Coherent quantum feedback - art. no. 022108

In the conventional picture of quantum feedback control, sensors perform me asurements on the system, a classical controller processes the results of t he measurements, and actuators supply semiclassical potentials to alter the behavior of the quantum system. In this picture, the sensors tend to destr oy coherence in the process of making measurements, and although the contro ller can use the actuators to act coherently on the quantum system, it is p rocessing and feeding back classical information. This paper proposes an al ternative method for quantum feedback control, in which the sensors, contro ller, and actuators are quantum systems that interact coherently with the s ystem to be controled. In this picture, the controller gets, processes, and feeds back quantum information. Controllers that operate using such quantu m feedback loops can perform tasks such as entanglement transfer that are n ot possible using classical feedback. Necessary and sufficient conditions a re presented for Hamiltonian quantum systems to be controllable and observa ble using both classical and quantum feedback.