  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.
