In this talk I will present recent progress in examining collective spin phenomena’s such as cavity protection, spectral hole burning and amplitude bi-stability in a hybrid solid state system consisting of a superconducting microwave cavity strongly coupled to an ensemble of electron spins in nitrogen-vacancy centers in diamond. I will show how the total de-coherence rate scales with the collective enhanced coupling strength in a broadened spin ensemble coupled to a single mode cavity, known as “cavity protection” effect. Furthermore, I will explain how the coherence times can be improved beyond the natural limit given by the cavity protection effect, by spectral-hole burning techniques. Long lived coherence is shown by examining Rabi oscillations with high visibility. The observed total decay rates are significantly reduced, a factor of forty compared to the spin ensemble line width and over a factor of three compared to the pure cavity dissipation rate. This verifies that a "new" class of states, dark states, can be employed for the coherent exchange of excitation's between the cavity and spin ensemble. This is the first step towards a solid-state microwave frequency comb realised by engineering multiple long-lived dark states. I will also show how this system is a versatile tool in studying strong non-linear effects such as collective amplitude bi-stability in a macroscopic spin ensemble coupled to a single mode cavity. This engineered hybrid system approach opens up the possibility for truly long lived quantum memories and solid-state microwave frequency combs but also a new route to cavity QED experiments with dense interacting ensembles where exotic quantum transport phenomena can be studied. Putz, S. et al. Protecting a spin ensemble against de-coherence in the strong-coupling regime of cavity QED. Nature Physics 10, 720–724 (2014). Amsüss, R. et al. Cavity QED with Magnetically Coupled Collective Spin States. 
Phys. Rev. Lett. 107, 060502 (2011).