High-coherence Phononic Resonators for Quantum Acoustic Applications

Date of Award

Fall 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Applied Physics

First Advisor

Rakich, Peter

Abstract

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.

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