N two-level atoms collectively coupled to a cavity — superradiant phase transition
The Dicke model describes N two-level atoms collectively coupled (strength g) to a single cavity mode. Above the critical coupling g_c = sqrt(omega*omega_0)/2, the system undergoes a superradiant phase transition: the cavity field develops macroscopic amplitude and atoms become collectively polarized. Dissipation (kappa) and drive push the system into a non-equilibrium steady state.