Coupling a superconducting quantum circuit to a phononic crystal defect cavity

TL;DR

This paper demonstrates coupling a phononic crystal defect cavity in lithium niobate to a superconducting quantum circuit, achieving strong phonon-photon interaction suitable for quantum information applications.

Contribution

It introduces a method to localize high-frequency acoustic modes in a lithium niobate phononic crystal defect cavity and couples them effectively to superconducting circuits.

Findings

Achieved a phonon-photon coupling rate of approximately 1.6 MHz.

Measured a mechanical quality factor around 3 x 10^4.

Demonstrated a cooperativity of about 4 when modes are tuned into resonance.

Abstract

Connecting nanoscale mechanical resonators to microwave quantum circuits opens new avenues for storing, processing, and transmitting quantum information. In this work, we couple a phononic crystal cavity to a tunable superconducting quantum circuit. By fabricating a one-dimensional periodic pattern in a thin film of lithium niobate and introducing a defect in this artificial lattice, we localize a 6 gigahertz acoustic resonance to a wavelength-scale volume of less than one cubic micron. The strong piezoelectricity of lithium niobate efficiently couples the localized vibrations to the electric field of a widely tunable high-impedance Josephson junction array resonator. We measure a direct phonon-photon coupling rate $g/2\pi \approx 1.6 \, \mathrm{MHz}$ and a mechanical quality factor $Q_\mathrm{m} \approx 3 \times 10^4$ leading to a cooperativity $C\sim 4$ when the two modes are tuned into resonance. Our work has direct application to engineering hybrid quantum systems for microwave-to-optical conversion as well as emerging architectures for quantum information processing.