Flux-mediated optomechanics with a transmon qubit in the single-photon ultrastrong-coupling regime

TL;DR

This paper proposes a superconducting circuit scheme that enables control of a mechanical resonator at the quantum level in the ultrastrong-coupling regime, facilitating advanced quantum state engineering and tests.

Contribution

It introduces a novel configuration combining a transmon qubit and a suspended beam in a SQUID loop to achieve tunable single-photon ultrastrong optomechanical coupling.

Findings

Achieves tunable ultrastrong optomechanical interaction with realistic parameters.

Demonstrates protocols for generating entangled states and Schrödinger cat states.

Identifies optimal conditions for ground-state cooling of the mechanical resonator.

Abstract

We propose a scheme for controlling a radio-frequency mechanical resonator at the quantum level using a superconducting qubit. The mechanical part of the circuit consists of a suspended micrometer-long beam that is embedded in the loop of a superconducting quantum interference device (SQUID) and is connected in parallel to a transmon qubit. Using realistic parameters from recent experiments with similar devices, we show that this configuration can enable a tuneable optomechanical interaction in the single-photon ultrastrong-coupling regime, where the radiation-pressure coupling strength is larger than both the transmon decay rate and the mechanical frequency. We investigate the dynamics of the driven system for a range of coupling strengths and find an optimum regime for ground-state cooling, consistent with previous theoretical investigations considering linear cavities. Furthermore, we numerically demonstrate a protocol for generating hybrid discrete- and continuous-variable entanglement as well as mechanical Schr\"{o}dinger cat states, which can be realised within the current state of the art. Our results demonstrate the possibility of controlling the mechanical motion of massive objects using superconducting qubits at the single-photon level and could enable applications in hybrid quantum technologies as well as fundamental tests of quantum mechanics.