  We show how an initially prepared quantum state of a radiation mode in a
cavity can be preserved for a long time using a feedback scheme based on the
injection of appropriately prepared atoms. We present a feedback scheme both
for optical cavities, which can be continuously monitored by a photodetector,
and for microwave cavities, which can be monitored only indirectly via the
detection of atoms that have interacted with the cavity field. We also discuss
the possibility of applying these methods for decoherence control in quantum
information processing.
