Phase driven unconventional superradiance phase transition in non-Hermitian cascaded quantum Rabi cavities

TL;DR

This paper explores phase-driven superradiance phase transitions in non-Hermitian cascaded quantum Rabi cavities, revealing unconventional transitions and critical behavior, with potential for experimental realization.

Contribution

It analytically derives superradiance phase boundaries in non-Hermitian systems and uncovers novel phase transitions absent in Hermitian counterparts.

Findings

Unconventional phase transition at specific phase couplings

Phase boundary independent of atom-photon coupling at certain phases

Identification of first- and second-order superradiance transitions

Abstract

This study investigates phase-driven symmetry breaking leading to superradiance phase transitions in cascaded non-Hermitian quantum Rabi cavities. Non-Hermiticity is introduced via the phase coupling $\varphi$ between the atom and the optical field. In the thermodynamic limit of the quantum harmonic oscillator, we analytically derive the superradiance phase boundary, validated by observables. An unconventional quantum phase transition without a Hermitian analogue arises when $|\varphi|=\frac{\pi}{4}$ or $|\varphi|=\frac{3\pi}{4}$, where the phase boundary is uniquely determined by the cavity coupling, at $\mathcal{J}=\frac{1}{2}$, independent of the atom-photon coupling strength $g$. For other $\varphi$, the phase boundary relies on both $\mathcal{J}$ and $g$, similar to the scenario observed in Hermitian systems. Furthermore, we identify phase-driven first- and second-order superradiance phase transitions, focusing on the quantum criticality of the second-order transition by determining the critical exponents and the universality class. The feasibility of experimental realization is also discussed, aiming to inspire further studies on non-Hermitian superradiance quantum phase transitions.