Cavity quantum electrodynamics in the non-perturbative regime

TL;DR

This paper develops a comprehensive quantum model for cavity QED that includes both dipole-field and dipole-dipole interactions, extending into the non-perturbative regime with strong coupling effects.

Contribution

It introduces a minimal quantum model applicable at arbitrary coupling strengths, unifying various predictions and characterizing different vacuum states in ultrastrong coupling regimes.

Findings

Identification of three classes of vacuum states: normal, superradiant, subradiant.

Extension of the Dicke model into the non-perturbative regime.

Reconciliation of previous contradictory predictions in cavity QED.

Abstract

We study a generic cavity-QED system where a set of (artificial) two-level dipoles is coupled to the electric field of a single-mode LC resonator. This setup is used to derive a minimal quantum mechanical model for cavity QED, which accounts for both dipole-field and direct dipole-dipole interactions. The model is applicable for arbitrary coupling strengths and allows us to extend the usual Dicke model into the non-perturbative regime of QED, where the dipole-field interaction can be associated with an effective finestructure constant of order unity. In this regime, we identify three distinct classes of normal, superradiant and subradiant vacuum states and discuss their characteristic properties and the transitions between them. Our findings reconcile many of the previous, often contradictory predictions in this field and establish a common theoretical framework to describe ultrastrong coupling phenomena in a diverse range of cavity-QED platforms.