Energy cat states induced by a parity-breaking excited-state quantum phase transition

TL;DR

This paper demonstrates how excited-state quantum phase transitions in a parity-breaking system can be used to create energy Schrödinger cat states, revealing new phases and requiring a generalized microcanonical ensemble for description.

Contribution

It introduces a method to engineer energy-cat states via ESQPTs in a generalized Rabi model with parity-breaking, revealing new excited phases and level crossings.

Findings

Identification of three excited-state phases induced by parity-breaking.

Observation of level crossings in the thermodynamic limit.

Necessity of a generalized microcanonical ensemble for equilibrium states.

Abstract

We show that excited-state quantum phase transitions (ESQPTs) in a system in which the parity symmetry is broken can be used to engineer an energy-cat state -- a Schr\"odinger cat state involving a quantum superposition of both different positions and energies. By means of a generalization of the Rabi model, we show that adding a parity-breaking term annihilates the ground-state quantum phase transition between normal and superradiant phases, and induces the formation of three excited-state phases, all of them identified by means of an observable with two eigenvalues. In one of these phases, level crossings are observed in the thermodynamic limit. These allow us to separate a wavefunction in two parts: one, with lower energy, trapped within one region of the spectrum, and a second one, with higher energy, trapped within another. Finally, we show that a generalized microcanonical ensemble, including two different average energies, is required to properly describe equilibrium states in this situation. Our results illustrate yet another physical consequence of ESQPTs.