Protecting a spin ensemble against decoherence in the strong-coupling regime of cavity QED

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(1-4). At present, the main source of decoherence in these systems is inhomogeneous spin broadening, which limits their performance for the coherent transfer and storage of quantum information(5-7). Here we study the dynamics of a superconducting cavity strongly coupled to an ensemble of nitrogen-vacancy centres in diamond. We experimentally observe how decoherence induced by inhomogeneous broadening can be suppressed in the strong-coupling regime-a phenomenon known as 'cavity protection'(5,7). 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 the cavity and spin ensemble by two orders of magnitude.