A hybrid quantum circuit consisting of a superconducting flux qubit coupled to both a spin ensemble and a transmission-line resonator

We propose an experimentally realizable hybrid quantum circuit for achieving a strong coupling between a spin ensemble and a transmission-line resonator via a superconducting flux qubit used as a data bus. The resulting coupling can be used to transfer quantum information between the spin ensemble and the resonator. More importantly, in contrast to the direct coupling without a data bus, our approach requires far less spins to achieve a strong coupling between the spin ensemble and the resonator (e.g., 3 to 4 orders of magnitude less). This drastic reduction of the number of spins in the ensemble can greatly improve the quantum coherence of the spin ensemble. This proposed hybrid quantum circuit could enable a long-time quantum memory when storing information in the spin ensemble.