Open Quantum Systems and Quantum Collision Models
Most quantum systems are not isolated, but rather interact continuously with the surrounding environment. When this happens the dynamics of the system is not described by the Schroedinger equation, but by a Completely Positive Trace Preserving map, which induces decoherence and dissipation into the dynamics of the system. A powerful method to describe open quantum systems is offered by quantum collision models, where the environment is described by a collection of smaller quantum systems, called ancillae, which interact piecewise with the open quantum systems. In this way the open dynamics is discretized both with respect to time and with respect to the set of degrees of freedom of the environment.