Resolution of superluminal signalling in non-perturbative cavity quantum electrodynamics

TL;DR

This paper shows that single-mode cavity models in ultra-strong coupling regimes are unphysical and proposes a multi-mode approach that resolves superluminal signalling, revealing new phenomena in cavity quantum electrodynamics.

Contribution

It demonstrates the inadequacy of single-mode models in non-perturbative regimes and introduces a multi-mode framework that accurately describes light propagation and intracavity dynamics.

Findings

Single-mode models allow superluminal signalling in ultra-strong coupling.

Multi-mode models predict different physical observables from single-mode models.

Coexistence of free photonic wavefronts and virtual photon bound states.

Abstract

Recent technological developments have made it increasingly easy to access the non-perturbative regimes of cavity quantum electrodynamics known as ultra or deep strong coupling, where the light-matter coupling becomes comparable to the bare modal frequencies. In this work, we address the adequacy of the broadly used single-mode cavity approximation to describe such regimes. We demonstrate that, in the non-perturbative light-matter coupling regimes, the single-mode models become unphysical, allowing for superluminal signalling. Moreover, considering the specific example of the quantum Rabi model, we show that the multi-mode description of the electromagnetic field, necessary to account for light propagation at finite speed, yields physical observables that differ radically from their single-mode counterparts already for moderate values of the coupling. Our multi-mode analysis also reveals phenomena of fundamental interest on the dynamics of the intracavity electric field, where a free photonic wavefront and a bound state of virtual photons are shown to coexist.