DISSIPATIVE DYNAMICS OF MULTI-STATE QUANTUM SYSTEMS IN THE WEAK TO STRONG COUPLING REGIME

In this thesis the dissipative dynamics of bistable quantum systems is studied within the path integral approach. The path integral representation of the propagator for quantum states and density matrices of discrete variable systems is described along with the Feynman-Vernon influence functional. The main approximations to the FV influence for the spin-boson model are introduced and applied to a bistable system beyond the two-level system approximation, the so-called double-doublet system. By the combined use of the Bloch-Redfield perturbative approach and of the path integral techniques, a phase diagram showing the various dynamical and dissipative regimes of the double-doublet system is established. Finally, the dynamics of general multi-level driven systems in the strong coupling regime and the escape problem from a quantum metastable state, starting form a nonequilibrium condition, are investigated.