Novel techniques for strong coupling between spin ensembles and cavity resonators

Spins in solids are a promising physical subsystem for the realization of hybrid quantum systems. We focus on experiments coupling spins to three dimensional cavities, a system where it is critical to achieve operation in the strong coupling regime. This has been achieved using two approaches: coupling to impurity ions in single-crystal Whispering Gallery photonic resonators, and by using a novel field focusing re-entrant cavity. The first approach has allowed us to investigate various impurities in sapphire, quartz, and YAG, as well as iron group ions in YSO. This method is characterised by relatively narrow photon linewidths, higher filling factors and lower impurity concentration. The second approach allowed strong coupling to P1 impurities in diamond and operation in the ultra-strong coupling regime with magnons in YIG. This method is designed to achieve spatial separation of the cavity magnetic and electric fields, relatively high filling factors with sub-mm crystals of any shape and a high concentration of magnetic ions, as well as arbitrary engineering of the cavity spectrum and field distribution.