On October 24, 2023, Lucy Yu, an applied physics student and a member of Yale’s Wright Lab, successfully defended the thesis, “Toward the Quantum Control of the Motional State of Superfluid 4He in a Optomechanical Resonator” (advisor: Jack Harris).
Lucy explained, “Demonstrating macroscopic quantum phenomena remains an outstanding scientific goal in experimental physics, which is instrumental for 1) the advance of fundamental quantum physics theory such as decoherence mechanism, 2) developing quantum-enhanced technologies, and 3) satisfying our intellectual curiosity of preserving quantum effects on macroscopic objects. Optomechanical systems where macroscopic mechanical motion coherently interacts with light that is of intrinsic quantum nature thus emerges as a prominent platform for realizing this scientific goal. This motivates my thesis work toward achieving quantum optical control on the motional state of nano-gram of superfluid helium in an optomechanical resonator.”
Lucy continues, I have not decided where to work next, whether to go back to China or staying the US. I am planning to start my new job (most likely in industry instead of academia) by January next year.”
The ability to generate and manipulate non-Gaussian macroscopic quantum states and multi-body entanglement with light is foundational to achieving quantum-enhanced technologies, and furthuring our understanding on macroscopic quantum effects and decoherence mechanism. Optomechanical devices which coherently couple the optical and mechanical degrees of freedom have emerged as a prominent platform for them in the past two decades.
I will present in this thesis my work toward realizing full optical quantum control of the motional state of superfluid 4He in a fiber Fabry-Perot cavity, where coherent coupling between the intensity of the optical field and the density fluctuation of ~ 1 ng of liquid helium is established via electrostrictive interaction. Measurements on phonon coherences are used to characterize our system using a heralded single photon detection scheme, where the detection of a single photon heralds the creation or annihilation of a single phonon.
The acoustic state is characterized by its phonon coherences up to the fourth-order, and found to be consistent with a thermal state in equilibrium to its bath via a Markovian coupling. Through post-selection on photon detection events, a k-phonon-subtracted or -added out-of-equilibrium state is heralded and characterized by its phonon coherences (for k ≤ 3).
Works presented in this thesis demonstrate the robust implementation of single photon counting for our system, providing access to the nonlinear quantum optomechanical effects induced by measurement backaction, and lays the foundation for implementing a full quantum protocol to generate, manipulate, store and read out an acoustic state on the single quantum level.