The Kerr-Induced Superradiant Tricritical Point

The interplay of interaction and dissipation in open quantum systems can forge phase transitions beyond conventional paradigms. In the canonical quantum Rabi model, we demonstrate that a photon-photon (Kerr) interaction transforms the celebrated superradiant phase transition, inducing a line that separates continuous, second-order behavior from discontinuous, first-order regimes. Strikingly, this is not a line of tricritical points. Instead, we prove that genuine tricriticality is an isolated phenomenon, emerging only at a single, mathematically exact dissipation rate: the ``sweet spot'' of $κ_t^2 = (\sqrt{13}-2)/3$. Our discovery pinpoints the precise and restrictive conditions for realizing a rare multicritical point in a foundational quantum optical system, identifying this critical dissipation as a unique gateway to a higher order of criticality.