Emergence of Rich Dissipative Phases in the Anisotropic Quantum Rabi Model Driven by the $\mathbf{A}^{2}$ Term

Abstract

The open quantum Rabi model is studied in this work, with the explicit $\mathbf{A}^{2}$ term incorporated. It is shown that anisotropy provides a generic and robust mechanism for establishing a genuine platform for observing dissipative phase transitions. The inclusion of the $\mathbf{A}^{2}$ term yields a significantly richer and asymmetric steady-state phase diagram, consisting of normal, superradiant, and bistable phases that intersect at tricritical points, while isolated bistable phases also emerge and the number of tricritical points is reduced. Notably, it is near the intersection of the two critical-line branches enclosing the superradiant phases, rather than at the tricritical points, that the $\mathbf{A}^{2}$ term fundamentally alters the scaling of photon-number fluctuations. Given the inherent role of the $\mathbf{A}^{2}$ term in light-matter interactions, our findings open a realistic route toward the experimental investigation and dynamical control of nonequilibrium critical phenomena in practical open quantum platforms.