Strong Coupling Between P1 Diamond Impurity Centres and 3D Lumped   Photonic Microwave Cavity
  We report strong coupling between an ensemble of N impurity (P1) centres in diamond and microwave photons using a unique double post re-entrant cavity. The cavity is designed so that the magnetic component of the cavity field is spatially separated from the electric component and focused into the small volume in which the diamond sample is mounted. The novelty of the structure simultaneously allows high magnetic filling factor (38.4\%) and low frequencies necessary to interact, at low magnetic field, with transitions in diamond such as those in NV$^{-}$ and P1 centres. Coupling strength (or normal-mode splitting) of 51.42 MHz, was achieved with P1 centres at 6.18 GHz and 220 mT in a centimetre-scale cavity, with a corresponding cooperativity factor of 4.7. This technique offers an alternative way, with some significant advantages, to couple 3D cavities to transitions in diamond and achieve the strong coupling necessary for applications to quantum information processing. 
Daniel L. Creedon, Jean-Michel Le Floch, Maxim Goryachev, Warrick G.   Farr, Stefania Castelletto, Michael E. Tobar
cond-mat.mes-hall cond-mat.mtrl-sci quant-ph