Regimes of Cavity-QED under Incoherent Excitation: From Weak to Deep Strong Coupling

TL;DR

This paper investigates the photon emission behavior of a two-level atom interacting with a quantized field across all light-matter coupling regimes, addressing theoretical challenges and gauge invariance issues in the ultrastrong and deep strong coupling regimes.

Contribution

It provides a comprehensive analysis of photon flux emission under incoherent excitation across all coupling strengths, including the deep strong coupling regime, ensuring gauge invariance and addressing limitations of standard models.

Findings

Photon flux emission rate varies with coupling strength and temperature.

Light-matter decoupling occurs in the deep strong coupling regime.

Results are gauge invariant across regimes.

Abstract

The prototypical system constituted by a two-level atom interacting with a quantized single-mode electromagnetic field is described by the quantum Rabi model (QRM). The QRM is potentially valid at any light-matter interaction regime, ranging from the weak (where the decay rates exceeds the coupling rate) to the deep strong coupling (where the interaction rate exceeds the bare transition frequencies of the subsystems). However, when reaching the ultrastrong coupling regime, several theoretical issues may prevent the correct description of the observable dynamics of such a system: (i) the standard quantum optics master equation fails to correctly describe the interaction of this system with the reservoirs; (ii) the correct output photon rate is no longer proportional to the intracavity photon number; and (iii) the appears to violate gauge invariance. Here, we study the photon flux emission rate of this system under the incoherent excitation of the two-level atom for any light-matter interaction strength, and consider different effective temperatures. The dependence of the emission spectra on the coupling strength is the result of the interplay between energy levels, matrix elements of the observables, and the density of states of the reservoirs. Within this approach, we also study the occurence of light-matter decoupling in the deep strong coupling regime, and show how all of the obtained results are gauge invariant.