  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 ensembles of nitrogen-vacancy (NV) centers and substitutional
nitrogen (P1) centers was observed. These results show that loop-gap resonators
are viable in the prospect of spin-based hybrid quantum systems, especially for
an ensemble quantum memory or a quantum transducer.
