Probing higher-order transitions through scattering of microwave photons in the ultrastrong-coupling regime of circuit QED

TL;DR

This paper proposes a method to detect higher-order virtual transitions in circuit QED by analyzing microwave photon scattering, revealing complex qubit-cavity interactions and asymmetric coupling effects.

Contribution

It introduces a feasible scattering-based approach to probe higher-order and dark states in the ultrastrong-coupling regime of circuit QED.

Findings

Higher-order transitions can be identified in scattering spectra.

Asymmetric coupling reveals dark antisymmetric states with Fano-lineshapes.

Scattering spectra reflect coherent qubit-cavity interactions.

Abstract

Higher-order transitions can occur in the ultrastrong-coupling regime of circuit QED through virtual processes governed by the counter-rotating interactions. We propose a feasible way to probe higher-order transitions through the scattering of propagating microwave photons incident on the hybrid qubit-cavity system. The lineshapes in the scattering spectra can indicate the coherent interaction between the qubits and the cavity, and the higher-order transitions can be identified in the population spectra. We further find that if the coupling strengths between the two qubits and the cavity are tuned to be asymmetric, the dark antisymmetric state with the Fano-lineshape can also be detected from the variations in the scattering spectra.