PhD Defense - Yizhi Luo

High-coherence phononic resonators for quantum acoustic applications

 

 

Long-lived phonons are a promising quantum resource, enabling advanced applications in quantum sensing, transduction, and memory.  While mechanical resonators spanning over a wide range of frequencies have been demonstrated, high-frequency (GHz) phonons are particularly sought after for quantum acoustic systems, as they permit ground-state operations at cryogenic temperatures and are more readily controlled using quantum optics and circuit-QED techniques. Recent advancements in silicon-based nanomechanical resonators have demonstrated lifetimes on the order of seconds; however, their coherence times are constrained to ~100 us due to pronounced surface interactions.

In this thesis, I will present a different type of phononic resonator: a micro-fabricated high-overtone bulk acoustic resonator ( uHBAR) based on high-purity quartz crystal. These resonators achieve Q-factors of 360 million at 12 GHz, translating to phonon coherence times of nearly 10 ms and record-setting f-Q products of 4.6X10^18 Hz. The coherence time was derived from the phonon spectral linewidth measured by a novel Brillouin-based laser spectroscopy technique. Complementary spectral and coherent ring-down measurements revealed negligible dephasing within these oscillators. Furthermore, surface-limited phonon dissipation was identified and found to extend far beyond surface roughness scattering, implicating subsurface defects—such as lattice distortions, dislocations, and elemental contamination—as significant contributors. By employing an optimized polishing process in conjunction with advanced material diagnostic techniques, including X-ray diffraction (XRD) and atomic force microscopy (AFM), the above record-high phonon coherence times have been realized. In conclusion, uHBARs not only provide an ideal platform for hosting high-coherence phonons, unlocking a wide spectrum of quantum applications, but also serve as a valuable testbed for investigating fundamental material properties and surface interactions.