Abstract
We report a Rydberg-atom experiment, demonstrating the dynamic suppression of spontaneous emission in a resonant microwave cavity driven by a resonant external field. If the Rabi frequency of the injected field is larger than the atomic and cavity line widths, the atoms almost completely decouple from the cavity mode due to suppressed field fluctuations at the sideband frequencies of the Mollow triplet. This was achieved with about 100 microwave photons in the cavity. The reduced decay rate is a consequence of non-Markovian dynamics induced by the finite cavity-response time.