  We give an introduction to feedback control in quantum systems, as well as an
overview of the variety of applications which have been explored to date. This
introductory review is aimed primarily at control theorists unfamiliar with
quantum mechanics, but should also be useful to quantum physicists interested
in applications of feedback control. We explain how feedback in quantum systems
differs from that in traditional classical systems, and how in certain cases
the results from modern optimal control theory can be applied directly to
quantum systems. In addition to noise reduction and stabilization, an important
application of feedback in quantum systems is adaptive measurement, and we
discuss the various applications of adaptive measurements. We finish by
describing specific examples of the application of feedback control to cooling
and state-preparation in nano-electro-mechanical systems and single trapped
atoms.
