Effects of quasiparticle tunneling in a circuit-QED realization of a strongly driven two-level system

TL;DR

This study investigates how quasiparticle tunneling affects the frequency response of a driven cavity coupled to a superconducting charge qubit, revealing regimes where charge sensitivity is enhanced or diminished.

Contribution

It introduces a master equation model including quasiparticle tunneling and demonstrates its impact on charge detection sensitivity in circuit-QED systems.

Findings

Quasiparticle tunneling causes a bimodal frequency shift behavior.

High drive strengths enable fast relaxation to sensitive states.

Charge sensitivity remains robust at high drives despite quasiparticles.

Abstract

We experimentally and theoretically study the frequency shift of a driven cavity coupled to a superconducting charge qubit. In addition to previous studies, we here also consider drive strengths large enough to energetically allow for quasiparticle creation. Quasiparticle tunneling leads to the inclusion of more than two charge states in the dynamics. To explain the observed effects, we develop a master equation for the microwave dressed charge states, including quasiparticle tunneling. A bimodal behavior of the frequency shift as a function of gate voltage can be used for sensitive charge detection. However, at weak drives the charge sensitivity is significantly reduced by non-equilibrium quasiparticles, which induce transitions to a non-sensitive state. Unexpectedly, at high enough drives, quasiparticle tunneling enables a very fast relaxation channel to the sensitive state. In this regime, the charge sensitivity is thus robust against externally injected quasiparticles and the desired dynamics prevail over a broad range of temperatures. We find very good agreement between theory and experiment over a wide range of drive strengths and temperatures.