Overcoming the No-Go Theorem Yields a Rich Dissipative Phase Diagram in the Open Quantum Rabi Model

Abstract: The open quantum Rabi model is studied in this work, with the explicit $\mathbf{A}^{2}$ term incorporated as required by the Thomas-Reich-Kuhn sum rule. It is shown that anisotropy provides a generic and robust mechanism for overcoming the no-go theorem in dissipative quantum systems, thereby 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.