Quantum feedback for preparation and protection of quantum states of light

Aiming on the preparation of a given physical system in a particular quantum state, we can follow one of several standard approaches. The most straightforward one is to expose the system to a specific coherent evolution that will deterministically bring it into the desired final state. However, in case of microscopic few-particle systems this approach requires very good control of physical parameters at the single particle level. Another method of the state production is based on projective quantum measurements of the system: Quantum Mechanics postulates that the system is randomly projected onto one of the eigenstates of the measurement operator M. By a proper choice of M, the system can end up in the desired state after the measurement. Being less demanding this method is however inherently non-deterministic. To steer the system towards the target state and thus to overcome the randomness of the measurement, we propose to use an active feedback on the system while the measurement is being performed.