Loss Mechanisms in High-coherence Multimode Mechanical Resonators Coupled to Superconducting Circuits

TL;DR

This paper investigates loss mechanisms in high-coherence multimode mechanical resonators used in circuit quantum acoustodynamics, identifying defect density and interfaces as key factors affecting coherence, and demonstrating record phonon lifetimes and coherence times.

Contribution

It provides the first detailed analysis of acoustic dissipation in HBAR resonators within cQAD systems, achieving record coherence times and demonstrating the impact of material interfaces on device performance.

Findings

Phonon lifetimes up to 400 μs measured.

Coherence times approaching 1 ms in the quantum regime.

Hybrid systems with high quantum coherence cooperativity achieved.

Abstract

Circuit quantum acoustodynamics (cQAD) devices have a wide range of applications in quantum science, all of which depend crucially on the quantum coherence of the mechanical subsystem. In this context, high-overtone bulk acoustic-wave resonators (HBARs) are particularly promising, since they have shown very high quality factors with negligible dephasing. However, the introduction of piezoelectric films, which are necessary for coupling to a superconducting circuit, can lead to additional loss channels, such as surface scattering and two-level systems (TLS). Here, we study the acoustic dissipation of HBAR resonators in cQAD systems and find that the defect density of the piezoelectric material and its interface with the bulk are limiting factors for the coherence. We measure acoustic modes with phonon lifetimes up to 400 $\mu$s and lifetime-limited coherence times approaching one millisecond in the quantum regime. When coupled to a superconducting qubit, this leads to a hybrid system with a large quantum coherence cooperativity of $C_{T_2}=1.1\times10^5$. These results represent a new milestone for the performance of cQAD devices and offer concrete paths forward for further improvements.