Supercritical Poincaré-Andronov-Hopf bifurcation in a mean field quantum laser equation
We deal with the dynamical system properties of a Gorini-Kossakowski-Sudarshan-Lindblad (GKSL) equation with mean-field Hamiltonian that models a simple laser by applying a mean field approximation to a quantum system describing a single-mode optical cavity and a set of two level atoms, each coupled to a reservoir. We prove that the mean field quantum master equation has a unique regular stationary solution. In case a relevant parameter $C_\mathfrak{b} $, i.e., the cavity cooperative parameter, is less than $1$, we prove that any regular solution converges exponentially fast to the equilibrium, and so the regular stationary state is a globally asymptotically stable equilibrium solution. We obtain that a locally exponential stable limit cycle is born at the regular stationary state as $C_\mathfrak{b} $ passes through the critical value $1$. Then, the mean-field laser equation has a Poincaré-Andronov-Hopf bifurcation at $C_\mathfrak{b} =1 $ of supercritical-like type. Namely, we derive rigorously, at the level of density matrices --for the first time--, the transition from a global attractor quantum state, where the light is not emitted, to a locally stable set of coherent quantum states producing coherent light. Moreover, we establish the local exponential stability of the limit cycle in case a relevant parameter is between the first and second laser thresholds appearing in the semiclassical laser theory. Thus, we get that the coherent laser light persists over time under this condition. In order to prove the exponential convergence of the quantum state, we develop a new technique for proving the exponential convergence in open quantum systems that is based in a new variation of constant formula. Applying our main results we find the long-time behavior of the von Neumann entropy, the photon-number statistics, and the quantum variance of the quadratures.