Finite-Temperature Decay Process In Quantum Open System

A realistic quantum system can rarely be isolated from its surroundingenvironment environment or called the bath completely.When it is coupled to the bathwith a large number of degrees of freedom, decoherence happens. There are usuallytwo distinct decoherence effects of the bath on the quantum system: when the energyexchange is allowed by the interaction between the open system and the bath, thesystem energy usually dissipates into the environment irreversibly and we name thiseffect quantum dissipation;another effect is called quantum dephasing, which occurswith no energy exchange, but an irreversible process of information loss happens inthe considered open system.Using Heisenberg operator method, we revisit an universal model for a quantumopen system with system-bath coupling. The analysis of the decay process for a Fockstate and a coherent statedemonstrate that Heisenberg operator method is very usefulin dealing with the problems in decay process of the open system. For finitetemperature, the calculations of the reduced density matrix and the mean excitationnumber for the open system show that an initial coherent state will evolve into antemperature-dependant coherent state after tracing over the bath variables. Also inshorttime limit, a temperature-dependant effective Hamiltonian for the open systemcharacterizes the decay process of the open system.