Strong coupling between P1 diamond impurity centers and a three-dimensional lumped photonic microwave cavity

We report strong coupling between an ensemble of N impurity (P1) centers in diamond and microwave photons using a unique double-post reentrant 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 the high magnetic filling factor (38.4%) and low frequencies necessary to interact, at low magnetic field, with transitions in diamond such as those in negatively charged nitrogen-vacancy and P1 centers. Coupling strength (or normal-mode splitting) of 51.42 MHz was achieved with P1 centers at 6.18 GHz and 220 mT in a centimeter-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.