Yiqi Wang successfully defends thesis, “Manipulating and measuring states of a superfluid optomechanical resonator in the quantum regime”

Left to right: Jack Harris (advisor), Yiqi Wang, Rob Schoelkopf (Applied Physics), and Shruti Puri (Applied Physics), Yoni Kahn (UIUC)
August 2, 2023

On Wednesday, August 2, Yiqi Wang, an applied physics student and a member of Yale’s Wright Lab, successfully defended the thesis “Manipulating and measuring states of a superfluid optomechanical resonator in the quantum regime” (advisor: Jack Harris).

Yiqi explained, “One often asked question is why we cannot have quantum phenomena in our daily life? It is worth questioning the applicability of quantum theory beyond certain scales and utilizing quantum features in large objects for applications. Carefully designed and well-controlled experiments can reveal quantum features in macroscopic objects. My thesis work focuses on using light to control and readout the state of a massive oscillator in an essentially quantum way. The experiment leverages the properties of superfluid helium and photon counting techniques to reach the required control. This nano gram oscillator was prepared in thermal states, post-selected states, coherent states and entangled states. The witness of quantum features in a large object paves the avenue for future quantum-enhanced applications and posting interesting questions in fundamental physics research.”

Yiqi will work as a HQI prize postdoc fellow at Harvard.

Thesis Abstract:

What is the largest and most tangible object to reveal purely quantum phenomena? Macroscopic mechanical devices in the quantum regime can play a crucial role in quantum communication, quantum sensing, and tests of quantum mechanics. In this talk, I will describe my work toward manipulating and measuring a nanogram superfluid optomechanical resonator in the quantum regime. The mechanical resonator is the density wave of superfluid helium-4 in a fiber-based Fabry-Perot cavity. The light as a gentle quantum “drumstick” is used to control the motion of the helium, while the helium in exchange imprints information about its motion on the emitted light. For such a large object, a myriad of different factors conspire to mask quantum effects. However, I can circumvent some of the obstacles by leveraging the material properties of superfluid helium and by using single-photon counting techniques. In the experiment, I manipulated and characterized the state of the mechanics through optomechanical coupling and by performing photon counting measurements on the scattered light. I measured this mechanical resonator’s second/third/fourth-order coherence functions while it was in a thermal state with less than three phonons. In addition, I drove this mechanical resonator to a nearly coherent state. The state had around two phonons’ worth of fluctuations while its amplitude corresponded to 4X104 phonons.  Following the DLCZ protocol, I conditionally prepared non-classical photon-phonon entangled states. These experiments explored quantum states in macroscopic objects and may lead to new quantum-enhanced applications.