Superradiant Quantum Phase Transition in Open Systems: System-Bath Interaction at the Critical Point

TL;DR

This paper investigates the behavior of the Dicke model near its quantum critical point when coupled to a bosonic environment, revealing that the environment does not alter the critical point or ground state but affects spectral lineshapes.

Contribution

It provides a comprehensive analysis of the open Dicke model at criticality without relying on common approximations, showing environment effects on spectral features.

Findings

Critical point remains unaffected by environment.

Ground-state condensates are robust against bath interactions.

Spectral lineshapes become asymmetric near the critical point.

Abstract

The occurrence of a second-order quantum phase transition in the Dicke model is a well-established feature. On the contrary, a comprehensive understanding of the corresponding open system, particularly in the proximity of the critical point, remains elusive. When approaching the critical point, the system inevitably enters first the system-bath ultrastrong coupling regime and finally the deepstrong coupling regime, causing the failure of usual approximations adopted to describe open quantum systems. We study the interaction of the Dicke model with bosonic bath fields in the absence of additional approximations, which usually relies on the weakness of the system-bath coupling. We find that the critical point is not affected by the interaction with the environment. Moreover, the interaction with the environment is not able to affect the system ground-state condensates in the superradiant phase, whereas the bath fields are infected by the system and acquire macroscopic occupations. The obtained reflection spectra display lineshapes which become increasingly asymmetric, both in the normal and superradiant phases, when approaching the critical point.