Strong and tunable couplings in flux-mediated optomechanics

TL;DR

This paper explores tunable strong couplings in a flux-mediated optomechanical system involving a SQUID with asymmetric Josephson junctions, demonstrating potential for reaching ultrastrong coupling regimes through magnetic field adjustments.

Contribution

It introduces a detailed quantum analysis of flux-mediated optomechanical interactions in asymmetric SQUIDs, highlighting tunable coupling regimes and the potential for ultrastrong single-photon coupling.

Findings

Radiation pressure and cross-Kerr interactions are modified by asymmetry.

Single-photon beam splitter interaction exists at finite asymmetry.

Magnetic field can enhance coupling strength to ultrastrong regimes.

Abstract

We investigate superconducting interference device (SQUID) with two asymmetric Josephson junctions coupled to a mechanical resonator embedded in the loop of the SQUID. We quantize this system in the case when the frequency of the mechanical resonator is much lower than the cavity frequency of the SQUID and in the case when they are comparable. In the first case, the radiation pressure and cross-Kerr type interactions arise and are modified by asymmetry. Cross-Kerr type coupling is the leading term at the extremum points where radiation pressure is zero. In the second case, the main interaction is single-photon beam splitter, which exists only at finite asymmetry. Another interaction in this regime is of cross-Kerr type, which exists at all asymmetries, but generally much weaker than the beam splitter interaction. Increasing magnetic field can substantially enhance optomechanical couplings strength with the potential for the radiation pressure coupling to reach the single-photon strong coupling regime, even the ultrastrong coupling regime, in which the single-photon coupling rate exceeds the mechanical frequency.