Tunable coupling of spin ensembles.

Spin ensembles are promising candidates for quantum memory units because they have long coherence time. Controlling the coupling between spin ensembles is necessary and important in quantum information processing. In this Letter, we propose a method to realize tunable coupling between spin ensembles by a superconducting flux qubit acting as a coupler. The resulting coupling can be used to high-fidelity speed up the adiabatic transfer of quantum information.