FEEDBACK IN OPEN QUANTUM-SYSTEMS

Open quantum systems continually lose information to their surrounding s. In some cases this information can be readily retrieved from the en vironment and put to good use by engineering a feedback loop to contro l the system dynamics. Two cases are distinguished: one where the feed back mechanism involves a measurement of the environment, and the othe r where no measurement is made. It is shown that the latter case can a lways replicate the former, but not vice versa. This emphasizes the qu antum nature of the information being fed back. Two approaches are use d to describe the feedback: quantum trajectories (which apply only for feedback based on measurement) and quantum Langevin equations (which can be used in either case), and the results are shown to be equivalen t. The obvious applications for the theory are in quantum optics, wher e the information is lost by radiation damping and can be retrieved by photodetection. A few examples are discussed, one of which is particu larly interesting as it has no classical counterpart.