  We report strong coupling between an ensemble of nitrogen-vacancy center
electron spins in diamond and a superconducting microwave coplanar waveguide
resonator. The characteristic scaling of the collective coupling strength with
the square root of the number of emitters is observed directly. Additionally,
we measure hyperfine coupling to 13C nuclear spins, which is a first step
towards a nuclear ensemble quantum memory. Using the dispersive shift of the
cavity resonance frequency, we measure the relaxation time T1 of the NV center
at millikelvin temperatures in a non-destructive way.
