  This article reviews efforts to build a new type of quantum device, which
combines an ensemble of electronic spins with long coherence times, and a
small-scale superconducting quantum processor. The goal is to store over long
times arbitrary qubit states in orthogonal collective modes of the
spin-ensemble, and to retrieve them on-demand. We first present the protocol
devised for such a multi-mode quantum memory. We then describe a series of
experimental results using NV center spins in diamond, which demonstrate its
main building blocks: the transfer of arbitrary quantum states from a qubit
into the spin ensemble, and the multi-mode retrieval of classical microwave
pulses down to the single-photon level with a Hahn-echo like sequence. A reset
of the spin memory is implemented in-between two successive sequences using
optical repumping of the spins.
