Coupling Impurity Spins in Diamond to a 3D Loop-gap Resonator*

We designed a loop-gap microwave resonator for applications of spin-based hybrid quantum systems, and tested it with impurity spins in diamond. Strong coupling with an ensemble of nitrogen-vacancy (NV) centers as well as that of nitrogen (P1) centers were observed. The external coupling rate can be tuned by changing the depth of the microwave SMA antenna pins. We also demonstrated time-domain measurements using standard pulse sequences of electron spin resonance. Large echo signals appear thanks to the large number of spins that are homogeneously coupled to the microwave magnetic field of the resonator. Coherence time T₂ and spin relaxation time T₁ were measured at 10 mK.