Quantum and Semiclassical Non-Hermitian Dicke Models without Nonreciprocity

The Dicke model, which describes the collective interaction between an ensemble of atoms and a single-mode photon field, serves as a fundamental framework for studying light-matter interactions and quantum electrodynamic phenomena. In this work, we investigate the manifestation of non-Hermitian effects in a generalized Dicke model, where two dissipative atom ensembles interact with a single-mode photon field. We explore the system in the semiclassical limit as a non-Hermitian Dicke model, revealing rich exceptional points (EPs) and diabolic points. Furthermore, we explore the quantum signature of EPs in the Hilbert space, relying on discrete photon numbers. The transition of photons from antibunching to bunching at steady state is unravelled. Our findings deepen the understanding of non-Hermitian physics in light-matter interaction, which is instructive for the design of advanced photonic devices.