Gauge-Independent Emission Spectra and Quantum Correlations in the Ultrastrong Coupling Regime of Open System Cavity-QED

TL;DR

This paper develops a gauge-invariant theoretical framework for analyzing emission spectra and quantum correlations in cavity-QED systems in the ultrastrong coupling regime, addressing limitations of previous models.

Contribution

It introduces a gauge-consistent truncation method and master equations for open systems, enabling accurate predictions of emission spectra and correlations in ultrastrong coupling.

Findings

Gauge-invariant emission spectra differ significantly from previous models.

Photon flux rates and quantum correlations show notable discrepancies with standard approaches.

Explicit calculations demonstrate the importance of gauge consistency in ultrastrong coupling regimes.

Abstract

A quantum dipole interacting with an optical cavity is one of the key models in cavity quantum electrodynamics (cavity-QED). To treat this system theoretically, the typical approach is to truncate the dipole to two levels. However, it has been shown that in the ultrastrong-coupling regime, this truncation naively destroys gauge invariance. By truncating in a manner consistent with the gauge principle, we introduce master equations {for open systems} to compute gauge-invariant emission spectra, photon flux rates, and quantum correlation functions which show significant disagreement with previous results obtained using the standard quantum Rabi model. Explicit examples are shown using both the dipole gauge and the Coulomb gauge.