Superharmonic Resonances in a Strongly Coupled Cavity-Atom System

TL;DR

This paper investigates superharmonic resonances in a strongly coupled superconducting qubit-cavity system, revealing nonlinear response changes and potential applications in quantum amplification and entangled light generation.

Contribution

It demonstrates the occurrence of superharmonic resonances and nonlinear response changes near the qubit-cavity crossing point, with experimental and theoretical analysis.

Findings

Sign change in Kerr coefficient near resonance crossing

Observation of superharmonic resonances at integer frequency ratios

Good qualitative agreement between experiment and theory

Abstract

We study a system consisting of a superconducting flux qubit strongly coupled to a microwave cavity. The fundamental cavity mode is externally driven and the response is investigated in the weak nonlinear regime. We find that near the crossing point, at which the resonance frequencies of the cavity mode and qubit coincide, the sign of the Kerr coefficient changes, and consequently the type of nonlinear response changes from softening to hardening. Furthermore, the cavity response exhibits superharmonic resonances when the ratio between the qubit frequency and the cavity fundamental mode frequency is tuned close to an integer value. The nonlinear response is characterized by the method of intermodulation and both signal and idler gains are measured. The experimental results are compared with theoretical predictions and good qualitative agreement is obtained. The superharmonic resonances have potential for applications in quantum amplification and generation of entangled states of light.