# Quantum phase transitions in the driven dissipative Jaynes-Cummings oscillator: from the dispersive regime to resonance

## TL;DR

This paper investigates quantum phase transitions in the driven dissipative Jaynes-Cummings oscillator, analyzing bistability, entanglement, and the breakdown of photon blockade across different regimes using quasidistribution functions.

## Contribution

It provides a detailed analysis of the transition from dispersive to resonance regimes, highlighting the role of quantum fluctuations and entanglement in phase transitions.

## Key findings

- Photon blockade breakdown in the dispersive regime
- Coexistence of cavity states with strong qubit-cavity entanglement
- Spontaneous dressed-state polarization at resonance

## Abstract

We follow the passage from complex amplitude bistability to phase bistability in the driven dissipative Jaynes-Cummings oscillator. Quasidistribution functions in the steady state are employed, for varying qubit-cavity detuning and drive parameters, in order to track a first-order dissipative quantum phase transition up to the critical point marking a second-order transition and spontaneous symmetry breaking. We demonstrate the photon blockade breakdown in the dispersive regime, and find that the coexistence of cavity states in the regime of quantum bistability is accompanied by pronounced qubit-cavity entanglement. Focusing on the role of quantum fluctuations in the response of both coupled quantum degrees of freedom (cavity and qubit), we move from a region of minimal entanglement in the dispersive regime, where we derive analytical perturbative results, to the threshold behaviour of spontaneous dressed-state polarization at resonance.