Critical phenomena in an extended Dicke model

TL;DR

This paper investigates an extended Dicke model, revealing a new third regime with unique spectral features and quantum phase transitions, bridging semi-classical and quantum analyses relevant for quantum technologies.

Contribution

It introduces a semi-classical and quantum analysis of an extended Dicke model, discovering a new regime and associated quantum phase transitions not present in the standard model.

Findings

Identification of a third regime with unique spectral discontinuities

Observation of two types of clustering indicating quantum phase transitions

Revival of order around the new phase transition in semi-classical dynamics

Abstract

Spectral characterization is a fundamental step in the development of useful quantum technology platforms. Here, we study an ensemble of interacting qubits coupled to a single quantized field mode, an extended Dicke model that might be at the heart of Bose-Einstein condensate in a cavity or circuit-QED experiments for large and small ensemble sizes, respectively. We present a semi-classical and quantum analysis of the model. In the semi-classical regime, we show analytic results that reveal the existence of a third regime, in addition of the two characteristic of the standard Dicke model, characterized by one logarithmic and two jump discontinuities in the derivative of the density of states. We show that the finite quantum system shows two different types of clustering at the jump discontinuities, signaling a precursor of two excited quantum phase transitions. These are confirmed using Peres lattices where unexpected order arises around the new precursor. Interestingly, Peres conjecture regarding the relation between spectral characteristics of the quantum model and the onset of chaos in its semi-classical equivalent is valid in this model as a revival of order in the semi-classical dynamics occurs around the new phase transition.