Ultrastrong coupling regime of non-dipolar light-matter interactions

TL;DR

This paper proposes a circuit-QED setup to achieve and study the ultrastrong coupling regime of nondipolar light-matter interactions, revealing new quantum phenomena and spectral features.

Contribution

It introduces a practical scheme to reach ultrastrong nondipolar coupling and demonstrates how to observe spectral collapse in a solid-state device.

Findings

Identification of a two-photon quantum Rabi model approximation

Detection of spectral collapse near the collapse point

Observation of changes in selection rules and spectral features

Abstract

We present a circuit-QED scheme which allows to reach the ultrastrong coupling regime of a nondipolar interaction between a single qubit and a quantum resonator. We show that the system Hamiltonian is well approximated by a two-photon quantum Rabi model and propose a simple scattering experiment to probe its fundamental properties. In particular, we identify a driving scheme that reveals the change in selection rules characterizing the breakdown of the rotating-wave approximation and the transition from strong to ultrastrong two-photon interactions. Finally, we show that a frequency crowding in a narrow spectral region is observable in the output fluoresce spectrum as the coupling strength approaches the collapse point, paving the way to the direct observation of the onset of the spectral collapse in a solid-state device.