Photon-by-photon feedback control of a single-atom trajectory

A longstanding goal in physics is to steer microscopic objects like single atoms into well defined states. One of the most powerful techniques for controlling classical systems is real-time feedback, in which continuously updated information about a system is used to control its evolution. An extension of feedback into the quantum domain is desirable as it could allow the production of nontrivial quantum states and protection against decoherence. Kubanek et al. now demonstrate real-time feedback control of the motion of a single rubidium atom trapped in an optical cavity. A dipole laser is used to steer the atom on timescales seventy times faster than the atom's oscillation period in the trap, resulting in extended trapping times.