Quantum Markovian master equation driven by coherent controls: a controllability analysis

The behavior of a coherently driven finite dimensional open quantum system (in the Markovian approximation) is studied from a control theoretical perspective. The irreversible character of such a system is formalized by showing that it can be accessible but neither small-time controllable nor controllable in finite time. When the generators of quantum dynamical semigroups are unital, then the reachable sets admit easy characterizations as they monotonically grow in time.