Cavity quantum acoustic device in the multimode strong coupling regime

TL;DR

This paper demonstrates a cavity quantum acoustic device that achieves multimode strong coupling between a transmon qubit and surface acoustic wave resonator, enabling advanced quantum acoustics experiments.

Contribution

It introduces a novel device integrating a flux tunable transmon with a high-Q surface acoustic wave resonator, operating in the multimode strong coupling regime.

Findings

Achieved qubit-cavity coupling of 6.5 MHz exceeding loss and linewidths.

Observed dispersive shifts consistent with generalized Jaynes-Cummings model.

Identified frequencies where phonon emission and qubit linewidth are suppressed.

Abstract

We investigate an acoustical analog of circuit quantum electrodynamics that facilitates compact high-Q (${>}20,000$) microwave-frequency cavities with dense spectra. We fabricate and characterize a device that comprises a flux tunable transmon coupled to a $300\,\mathrm{\mu m}$ long surface acoustic wave resonator. For some modes, the qubit-cavity coupling reaches $6.5\,\mathrm{MHz}$, exceeding the cavity loss rate ($200\,\mathrm{kHz}$), qubit linewidth ($1.1\,\mathrm{MHz}$), and the cavity free spectral range ($4.8\,\mathrm{MHz}$), placing the device in both the strong coupling and strong multimode regimes. With the qubit detuned from the cavity, we show that the dispersive shift behaves according to predictions from a generalized Jaynes-Cummings Hamiltonian. Finally, we observe that the qubit linewidth strongly depends on its frequency, as expected for spontaneous emission of phonons, and we identify operating frequencies where this emission rate is suppressed.