUSY, gluino pair-production gives rise to multijet final states in models that allow for violation of R-parity. Two types of spectra are evaluated – those that have an effective gluino decay to jets and those in which the lightest SUSY particle is a neutralino through which a gluino can cascade into a higher jet multiplicity. A counting experiment is performed and results are presented for all possible R-parity violating branching fractions to various quark flavors. The relevant theory space is probed in an unprecedented way in a branching ratio space with full generality. This represents the most sensitive result for this kind of supersymmetry to date.
Rongrong Ma's picture
Rongrong Ma
John Harris
Assistant Physicist
Brookhaven National Laboratory
Jet measurements in pp and Pb-Pb collisions in ALICE
Lattice-QCD predicts the existence of a new form of hot, dense matter called the Quark Gluon Plasma (QGP) above a critical energy density. Such matter is believed to be created in relativistic heavy-ion collisions, where sufficient energy is expected to be deposited by colliding ions in a limited volume. To study the properties of the QGP, high transverse momentum (pT) partons produced at the early stage of the collisions are used as probes. Since partons are not directly measurable, jet reconstruction, which assembles the hadron fragments of the partons, provides an experimental tool to reconstruct the parton kinematics. When traversing the colored medium, partons lose energy via both induced gluon radiation and elastic scatterings. Consequently jet structure is modified relative to jets generated in vacuum. The inclusive differential jet cross section in pp collisions at sqrt(s) = 2.76 TeV is measured, which serves as a reference for jet measurements in Pb–Pb collisions at the same psNN. Two jet cone radii R = 0.2 and 0.4 are used to reconstruct jets, and the ratio of the cross sections is formed. The good agreement between these results and perturbative QCD calculations at Next-to-Leading Order confirms the validity of the theoretical calculations in a new energy regime. Performing the same analysis in Pb–Pb collisions is challenging because of the overwhelming background. Therefore, a novel approach using the difference of the jet distributions recoiling from trigger hadrons in two disjoint pT intervals is developed to remove the contribution of background jets. In this thesis, the azimuthal correlations between trigger hadrons and recoil jets in selected kinematics are studied, and seen to remain essentially the same in pp and Pb–Pb collisions, implying that jets are not further deflected in the medium.
Tianqi Shen's picture
Tianqi Shen
Corey O'Hern
Quantitative Research
Laurion Capital
Contact Percolation, Fragility and Frictional Packings
This thesis presents four computational and theoretical studies of the structural, mechanical, and vibrational  properties of purely repulsive  disks, dimer-, and ellipse-shaped particles with and without friction.  The first study investigated the formation of interparticle contact networks below jamming onset at packing fraction φJ , where the pressure of the system becomes nonzero.  We generated ensembles of static packings of frictionless disks over a range of packing fraction.  We find that the network of interparticle contacts forms a system spanning cluster at a critical packing fraction φP < φJ . The contact percolation transition also signals the onset of cooperative non-affine particle motion and non-trivial response to applied stress.   For the second project, we performed  molecular dynamics simulations of dense liquids composed of bidisperse dimer- and ellipse-shaped particles over a wide range of temperature and packing fraction.  We measured structural relaxation times for the translational and rotational degreees of freedom. We find that the slow dynamics for dense liquids composed of dimer- and ellipse-shaped particles are qualitatively the same, despite the fact that zero- temperature static packings of dimers are isostatic, while static packings of ellipses are hypostatic. We also show that the fragility of the structural relaxation time decreases with increasing aspect ratio for both dimer- and ellipse-shaped particles.   For the third project, we developed a novel method to calculate and predict the average contact number as a function of the static friction coefficient for disk packings. We employed a novel numerical method that allowed us to enumerate  sets of packings with m = N 0 − Nc missing contacts relative to the isostatic value N 0. We show that the probability Pm(µ) to obtain a static packing with m missing contacts at µ can be expressed as a power series in µ.  Using Pm(µ),  we find that the average contact number versus µ agrees quantitatively with that from simulations of the Cundall-Strack model for frictional disks. In the final project, we performed calculations of the structure of the basin volumes of mechanically stable packings in configuration space as a function packing fraction.  Using the basin volumes, we show that  the probability  to obtain a given MS packing depends strongly on the packing fraction of the initial  configuration.
Degree Date: December, 2013
John Barry's picture
John Barry
David DeMille
Technical Staff - Lincoln Laboratory
MIT
Laser cooling and slowing of a diatomic molecule
Laser cooling and trapping are central to modern atomic physics. It has been roughly three decades since laser cooling techniques produced ultracold atoms, leading to rapid advances in a vast array of fields and a number of Nobel prizes. Prior to the work presented in this thesis, laser cooling had not yet been extended to molecules because of their complex internal structure. However, this complexity makes molecules potentially useful for a wide range of applications. The first direct laser cooling of a molecule and further results we present here provide a new route to ultracold temperatures for molecules. In particular, these methods bridge the gap between ultracold temperatures and the approximately 1 Kelvin temperatures attainable with directly cooled molecules (e.g. with cryogenic buffer gas cooling or decelerated supersonic beams). Using the carefully chosen molecule strontium monofluoride (SrF), decays to unwanted vibrational states are suppressed. Driving a transition with rotational quantum number R=1 to an excited state with R'=0 eliminates decays to unwanted rotational states. The dark ground-state Zeeman sublevels present in this specific scheme are remixed via a static magnetic field. Using three lasers, this scheme should allow more than 100,000 photon absorption/emission cycles before molecules are lost via unwanted decays. This number of cycles should be sufficient to load a magneto-optical trap (MOT) of molecules. In this thesis, we demonstrate transverse cooling of a SrF beam, in both Doppler and a Sisyphus-type cooling regimes. We also realize longitudinal slowing of a SrF beam. Finally, we detail current progress towards trapping SrF in a MOT. Ultimately, this technique should enable the production of large samples of molecules at ultracold temperatures for molecules chemically distinct from competing methods.
Prasenjit Dutt's picture
Prasenjit Dutt
R. Shankar

Strongly correlated quantum transport out-of-equilibrium
The revolutionary advances in nanotechnology have facilitated the precise control and manipulation of mesoscopic systems where quantum effects are pronounced. Typical experimental settings are capable of driving these systems far from equilibrium, where linear response theory is inadequate. We study transport through quantum-impurity systems in the regime of strong correlations and determine the effects of large temperature and potential gradients on its many-body physics. We introduce a method whereby the system in steady-state can be mapped onto an effective-equilibrium problem when formulated in terms of its Lippmann Schwinger operators. A rigorous connection with the Schwinger-Keldysh formalism is established at the level of the Green's functions and equivalence of the two frameworks is demonstrated. Furthermore, we develop necessary calculational tools and techniques, such as a novel perturbative framework for evaluating Green's functions. This machinery is used to analyze the fate of the Abrikosov-Suhl resonance under the influence of large potential biases and/or significant thermal gradients. Physical signatures such as the breaking of particle-hole symmetry due to the combined influence of thermal and potential gradients is predicted and existing experimental results analyzed. The AC dynamics near equilibrium of the quantum RC circuit with an arbitrary number of conduction channels is also investigated. We determine the relaxation resistance and mesoscopic capacitance in various parameter regimes and discuss its low-energy behavior. In the Coulomb-blockade regime it is observed that channel-mixing and/or asymmetry in coupling strengths causes the system to flow to an effective one-channel system at low energies.
Merideth Frey's picture
Merideth Frey
Sean Barrett
Professor of Physics
Sarah Lawrence College
Using Novel Pulse Sequences for Magnetic Resonance Imaging of Phosphorus-31 in Hard and Soft Solids
Since its invention in 1973, magnetic resonance imaging (MRI) has become an invaluable tool for clinical medicine, fundamental biomedical research, the physical sciences, and engineering. The vast majority of all MRI studies, in medicine and beyond, detect only the signal from a single nuclear isotope, H-1, in liquid water. Extending the reach of MRI to the study of other elements, and to hard or soft solids, opens new frontiers of discovery. In practice, however, the slower motion of the nuclei in solid environments compared to H-1 in water results in much broader magnetic resonance (MR) spectra, limiting both the attainable spatial resolution and the signal-to-noise. Our lab recently discovered a novel nuclear magnetic resonance (NMR) pulse sequence while doing fundamental research related to the `spins in semiconductors' approach to quantum computing. This sequence can greatly narrow the MR linewidth of solids, and it opens a new path to do high-resolution MRI of various nuclei in solids. In this thesis work, I use our pulse sequence to take the highest resolution MR images of P-31 in hard and soft solids using a conventional animal MRI system. We also have developed strategies to greatly enhance the imaging speed by making use of sparse MRI techniques as well as a new algorithm developed in our lab to do fast and accurate image reconstruction from sparse data. For future work, we propose ways to enhance spatial resolution and speed up imaging as well as discuss the potential applications of this work to a wider range of scientific problems.
Kurtis Geerlings's picture
Kurtis Geerlings
Michel Devoret
Research Manager
Gentex Corporation
Improving Coherence of Superconducting Qubits and Resonators
Superconducting qubits and resonators with quality factors exceeding 10^7 are of great interest for quantum information processing applications. The improvement of present devices necessarily involves the consideration of participation ratios, which budget the influence of each physical component in the total energy decay rate. Experiments on compact resonators in which participation ratios were varied has demonstrated the validity of this method, yielding a two-fold improvement in quality factor. Similar experiments on compact transmon qubit devices led to a three-fold improvement over previous transmons, validating the method of participation ratios for qubits as well. Through the use of a 3D cavity, a further minimization of the participation of surface components combined with the removal of unnecessary components, produced an additional ten-fold increase in coherence times. Finally, the fluxonium qubit was redesigned in a similar minimalist environment with an improved superinductance, thus combining the advantages of the 3D architecture with the natural insensitivity to dissipation of the fluxonium, resulting in another ten-fold increase in relaxation times. This large increase in relaxation and coherence times enables experiments that were previously impossible, thus preparing the field of quantum information to advance on other fronts.
Aaron Mertz's picture
Aaron Mertz
Eric Dufresne
Director, Science & Society Program
Aspen Institute
Collective Mechanics of Epithelial Cells
Cell–cell and cell–matrix adhesions play essential roles in the function of tissues, yet little is known about how crosstalk between these two adhesion types regulate cells' material properties and active processes. This dissertation combines experiment and theory to reveal how colonies of cells apply forces to the extracellular matrix. Using traction force microscopy, we measure forces transmitted to the substrate by colonies of epithelial cells with strong and weak cadherin-based intercellular adhesions. A minimal physical model in which cell–cell adhesions modulate the physical cohesion between contractile cells is sufficient to recreate the spatial rearrangement of traction forces observed experimentally with varying strength of cadherin-based adhesions. In the limit of very cohesive colonies with strong cell–cell adhesions, our experimental results reveal the emergence of an effective surface tension for cell colonies above a critical size. A minimal model incorporating strong adhesion between cells predicts the observed scaling relationship between total traction force and colony size and that apparent surface tension is determined by the contractility of the actomyosin network. This dissertation delineates the importance of cadherin-based cell–cell adhesions in coordinating mechanical activity and material properties of epithelial tissues. Our findings have implications for the mechanical regulation of epithelial cells during development, homeostasis, and disease.
Konstantin Nesterov's picture
Konstantin Nesterov
Yoram Alhassid
postdoctoral research associate
University of Wisconsin-Madison
Mesoscopic Effects in Chaotic Metallic Nanoparticles
We study electron interactions in a nanosized chaotic metallic grain with a large Thouless conductance. We use the so-called universal Hamiltonian, which describes the low-energy physics of such a grain. The noninteracting part of this model fluctuates and is described by random-matrix theory, while its interaction terms are universal and do not fluctuate. Of particular interest are grains in the fluctuation-dominated regime of pairing correlations, where the grain is so small that its single-particle mean level spacing is comparable to or larger than the bulk pairing gap. We first study the heat capacity and spin susceptibility of a grain in the absence of spin-orbit scattering. The universal Hamiltonian of such a grain describes the competition between pairing and exchange correlations. We study this competition in odd-even particle-number effects in the thermodynamic observables and investigate the mesoscopic fluctuations of these observables. We find that odd-even effects can be suppressed and shifted to lower temperatures by the exchange interaction. In the fluctuation-dominated regime, mesoscopic fluctuations can wash out these effects completely, and the fluctuations of the spin susceptibility may be especially large. In a second study, we focus on the magnetic-field response of the discrete energy levels in a grain with strong spin-orbit scattering. In this case, the only nontrivial interaction term in the universal Hamiltonian is the pairing interaction. We show that the linear part of the response, parametrized by g-factors, is independent of the strength of the pairing interaction. On the other hand, the quadratic part, parametrized by the level curvatures, is sensitive to pairing correlations and can be used to probe them.
Alice Ohlson's picture
Alice Ohlson
John Harris
Associate Senior Lecturer
Lund University
Investigating Parton Energy Loss with Jet-hadron Correlations and Jet vn at STAR
A strongly-coupled, deconfined state of quarks and gluons, known as the Quark-Gluon Plasma (QGP), is created in high-energy collisions of heavy nuclei. The QGP can be probed by high-momentum quarks and gluons (collectively known as partons) that are produced in hard scatterings early in the collision. The partons traverse the QGP and fragment into collimated "jets" of hadrons. Studies of parton energy loss within the QGP, or medium-induced jet quenching, can lead to insights into the interactions between a colored probe (a parton) and the colored medium (the QGP). Two analyses of jet quenching in relativistic heavy ion collisions are presented here. In the jet-hadron analysis, it is shown that jets that traverse the QGP are softer (consisting of fewer high-momentum fragments and more low-momentum constituents) than jets in p+p collisions. There are also indications that the shapes of the distributions of charged hadrons about the jet axis are modified by interactions with the QGP. Additionally, a measurement of jet v2 demonstrates that the parton energy loss depends on the length of the parton's path through the QGP. The data analyzed here were collected in sqrt(s_NN) = 200 GeV Au+Au and p+p collisions at the STAR detector at the Relativistic Heavy Ion Collider.
Kinga Partyka's picture
Kinga Partyka
Bonnie Fleming
Strategic Accounts Lead, Data Scientist
DataRobot
Exclusive Muon-Neutrino Charged Current mu+Np topologies in ArgoNeuT
Neutrinos remain among the least understood fundamental particles even after decades of study. As we enter the precision era of neutrino measurements bigger and more sophisticated detectors have emerged. The leading candidate among them is a Liquid Argon Time Projection Chamber (LArTPC) detector technology due to its bubble-like chamber imaging, superb background rejection and scalability. It is a perfect candidate that will aim to answer the remaining questions of the nature of neutrino and perhaps our existence. Studying neutrinos with a detector that employs detection via beautiful images of neutrino interactions can be both illuminating and surprising. The analysis presented here takes the full advantage of the LArTPC power by exploiting the first topological analysis of charged current muon neutrino $\mu+Np$, muon and any number of protons, interactions with the ArgoNeuT LArTPC experiment on an argon target. The results presented here are the first that address the proton multiplicity at the vertex and the proton kinematics. This study also addresses the importance of nuclear effects in neutrino interactions. Furthermore, the developed here reconstruction techniques present a significant step forward for this technology and can be employed in the future LArTPC detectors.
William Pontius's picture
William Pontius
Thierry Emonet
Senior Software Engineer
Mighty.com
The Molecular Origins and Functional Role of Noise in a Simple Sensory Network
Biological pathways perform calculations with often-small numbers of constituent molecules, leading to potentially significant variability in their output. In this thesis, I use the chemotaxis pathway of the bacterium Escherichia coli as a model to investigate the molecular origins of large temporal fluctuations and their consequences for cellular behavior. The bacterial chemotaxis pathway is a simple sensory network that performs temporal comparisons of external chemical stimuli, enabling the bacterium to perform a random walk biased toward increasingly favorable conditions. In this thesis, I first analyze measurements of living cells and argue that the statistics of pathway noise and the cellular response to stimuli, which both arise from the same biochemical pathway, are intrinsically linked. I then use theoretical models to argue that noise in the bacterial chemotaxis pathway may have significant positive consequences for the behavior of the cell: by coordinating the behavior of independent, stochastically switching flagellar motors, noise may enable the cell to respond more quickly to stimuli, track weak chemoattractant gradients more effectively, and explore sparse environments more efficiently. Finally, I construct a detailed, calibrated stochastic model of the mechanism through which the chemotaxis system adapts to persistent stimuli and identify the specific architectural features—densely clustered chemoreceptors and an enzyme localization mechanism—that give rise to large pathway fluctuations. I further argue that these features giving rise to pathway noise also underlie other well-known properties of the chemotaxis system: precise adaptation and functional robustness to expression levels of the pathway constituents. The simplicity of the bacterial chemotaxis system and the ubiquity of many of its architectural features suggest that these results will be relevant to the study of pathway noise in other sensory systems and throughout biology.
Matthew Reed's picture
Matthew Reed
Robert Schoelkopf
Research Staff Physicist
HRL Laboratories
Entanglement and Quantum Error Correction with Superconducting Qubits
A quantum computer will use the properties of quantum physics to solve certain computational problems much faster than otherwise possible. One promising potential implementation is to use superconducting quantum bits in the circuit quantum electrodynamics (cQED) architecture. There, the low energy states of a nonlinear electronic oscillator are isolated and addressed as a qubit. These qubits are capacitively coupled to the modes of a microwave-frequency transmission line resonator which serves as a quantum communication bus. Microwave electrical pulses are applied to the resonator to manipulate or measure the qubit state. State control is calibrated using diagnostic sequences that expose systematic errors. Hybridization of the resonator with the qubit gives it a nonlinear response when driven strongly, useful for amplifying the measurement signal to enhance accuracy. Qubits coupled to the same bus may coherently interact with one another via the exchange of virtual photons. A two-qubit conditional phase gate mediated by this interaction can deterministically entangle its targets, and is used to generate two-qubit Bell states and three-qubit GHZ states. These three-qubit states are of particular interest because they redundantly encode quantum information. They are the basis of the quantum repetition code prototypical of more sophisticated schemes required for quantum computation. Using a three-qubit Toffoli gate, this code is demonstrated to autonomously correct either bit- or phase-flip errors. Despite observing the expected behavior, the overall fidelity is low because of decoherence. A superior implementation of cQED replaces the transmission-line resonator with a three-dimensional box mode, increasing lifetimes by an order of magnitude. In-situ qubit frequency control is enabled with control lines, which are used to fully characterize and control the system Hamiltonian.
Charles Riley's picture
Charles Riley
Jack Sandweiss
Manager, Finance & Strategy
ConnectGen LLC
Searching for Local Parity Violation in Heavy Ion Collisions
Parity violation of the strong interaction is prohibited globally, however, it may be possible for parity to be violated locally in hot, dense, and deconfined QCD matter created in heavy ion collisions. Microscopic parity-odd domains in QCD are the consequence of topologically non-trivial configurations of gauge fields, and may be observable in heavy ion collisions due to the so called Chiral Magnetic Effect (CME). The CME predicts that provided a strong magnetic field (produced in a non-central collisions) and a P-odd bubble, the resultant hadron distribution should reflect a separation of charge parallel to the system's orbital angular momentum. In this talk, I present several measurements that identify this charge separation in heavy ion collisions collected using the STAR detector at RHIC. This analysis considers other possible sources that contribute to the measurement, and attempts to differentiate the possible backgrounds that obscure the evidence for local parity violation.
Flavius Schackert's picture
Flavius Schackert
Michel Devoret
Software Engineer, Machine Learning
Facebook, Inc.
A Practical Quantum-Limited Parametric Amplifier Based on the Josephson Ring Modulator
This dissertation has addressed the problem of developing the Josephson Parametric Converter (JPC) as a practical phase-preserving microwave parametric amplifier operating at the quantum limit of added noise. The device consists of two superconducting resonators coupled through the Josephson Ring Modulator (JRM), which in essence consists of a loop of four identical Josephson tunnel junctions, threaded by an applied magnetic flux. The nonlinearity of the JRM is of the tri-linear form XYZ without spurious nonlinear terms and involving only the minimal number of modes, thus placing the JPC close to the ideal non-degenerate parametric amplifier. This pure form of the nonlinearity is confirmed here by the observation of coherent attenuation (CA), the time-reversed process of three-wave parametric amplification, with signal, idler, and pump modes in the fully nonlinear regime. The design developed in this dissertation allows fabrication of the amplifier in a single lithography step, greatly simplifying parameter adjustments from one device to the next. Measured device characteristics and amplifier performances are presented, and limitations linked to the junction energy E_J and the circuit parameters discussed. The use of these JPCs in the readout of superconducting qubits is shown to lead to almost ideal quantum measurements, the measurement efficiency can approach the ideal value of 1.
Adam Sears's picture
Adam Sears
Robert Schoelkopf

Extending Coherence of Superconducting Qubits: from microseconds to milliseconds
Circuit quantum electrodynamics (Circuit QED) is the extremely successful framework for studying quantum devices developed along with the transmon, a superconducting charge qubit with an insensitivity to several types of dephasing. It involves the description of superconducting qubits and harmonic oscillators as quantized circuits. This thesis describes the implementation of two experiments that reduce circuit QED to its simplest components. Both experiments utilize elements that are known to have low dissipation: excited electron spin defects in crystals may take seconds to decay at cryogenic temperatures and the Josephson junction in superconducting qubits is nearly lossless; we begin by discussing the proper perspective for the remaining lossy elements. In the first experiments, a collection of magnetic dipoles is coupled as an ensemble to a superconducting resonator to investigate their suitability as a quantum memory; in the second a transmon “artificial atom” is placed inside a three-dimensional superconducting box. We further extend the study of the “3D transmon” and the harmonic oscillator modes of its rectangular waveguide cavity in terms of a new description of their hybridization (Black-box Quantization). Finally, we identify and resolve issues of photon induced dephasing in the first new devices. This thesis follows the evolution of superconducting qubits from coherence times of several microseconds to nearly a millisecond.
Degree Date: May, 2013
Sourpouhi Bedikian's picture
Sourpouhi Bedikian
Sarah Demers

A Search for the Charged Higgs: Using Tau Polarimetry with Proton-Proton Collisions at the ATLAS Detector
A search for a 130 GeV charged Higgs boson in ttbar events containing a tau lepton is presented. Tau polarimetry is used in order to distinguish the signal, t → H±b → τνb, from the dominant Standard Model background, t → W±b → τνb. The signal extraction is performed by a log-likelihood template fit. The dataset corresponds to an integrated luminosity of 4.6 fb^−1 of sqrt(s)=7 TeV proton-proton collisions collected with the ATLAS detector in 2011 at the Large Hadron Collider. Assuming the H± decays exclusively to τν, a limit is placed on the BR(t → H±b) of less than 16% at the 95% confidence level. This is the first search for physics beyond the Standard Model to use τ polarimetry at a hadron collider.
Christopher Gilbreth's picture
Christopher Gilbreth
Yoram Alhassid
Sr. Advanced Physicist
Honeywell Quantum Solutions
Ultracold Fermi Gases: Effective Interactions and Superfluidity
Cold atomic Fermi gases are clean, highly experimentally tunable systems with connections to many different fields of physics. However, in the strongly-interacting regime they are nonperturbative and difficult to study theoretically. One challenge is to calculate the energy spectra of few-body cold atom systems along the crossover from a gas described by a Bose-Einstein condensate (BEC) to a gas described by Bardeen-Cooper-Schrieffer (BCS) theory. The configuration-interaction (CI) method is widely used for such problems, but the finite model spaces employed require carefully chosen interactions with good convergence characteristics. We study a recently introduced effective interaction in the CI approach for the unitary Fermi gas, extending it to the BEC-BCS crossover and examining its properties analytically and numerically. We find it exhibits fast convergence away from the unitary limit, which allows us to calculate the low-lying energy spectrum of three- and four- particle systems along the crossover. For larger systems of cold atoms, the superfluid phase transition is a subject of principle interest, but is still incompletely understood. Realistic ab initio calculations of the heat capacity across the superfluid phase transition have not to date been achieved, and the nature of the pseudogap effect in the unitary regime is still a subject of debate. We apply the auxiliary field quantum Monte Carlo (AFMC) method to shed light on the superfluid phase transition by studying a finite-size trapped gas in the canonical ensemble. The AFMC method permits fully nonperturbative calculations of strongly-interacting systems without introducing uncontrolled approximations, but can be computationally intensive. Our calculations are made feasible by introducing a new stabilization technique to improve the scaling of the method. Applying this method, we present new results concerning the signatures of the superfluid phase transition and pseudogap phase in this system.
Archana Kamal's picture
Archana Kamal
Michel Devoret
Assistant Professor
Universty of Massachusetts Lowell
Nonreciprocity in active Josephson junction circuits
I will present different flavors of nonreciprocal photon dynamics realized using active parametric circuits based on Josephson junctions. The motivation stems from developing non-magnetic alternatives to existing nonreciprocal devices, invariably employing magnetic materials and fields and hence limited in their application potential for use with on-chip microwave superconducting circuits. The main idea rests on the fact the “pump” wave (or the carrier) in an active nonlinear system changes the phase of a small modulation signal just as the magnetic field rotates the polarization of the wave propagating in a Faraday medium. All the implementations discussed will draw from the basic idea of chaining together discrete parametric processes with an optimal phase difference between the respective pumps to realize nonreciprocity. Though discussed specifically for microwave applications using Josephson junctions as a platform, these ideas are generic enough to be adopted for any nonlinear system implementing frequency mixing.
Zuhair Khandker's picture
Zuhair Khandker
Walter Goldberger
Postdoctoral Researcher
University of Illinois at Urbana-Champaign
Embedding Methods for Conformal Field Theor
Conformal field theories (CFTs) are highly constrained by symmetry. For instance, based on symmetry considerations alone one can derive constraints on the form of correlators and on the scaling dimensions of certain operators. However, the full implications of symmetry are still far from understood, i.e. we still do not know the extent to which conformal symmetry constrains the space of all possible CFTs. In the non-supersymmetric setting, the embedding-space formalism for CFTs has proven to be an important tool for addressing these questions. After starting with a very general introduction to conformal symmetry, CFTs, and the embedding formalism, I will discuss the generalization of embedding methods to the supersymmetric setting for applications to four-dimensional N=1 superconformal field theories and focus on one such application, constructing manifestly-covariant current correlators.
Nicholas Masluk's picture
Nicholas Masluk
Michel Devoret
Research Staff Member / Manager
IBM
Purifying the environment of the fluxonium artificial atom
Fluxonium is a highly anharmonic artificial atom, which makes use of an array of large Josephson junctions to shunt the junction of a Cooper-pair box for protection from charge noise. At microwave frequencies the array forms a "superinductance", a superconducting inductance whose impedance exceeds the resistance quantum h/(2e)^2 = 6.5 kOhm. The first excited state transition frequency is widely tunable with flux, covering more than five octaves, yet the second excited state remains well within one octave. This unique spectrum permits a dispersive readout over the entire flux tunable range, in contrast to the flux qubit. By measuring the energy relaxation time of the qubit over the full range of flux dependent transition energies, it is possible to determine the dominant loss mechanisms, and therefore implement design changes to reduce their contribution. The losses in several fluxonium samples is explored, with progressive improvements made towards reducing capacitive loss, the dominant loss mechanism. Additionally, the detailed characterization of Josephson junction array superinductances is examined.
John Murray's picture
John Murray
Xiao-Jing Wang
Associate Professor of Psychiatry, Physics, and Neuroscience
40 Temple Street, Suite 6E
Research Website
Specialization of neural circuits for cognition: linking structure to function through dynamics
Richard Wall's picture
Richard Wall
Paul Tipton
Physics Teacher
The Hewitt School
A study of heavy flavor quarks produced in association with top quark pair events at sqrt(s) = 7 TeV using the ATLAS detector
In this thesis, we show evidence for the production of ttbar + b + X and ttbar + c + X, together refered to as ttbar + HF, at the Large Hadron Collider. A sample of dilepton ttbar candidate events with three or more b-tagged jets is used to isolate a $\ttbar$ sample rich in extra heavy flavor jets. A fit to the vertex mass distribution for the b-tagged jets in this region is performed to extract the flavor composition of the additional b-tags. This measurement is converted to a cross-section for ttbar + HF production using a correction factor from Monte Carlo simulation. The cross-section for ttbar events with at least one additional jet is also measured. The final result is quoted as a ratio of cross-sections within the visible ATLAS acceptance to reduce the overall systematic uncertainty. Using 4.7 fb^-1 of data collected during the 2011 run, we find the ratio of sigma(ttbar + HF) / sigma(ttbar + jet) to be 7.9 +/- 1.4 (stat.) +5.5,-2.1 (syst.)%, compared to the leading-order Standard Model expectation of 4.1 +/- 1.3%.
Brian Walsh's picture
Brian Walsh
Tobias Golling
Economist
World Bank
Research Website
SEARCH FOR STOP SQUARK PAIR PRODUCTION IN FINAL STATES WITH ONE ISOLATED LEPTON, JETS, AND MISSING TRANSVERSE MOMENTUM IN SQRT(S) = 7 TeV COLLISIONS USING 4.7 fb^(-1) OF ATLAS DATA
A cut-and-count search for stop squark pair production in pp collisions at √s = 7 TeVwith 4.71 fb−1 of 2011 ATLAS data is presented. Events with one isolated lepton, four or more jets, and missing transverse energy are selected to study a simplified model of stop squark decay to one top quark and one neutralino χ ̃01. Data is seen to be compatible with Standard Model-only hypothesis, and model-dependent as well as model-independent exclusion limits are set.
Degree Date: December, 2012
Steven Eckel's picture
Steven Eckel
Steve Lamoreaux
Research Scientist
National Institute of Standards and Technology
A Search for the Electron EDM using Europium-Barium Titanates
The discovery of a permanent electric dipole moment (EDM) of a fundamental particle would prove a great discovery in modern physics, as such an EDM would violate some of the core symmetries of the fundamental forces of nature. Many models that go beyond the standard model of particle physics produce EDMs with magnitudes approaching the level detectable by the next generation of experiments. One possibility for such an experiment involves the use of a solid sample at low temperatures. In a paramagnetic material, the unpaired electrons, if they posses an EDM, can interact with the polarization of the sample and produce a magnetization that can be detected. I discuss an incarnation of such an experiment based on mixed europium-barium titanates. Such an experiment offers several advantages over other solid-state and atomic EDM searches including larger EDM induced interactions and the ability to measure without an applied electric field. This experiment has produced the world's best limit on the electron's EDM to date from a solid sample, at $|d_e|<6.05\times10^{-25}\ecm$ (90\% confidence limit). While this limit represents an improvement in the realm of solid-state experiments, it is not yet competitive with similar molecular and atomic experiments. However, there are many possibilities that could produce a superior solid-state experiment, and these will be discussed.
Matthew Phillips's picture
Matthew Phillips
Gordon M. Shepherd

Sensory Processing in the Olfactory Bulb: From Lateral Inhibition to Behavior
Sensory perception is the fundamental process by which all organisms gain knowledge of their world. Perceptions arise from stimulations of sensory receptors that are translated into neuronal signals. These signals become processed and refined greatly before reaching conscious awareness in higher brain areas. Understanding the mechanisms and functions of this processing is a significant and major challenge for science. The use of model systems, such as the mammalian olfactory system, is helpful in this task due to simplified circuit organizations relative to other sensory systems. This dissertation aims to address three critical questions in olfactory processing. • First, odor sampling is fundamentally linked with respiration in mammals. Yet very little is known about the network activities and synaptic and circuit processing that result from respiration. Does respiration drive network activity and actively refine sensory representations in the olfactory system? • Second, olfactory circuits are organized, as in many sensory systems, within columns. However, it is not known if olfactory columns interact or are individual processing elements. Do olfactory columns interact combinatorially or in an organized manner? • Finally, when olfactory processing is impaired, either by age or disease, great risks to humans can result. Can we use animal model systems to recapitulate olfactory processing impairments found in humans from aging and Alzheimer’s disease? In this dissertation physiological, anatomical and behavioral measurements of sensory processing within olfactory circuits at the synaptic, neuronal, circuit, network and behavioral levels will be presented. The results demonstrate that respiration is an active process which generates network activity and refines sensory representations through functionally distinct circuits; that olfactory columns are not separate parallel processing structures, but interact non-stochastically; and finally, that the evolutionally time course of sensory and cognitive processing abilities must be carefully measured in Alzheimer’s disease modes to distinguish between ageing and disease-related impairments.
Hasuk Francis Song's picture
Hasuk Francis Song
Karyn Le Hur
Research Scientist
DeepMind
Entanglement in Quantum Many-Body Systems
The scaling of entanglement entropy and, more recently, the full entanglement spectrum have become useful tools for characterizing certain universal features of quantum many-body systems. We motivate the importance of entanglement in the study of many-body systems by considering the “gratuitously big” size of Hilbert space and the need for generic ansatzes that efficiently represent useful wave functions. In addition, we study the scaling of the entanglement entropy in the two-dimensional spin-1/2 Heisenberg antiferromagnet, where our work and other recent work indicate that a subleading logarithmic term contains universal information about the number of Goldstone modes in the symmetry-broken phase. Although entanglement entropy is difficult to measure experimentally, we show that for systems that can be mapped to non-interacting fermions both the von Neumann entanglement entropy and generalized R\'{e}nyi entropies can be related exactly to the cumulants of number fluctuations, which are accessible experimentally. In principle, this also extends to the full entanglement spectrum. Such systems include free fermions in all dimensions, the integer quantum Hall states and topological insulators in two dimensions, strongly repulsive bosons in one-dimensional optical lattices, and the spin-1/2 XX chain, both pure and strongly disordered. The same formalism can be used for analyzing entanglement entropy generation in quantum point contacts with non-interacting electron reservoirs. Beyond the non-interacting case, we show that in analogy to the full counting statistics used in mesoscopic transport, fluctuations give important information about many-body systems including the location of quantum critical points.
Degree Date: May, 2012
Hanghui Chen's picture
Hanghui Chen
Sohrab Ismail-Beigi
Postdoctoral Associate
Columbia University
A first principles study of oxide interfaces
Both theoretically and experimentally, enormous progress has been made toward understanding and controlling materials at the atomic scales. The advances in thin film deposition techniques make it possible to grow artificially designed heterostructures that do not exist in nature. The theoretical developments of ab initio calculations combined with the rapid increase of computational capability enable scientists to analyse data, make predictions and even design experiments from numerical simulations. In this thesis, as a good demonstration, we use Density Functional Theory to investigate structural, electronic and magnetic properties of oxide interfaces. Transition metal oxides are complex materials in which charge, spin, orbital and lattice are all coupled to each other, resulting in many competing phases. Oxide interfaces are more intriguing because unexpected novel phenomena emerge that are absent in both bulk constituents. We study three interesting and representative oxide interfaces. The first is the interface between band insulators SrTiO3 and LaAlO3. A comprehensive investigation of electronic and atomic reconstructions at the interface is presented and different scenarios of formation of two dimensional electron gas at the interface are provided. The second example is the interface between ferroelectrics and manganites. The couplings of ferroelectric polarization to charge, spin and orbital occupancy in manganites are studied in details. Comparisons to existent experiments and predictions for new experiments are presented. The last interface investigated is the one between simple metals and binary oxides. We show that when the interface structure is carefully engineered, a pair of unstable metal-O bonds could induce ferroelectricity in otherwise paraelectrics. If this interfacial ferroelectricity is combined with magnetic insulators, multiferroics can be formed in ultra thin films.
Andrew Jayich's picture
Andrew Jayich
Jack Harris
Postdoctoral Associate
University of California, Santa Barbara
Laser cooling a 261 kHz harmonic oscillator
Optomechanics is a diverse field where mechanical harmonic oscillators are coupled to an optical field via radiation pressure. The mechanical devices range from atom clouds and nanotubes up to the kilogram scale mirrors used in the LIGO observatories. The starting point for many interesting measurements begins with the mechanical device in its ground state, where it has less than a phonon worth of energy. In Jack Harris's lab we are working to cool a mechanical membrane to its ground state. This talk will disscuss an optomechanical system where a high finesse cavity is coupled to a millimeter scale SiN membrane that is anchored to a He3 fridge at 400mK. We have laser cooled to an occupancy of about 15 phonons. Readout of lower occupancies is limited by shaking of elements in the fridge. Steps are being implmented to overcome this hurdle and reach the ground state.
Benjamin Kaplan's picture
Benjamin Kaplan
Paul L. Tipton
Postdoctoral Associate
NYU
Testing the Standard Model of Particle Physics: A Search for New Phenomena in Multilepton Events with the ATLAS detector at the LHC
An excess of events over the Standard Model predictions in final states with three or more high momentum charged leptons would constitute evidence of new physical processes. Presented is a generic search for new phenomena in such events. To reduce the largest Standard Model backgrounds, events consistent with the production of a Z boson are rejected. Events are selected from 1.02 fb$^{-1}$ of proton-proton collision data recorded by the ATLAS detector at $\sqrt{s}$ = 7 TeV. There is no requirement on missing energy or jet activity. A total of 25.9 $\pm$ 3.8(stat) $\pm$ 4.3(syst) events are expected in the signal region from Standard Model sources, and 31 events are observed. In a second signal region, which uses a tighter lepton momentum selection criteria, 4.9 $\pm$ 1.6(stat) $\pm$ 0.9(syst) are expected from Standard Model processes, and 6 events are observed. The observed p-values for consistency with the Standard Model expectation are 27\% and 33\% in the two signal regions. A model of the pair production of doubly-charged Higgs bosons is used as a benchmark for determining fiducial limits. The observed fiducial cross-section limits are 38 fb and 14 fb in the two signal regions. Cross-section upper limits at 95\% confidence are also set at 41 (34) fb for the pair production of 200 (300) GeV excited electron neutrinos.
Sarah Lockwitz's picture
Sarah Lockwitz
Paul Tipton
Postdoctoral Associate
Fermilab
A Search for the Standard Model Higgs Boson in CDF II Data
This dissertation presents a search for the standard model Higgs boson in the associated production process proton anti-proton to ZH to electron positron b quark anti-b quark. Data amounting to an integrated luminosity of 7.5/fb at a center of mass energy of 1.96 TeV collected at the Collider Detector at Fermilab (CDF) at the Tevatron are analyzed. Two objectives are pursued in the methods applied: maximize acceptance, and distinguish the signal from background. The first aim is met by applying a neural-network-based electron identification and considering multiple electron triggers in an effort to improve Z boson acceptance. In an attempt to maximize the Higgs acceptance, three b quark identification schemes are used allowing for varying event conditions. The latter goal is met by employing more multivariate techniques. First, the dijet mass resolution is improved by a neural network. Then, both single variables and boosted decision tree outputs are fed into a segmented final discriminant simultaneously isolating the signal-like events from the Z with additional jets background and the kinematically different top anti-top background. Good agreement is seen with the null hypothesis and upper production cross section times branching ratio (BR(H to b anti b)) limits are set for 11 mass hypotheses between 100 and 150 GeV/c^2 at the 95% confidence level. For a Higgs boson mass of 115 GeV/c^2, this channel sets an observed (expected) upper limit of 3.9 (5.8) times the standard model value of production cross section times branching ratio. The inclusion of this channel within the combined CDF and Tevatron limits is discussed.
Vladimir Manucharyan's picture
Vladimir Manucharyan
Michel Devoret
Postdoctoral Associate
Harvard
Superinductance: a New Element for Quantum Circuits
We harness the phenomenon of kinetic inductance of a superconductor for the purposes of quantum information processing with superconducting circuits. In the present work, kinetic inductance of an array of Josephson tunnel junctions with carefully chosen parameters exceeds its geometric (magnetic) inductance by four orders of magnitude. Using such inductance, one can construct electrical circuits, in which quantum electrodynamics of charges and fluxes is governed by an effective fine structure constant value over a unity. We refer to this fundamentally new quantum circuit element as superinductance.   In order to experiment with superinductance we use it to shunt a small-capacitance Josephson tunnel junction to form a new superconducting artificial atom, dubbed fluxonium. We tune this atom using magnetic flux threading the fluxonium loop, and communicate with it by coupling the small junction capacitively to a microwave cavity, following a well-established approach of circuit quantum electrodynamics. With an adequate choice of junction parameters, the low energy spectrum of fluxonium is quite unique: it almost corresponds to the inductive energy of the array charged with an integer number of flux quanta. From the measurement of the transition spectrum of fluxonium we established that the junction array indeed behaves as a linear inductance of quoted magnitude for a range of frequencies exceeding 10 GHz. The quality factor of fluxonium transitions reaches 10 5, and our analysis show that it is likely not limited at present by the losses in the superinductance. Finally, inhomogeneous broadening of fluxonium transitions revealed the presence of coherent quantum phase-slip across the Josephson junction array. This phenomenon limits the lower operating frequency of a superinductance; in the present experiment this frequency was below 1 MHz and according to our analysis could easily be suppressed by several orders of magnitude after a small adjustment of the array junction parameters.
Carl Schreck's picture
Carl Schreck
Corey O'Hern
Senior Bioinformatics Scientist
Center for Pediatric Genomic Medicine at Children's Mercy Hospital
Mechanical and vibrational properties of model granular media
This thesis describes comprehensive computational and theoretical studies of the mechanical and vibrational properties of athermal particulate systems, such as granular media, foams, and emulsions, modeled as frictionless particles with spherical or ellipsoidal shapes. First, we investigate the mechanical properties of static packings of ellipsoidal particles. While amorphous packings of spherical particles are isostatic at jamming onset and possess the minimum contact number $z=z_{\rm iso}$ required for mechanical stability, packings of ellipsoidal particles are generally hypostatic with $z < z_{\rm iso}$. To investigate the deviation in contact number from the isostatic value, we measured the dynamical matrix of static packings of ellipsoidal particles. At jamming onset, we find that the harmonic response of these systems vanishes, and the potential energy scales as $\delta^4$ for perturbations $\delta$ along special directions in configuration space called `quartic modes'. Quartic modes give rise to novel power-law scaling of the static shear modulus and their number matches the deviation in the contact number from the isostatic value. Second, we investigated the dependence of the structural and mechanical properties of static bidisperse disk packings to two packing generation protocols. The first involved thermally quenching liquid configurations to zero temperature followed by compression to reach packing fractions $\phi_J$ at jamming onset. The second involved breaking the system up into a square grid and initializing alternating squares with small and large particles followed by compression to $\phi_J$. We find that {\it isostatic} packings exist over a range of packing fractions for large systems, in contrast to the view in the community that amorphous mechanically stable disk packings exist at a single packing fraction. We also compare the structural and mechanical properties of isostatic and hyperstatic packings. In addition, we also study the vibrational response of mechancially stable disk packings to thermal fluctuations. We find that, in contrast to atomic and molecular systems, systems that interact via repulsive contact forces possess no harmonic regime in the large system limit for all values of overcompression studied, and at jamming onset for all system sizes. To show this, we performed fixed energy simulations following perturbations with amplitude $\delta$ along all eigendirections of the dynamical matrix. The fluctuations abruptly spread to all modes for extremely small perturbation amplitudes, $\delta > \delta_c$, where $\delta_c$ is the amplitude at which a single contact breaks, and to a continuous frequency band for $\delta>\Delta\phi$. We also show that the density of vibrational modes deviates strongly from that predicted from the dynamical matrix in the nonharmonic regime.  
Benjamin Zwickl's picture
Benjamin Zwickl
Jack Harris
Assistant Professor of Physics
Rochester Institute of Technology
Progress Toward Observation of Radiation Pressure Shot Noise
Abstract: It has been over 100 years since the first conclusive demonstration of radiation pressure by Lebedev and Nichols and Hull. Cavity optomechanical systems---high finesse optical cavities coupled to mechanical resonators---are good testing grounds for the mechanical properties of light. The system described in this dissertation is a 7 mm long cavity coupled to a 1 mm square, 50 nm thick silicon nitride membrane. Like many similar optomechanical systems, ranging from LIGO to microtoroids, this work has moved beyond detecting the steady state force of light on a mirror to a rich array of dynamical effects. Classical effects include shifts in the mechanical resonant frequency and optical damping, both of which are demonstrated in this thesis. The (relatively) strong coupling between the light and mechanical resonator can, in principle, demonstrate effects beyond classical mechanics and classical light. This thesis represents an attempt to directly measure random quantum fluctuations in the force of light reflecting from a surface, an effect we call the radiation pressure shot noise. A correlation measurement scheme developed theoretically by Børkje et al. (Borkje2010) was implemented. The scheme is capable of distinguishing the effects of the random thermal force from the random radiation pressure shot noise. Successful suppression of thermal effects was demonstrated, though unfortunately not to the level required to measure the radiation pressure shot noise. In spite of not accomplishing this major physics goal, much was learned about this measurement scheme and its potential for future measurements of the radiation pressure shot noise.  
Degree Date: December, 2010
Rona Ramos's picture
Rona Ramos
Sean Barrett
Graduate Services Coordinator; Lecturer Physics
SPL 35A
Novel Pulse Sequences Exploiting the Effects of Hard π-pulses
In magnetic resonance and other spectroscopies, the strong pulses used to control coherent spin evolution are often approximated as instantaneous delta function rotations. However, small corrections to the delta function model can cause surprising departures from the conventional theory in standard multipulse NMR experiments using strong π-pulses. In this dissertation, we report the exploration of the small correction terms resulting from the finite duration of realistic pulses, however strong, using average Hamiltonian theory. Investigation of role these terms could play in standard NMR experiments led to the design and demonstration of a new class of spin echoes. We present analogs of the original free induction decay (FID), Hahn echo, and CPMG echoes whose experimental design is based on terms typically ignored when strong pulses are used. Variants on the original magic echo are demonstrated as well as the quadratic echo, based on both the zeroth- and first-order average Hamiltonian expressions and which has no classic NMR spin echo analog. Finally, we present alternative approaches to overcoming the line broadening effect of dipolar interactions in solids. Using a variation on the quadratic echo pulse sequence as a building block, we develop a new approach to line-narrowing and magnetic resonance imaging of solids which allows control of both the Zeeman and dipolar phase wrapping.
Degree Date: December, 2009
Ke Han's picture
Ke Han
Jack Sandweiss

Search for Stable Strange Quark Matter in Lunar Soil
Strange Quark Matter (SQM) is a proposed state of hadronic matter made up of roughly one-third each of up, down, and strange quarks in a single hadronic bag. For the last three decades, a wide range of experimental searches for strangelets (small lumps of SQM with baryon number less than 106) have been conducted but all failed to give a definite answer to the existence of SQM. We report results from a search for strangelets in a lunar soil sample, in which the predicted strangelet concentration is about 104 times higher than that on the Earth. The search used the tandem Van-de-Graaff accelerator at Yale as an ultra-sensitive mass spectrometer. We have searched for strangelets with nuclear charges 5, 6, 8. 9, and 11 over a range in mass from A = 42 to A = 70 amu. No strangelets were found in the experiment. For strangelets with nuclear charge 8, usually called strangelet oxygen, a concentration in the lunar soil sample higher than 10-16 per normal atom is excluded at the 95% confidence level. The implied limits on the strangelet flux in cosmic rays are the most sensitive to date for the covered range and are relevant to both recent theoretical flux predictions and a strangelet candidate event found by the Alpha Magnetic Spectrometer collaboration. 
Degree Date: May, 2008
Yong Jiang's picture
Yong Jiang
David DeMille

Research Website
Progress Towards Searching for Electron Electric Dipole Moment Using PbO
Observation of a non-zero electron electric dipole moment (EDM) will be explicit evidence for physics beyond the Standard Model. There is significant interest in developing experiments that can probe beyond the current limits for the electron EDM. An experiment to look for the EDM of the electron using the metastable a(1) [3Σ+] state of the PbO molecule has been implemented at Yale University. We populate the a(1) [3Σ+] state of 208PbO using laser-microwave double resonance, and we detect fluorescence, using quantum beat spectroscopy to extract minute frequency changes due to the electron EDM. The experimental method and setup will be described in this thesis. We have demonstrated the ability to manipulate the internal molecular state in such a way as to produce the desired states for our EDM experiment. Efforts were carried out to optimize the state excitation efficiency using an adiabatic following scheme. We performed various experiments to confirm our understanding of molecular state evolution dynamics in a variety of experimental configurations. Our experiment improved the accuracy of previously measured molecular constants of PbO, which cast light on the feasibility of future systematic error checking and reduction. Due to various technical issues, the sensitivity to an electron EDM in this generation of EDM experiment is far less than expected. Two novel proposals for a second generation EDM experiment are considered.
Degree Date: May, 1963
Charles Baltay's picture
Charles Baltay
Horace Dwight Taft
Eugene Higgins Professor of Physics
WL 213
Research Website
A study of antiperon production in antiperon-proton reactions