Listening to Bulk Crystalline Vibrations with Superconducting Qubits*

Superconducting circuits are a versatile platform for quantum computing owing to their scalability and reconfigurability, integration with high quality microwave resonators, and demonstrated ease in interfacing with hybrid quantum platforms. Recent work has demonstrated strong coupling to long-lived phonons of bulk acoustic waves (BAWs) of pristine crystalline substrates [1]. Despite achieving single-phonon control, a superconducting qubit piezoelectrically coupled to BAWs had a significantly reduced lifetime (T₁ = 7 us) compared to a naked transmon qubit (T₁ = 40 us), suggesting that uncontrolled coupling to BAW modes may be complicit in the qubit’s decoherence.
Here, we use the lifetime of a superconducting qubit to measure the local density of acoustic states in sapphire as a proxy for electro-mechanical decoherence. Varying the crystalline geometry can be used to systematically modify the qubit’s lifetime in order to ascertain how the coupling geometry leads to a mechanical Purcell effect. Our work offers insights into the design of compact quantum memories based on BAW resonators.
[1] Y. Chu, et al. Nature 563, 666-670 (2018)