Studies of High-Q Phononic Resonator for Quantum Acoustic Applications*

High-Q phononic resonators hold tremendous potential as the basis for solid-state quantum storage, however, many technical challenges must be addressed before we can harness their remarkable storage times as the basis for high-fidelity quantum memories and sensor technologies. Long coherence time is difficult to achieve without a good understanding of the material quality as well as the surface losses. In this work, we apply a new non-invasive laser-based spectroscopy method to the study of phonon coherence in high-purity crystals. Using a novel reflow-based fabrication technique, we shape the faces of pristine crystalline quartz substrates to form a stable phononic resonator that supports high Q-factor bulk acoustic modes. We interrogate the modes of this phononic resonator using stimulated Brillouin scattering measurements. To differentiate the geometric loss (e.g. clipping loss and surface scattering loss) from the material loss (e.g. dislocations, impurities, internal stress, etc.) we use the fabricated device topography as inputs to a new acoustic mode solver that computes the geometric component of loss. Using these new spectroscopic techniques and device simulations in conjunction with quantitative measurements of defects and impurities, we work to build record-breaking high-Q phononic resonators for high-coherence quantum acoustic technologies.