AcademicArticleSCO_85054547579

abstract

• © 2018 American Physical Society. 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 semiclassical and quantum analysis of the model. In the semiclassical regime, we show analytic results that reveal the existence of a third regime in addition of the two characteristics 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 the spectral characteristics of the quantum model and the onset of chaos in its semiclassical equivalent is valid in this model as a revival of order in the semiclassical dynamics occurs around the new phase transition.