  We propose a quantum memory scheme to transfer and store the quantum state of
a superconducting flux qubit (FQ) into the electron spin of a single
nitrogen-vacancy (NV) center in diamond via yttrium iron garnet (YIG), a
ferromagnet. Unlike an ensemble of NV centers, the YIG moderator can enhance
the effective FQ-NV-center coupling strength without introducing additional
appreciable decoherence. We derive the effective interaction between the FQ and
the NV center by tracing out the degrees of freedom of the collective mode of
the YIG spins. We demonstrate the transfer, storage, and retrieval procedures,
taking into account the effects of spontaneous decay and pure dephasing. Using
realistic experimental parameters for the FQ, NV center and YIG, we find that a
combined transfer, storage, and retrieval fidelity higher than 0.9, with a long
storage time of 10 ms, can be achieved. This hybrid system not only acts as a
promising quantum memory, but also provides an example of enhanced coupling
between various systems through collective degrees of freedom.
