PhD Defense - Yiqi Wang

Manipulating and Measuring States of a Superfluid Optome- chanical Resonator in the Quantum Regime

What is the largest and most tangible object to reveal purely quantum phe- nomena? Macroscopic mechanical devices in the quantum regime can play a crucial role in quantum communication, quantum sensing, and tests of quan- tum 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 im- print 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 ex- periment, 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 co- herent 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.