Enhancement of the optomechanical coupling and Kerr nonlinearity using the Josephson Capacitance of Cooper-pair box

TL;DR

This paper proposes a method to significantly enhance optomechanical coupling in a circuit system using a Josephson junction Cooper-pair box, enabling ultrastrong coupling and tunable nonlinearities for advanced quantum applications.

Contribution

It introduces a scheme to boost optomechanical coupling by tuning Josephson capacitance, achieving ultrastrong coupling and enabling pure Kerr nonlinearity for phonon counting.

Findings

Enhanced optomechanical coupling by up to seven orders of magnitude.

Tunable system exhibiting pure cross-Kerr nonlinearity.

Potential for phonon counting applications.

Abstract

We propose a scheme for enhancing the optomechanical coupling between microwave and mechanical resonators by up to seven orders of magnitude to the ultrastrong coupling limit in a circuit optomechanical setting. The tripartite system considered here consists of a Josephson junction Cooper-pair box that mediates the coupling between the microwave cavity and the mechanical resonator. The optomechanical coupling can be modified by tuning the gate charge and the magnetic flux bias of the Cooper-pair box which in turn affect the Josephson capacitance of the Cooper-pair box. We additionally show that with suitable choice of tuning parameters, the optomechanical coupling vanishes and the system exhibits purely a cross-Kerr type of nonlinearity between the cavity and the mechanical resonator. This allows the system to be used for phonon counting.