  Hybrid quantum systems based on spin ensembles coupled to superconducting
microwave cavities are promising candidates for robust experiments in cavity
quantum electrodynamics (QED) and for future technologies employing quantum
mechanical effects. Currently the main source of decoherence in these systems
is inho- mogeneous spin broadening, which limits their performance for the
coherent transfer and storage of quantum information. Here we study the
dynamics of a superconducting cavity strongly coupled to an ensemble of
nitrogen-vacancy centers in diamond. We experimentally observe for the first
time, how decoherence induced by a non-Lorentzian spin distribution can be
suppressed in the strong-coupling regime - a phenomenon known as "cavity
protection". To demonstrate the potential of this effect for coherent control
schemes, we show how appropriately chosen microwave pulses can increase the
amplitude of coherent oscillations between cavity and spin ensemble by two
orders of magnitude.
