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. In particular, 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., three to four orders of magnitude less). This proposed hybrid quantum circuit could enable a long-time quantum memory when storing information in the spin ensemble, and allows the possibility to explore nonlinear effects in the ultrastrong-coupling regime. 
Ze-Liang Xiang, Xin-You Lu, Tie-Fu Li, J. Q. You, and Franco Nori
quant-ph cond-mat.mes-hall cond-mat.supr-con