arXiv subjects
Publications and source records attributed to F. Fagnola.
We show that a bipartite Gaussian quantum system interacting with an external Gaussian environment may possess a unique Gaussian entangled stationary state and that any initial state converges towards this stationary state. We discuss dependence of entanglement on temperature and interaction strength and show that one can find entangled stationary states only for low temperatures and weak interactions.
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.
We show that, for a Quantum Markov Semigroup (QMS) with a faithful normal invariant state, the atomicity of the decoherence-free subalgebra and environmental decoherence are equivalent. Moreover, we characterize the set of reversible states and explicitly describe the relationship between the decoherence-free subalgebra and the fixed point subalgebra for QMSs with the above equivalent properties.
We find the structure of generators of norm continuous quantum Markov semigroups on B(h) that are symmetric with respect to the scalar product tr(ρ^{1/2}xρ^{1/2}y) induced by a faithful normal invariant state invariant state ρand satisfy two quantum generalisations of the classical detailed balance condition related with this non-commutative notion of symmetry: the so-called standard detailed balance condition and the standard detailed balance condition with an antiunitary time reversal.