Quantum electromechanics of a hypersonic crystal

TL;DR

This paper demonstrates a hypersonic electromechanical crystal that couples microwave photons with high-frequency phonons, achieving strong coupling and near-ground-state cooling, paving the way for hybrid quantum circuits.

Contribution

It introduces a novel hypersonic crystal device embedding a vacuum-gap capacitor within a phononic crystal, enabling efficient microwave-phonon coupling at gigahertz frequencies.

Findings

Large cooperative coupling (C ≈ 30) achieved between microwave and hypersonic phonons.

Hypersonic mode has a decay time of 2.3 ms and is close to its quantum ground state.

Device operates effectively at 10 mK, suitable for quantum applications.

Abstract

Radiation pressure within engineered structures has recently been used to couple the motion of nanomechanical objects with high sensitivity to optical and microwave electromagnetic fields. Here, we demonstrate a form of electromechanical crystal for coupling microwave photons and hypersonic phonons by embedding the vacuum-gap capacitor of a superconducting resonator within a phononic crystal acoustic cavity. Utilizing a two-photon resonance condition for efficient microwave pumping and a phononic bandgap shield to eliminate acoustic radiation, we demonstrate large cooperative coupling ($C \approx 30$) between a pair of electrical resonances at $10$GHz and an acoustic resonance at $0.425$GHz. Electrical read-out of the phonon occupancy shows that the hypersonic acoustic mode has an intrinsic energy decay time of $2.3$ms and thermalizes close to its quantum ground-state of motion (occupancy $1.5$) at a fridge temperature of $10$mK. Such an electromechanical transducer is envisioned as part of a hybrid quantum circuit architecture, capable of interfacing to both superconducting qubits and optical photons.