Resolution of superluminal signalling in non-perturbative cavity quantum electrodynamics

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.

Resolution of Superluminal Signalling in Non-Perturbative Cavity Quantum Electrodynamics

The paper "Resolution of superluminal signalling in non-perturbative cavity quantum electrodynamics" addresses the challenges of modelling light-matter interactions within the ultra and deep strong coupling regimes. These regimes, achieved through large light-matter couplings in solid-state cavity quantum electrodynamics (QED), present unique phenomena that deviate significantly from traditional descriptions, particularly when employing a single-mode cavity approximation.

Numerical Findings and Physical Observables

Our multi-mode analysis reveals that the widely utilized single-mode cavity approximation falls short in describing the non-perturbative regimes of cavity QED. Specifically, such models become unphysical, permitting phenomena such as superluminal signalling. The authors provide evidence that multi-mode descriptions, which honour light propagation at finite speeds, offer observable predictions that diverge notably from their single-mode counterparts, even at moderate coupling strengths.

The paper examines the quantum Rabi model as a testbed for these explorations. It demonstrates that modelling the electromagnetic field with multiple modes not only restores physical soundness to the model but also unveils fundamental phenomena within the dynamics of the intracavity electric field. The coexistence of a free photonic wavefront and a bound state of virtual photons is distinctly observed.

Implications for Quantum Models

From a theoretical perspective, this paper highlights the intrinsic problems of single-mode descriptions in the ultra and deep strong coupling regimes. The results underscore the necessity for multi-mode considerations in systems where light-matter coupling energy becomes comparable to bare modal frequencies. This insight is critical for developing accurate quantum models in such regimes, which may impact various applications in quantum technology and photonics.

Speculative Future Developments

Given the paper’s insights, advancements in QED at ultra and deep strong coupling regimes can be anticipated. Future research may focus on expanding multi-mode models to more complex architectures or exploring new quantum phenomena that arise from such strong couplings. These developments could steer innovations not only in quantum sensing and computing but also in designing systems that exploit these unique light-matter interaction properties.

In conclusion, the authors demonstrate that multi-mode cavity approaches are indispensable for accurately capturing non-perturbative dynamics in cavity QED, challenging prevalent single-mode approximations and providing fresh perspectives on ultra and deep strong coupling phenomena.