Relaxation breakdown and resonant tunneling in ultrastrong-coupling cavity QED

TL;DR

This paper investigates how ultrastrong coupling in cavity QED affects system relaxation, revealing that while it inhibits relaxation, polaronic multi-photon resonances can restore fast relaxation through cavity-mediated tunneling.

Contribution

It provides a comprehensive analysis of relaxation dynamics in ultrastrong-coupling cavity QED, introducing a generalized rotating-wave approximation in the polaron frame and exploring multi-photon resonances.

Findings

Ultrastrong coupling suppresses relaxation due to tunneling rate inhibition.

Polaronic multi-photon resonances restore fast relaxation.

Analytical diagonalization of the Rabi model in the polaron frame.

Abstract

We study the open relaxation dynamics of an asymmetric dipole that is ultrastrongly coupled to a single electromagnetic cavity mode. By using a thermalizing master equation for the whole interacting system we derive a phase diagram of the Liouvillian gap. It emerges that the ultrastrong coupling inhibits the system relaxation toward the equilibrium state due to an exponential suppression of the dipole tunneling rate. However, we find that polaronic multi-photon resonances restore fast relaxation by a cavity-mediated dipole resonant tunneling process. Aside of the numerical evidences, we develop a fully analytical description by diagonalizing the Rabi model through a generalized rotating-wave approximation, valid in the so-called polaron frame. The relaxation physics of such ultrastrong-coupling systems is then reduced to a multi-photon polaron version of the standard text-book dressed states picture. At the end we discuss an extension to a multi-well dipole that can set the basis of a cascaded resonant tunnelling setup in the ultrastrong coupling regime.