Gauge ambiguities imply Jaynes-Cummings physics remains valid in ultrastrong coupling QED

TL;DR

This paper demonstrates that gauge ambiguities in quantum electrodynamics allow the Jaynes-Cummings model to remain valid in the ultrastrong coupling regime, challenging previous assumptions about the breakdown of simpler models.

Contribution

The study shows that different gauge choices lead to distinct models in ultrastrong coupling, and that a gauge-specific Jaynes-Cummings model can outperform the quantum Rabi model in predictions.

Findings

The gauge choice affects the predicted physics in ultrastrong coupling.

A gauge-specific Jaynes-Cummings model accurately predicts ground state properties.

Ground state entanglement is not necessarily high in ultrastrong coupling regimes.

Abstract

Ultrastrong-coupling between two-level systems and radiation is important for both fundamental and applied quantum electrodynamics (QED). Such regimes are identified by the breakdown of the rotating-wave approximation, which applied to the quantum Rabi model (QRM) yields the apparently less fundamental Jaynes-Cummings model (JCM). We show that when truncating the material system to two levels, each gauge gives a different description whose predictions vary significantly for ultrastrong-coupling. QRMs are obtained through specific gauge choices, but so too is a JCM without needing the rotating-wave approximation. Analysing a circuit QED setup, we find that this JCM provides more accurate predictions than the QRM for the ground state, and often for the first excited state as well. Thus, Jaynes-Cummings physics is not restricted to light-matter coupling below the ultrastrong limit. Among the many implications is that the system's ground state is not necessarily highly entangled, which is usually considered a hallmark of ultrastrong-coupling.