Investigating the coherence properties of HBARs with classical and quantum sensors

High overtone bulk acoustic wave resonators (HBARs) have been successfully integrated with superconducting qubits into hybrid quantum systems with many applications in quantum information technologies [1,2]. Their success strongly relies on the excellent coherence properties of the mechanical modes. Nevertheless, a full understanding of the limiting loss mechanisms affecting multimode HBAR devices is still missing. In the present work, we use microwave antennas to characterize the quality factors of hundreds of HBAR modes in a several-GHz frequency span. We observe near state-of-the-art quality factors for our devices, ranging from 1 to 10 million at cryogenic temperatures, resulting in frequency-quality factor products of fxQ~10¹⁶. The systematic comparison between HBARs of different thickness allows us to identify surface roughness as one of the limiting loss mechanisms affecting the higher frequency modes. We also compare these results to measurements of the phonon lifetimes in the single-excitation regime using a flux-tunable superconducting qubit, finding good agreement between the results of measurements in the classical and quantum regimes.

[1] Y. Chu, et.al. Science 358 (6360), 199-202 (2017).

[2] U. von Luepke, et.al. Nature Physics 20, 564-570 (2024)