Dynamical Evolution

The dynamics of an open quantum system differs from a unitary evolution so that it can describe irreversible phenomena like dissipation and decoherence, with the latter a typical quantum effect. An important special class of open-system evolutions is given by quantum dynamical semigroups, whose mathematical structure is determined by the property of complete positivity and will be exposed in detail. To put into evidence the general features of a reduced dynamics we will further consider examples in which it can be exactly determined. The projection operator technique is finally introduced as general perturbative strategy for the derivation of master equations obeyed by an arbitrary reduced dynamics.