We give a basic account of quantum filtering and control from the point of view of quantum probability and information theory. We describe and prove the "no-cloning" principle, and Heisenberg's related principle which says that the leakage of information to the environment necesserily implies. a deterioration of the quantum state. A typical challenge to quantum technology issuing from this principle is to recover the leaked information and to feed it back into the system under control in order to preserve its state. We show that in the situation of discrete time and complete recovery such feedback control indeed enables us to stabilize any particular quantum state, pure or mixed. We illustrate this fact by an example where it is actually well-known: the fluorescent two-level atom (in a discrete time setting).