  We report the experimental realization of a hybrid quantum circuit combining
a superconducting qubit and an ensemble of electronic spins. The qubit, of the
transmon type, is coherently coupled to the spin ensemble consisting of
nitrogen-vacancy (NV) centers in a diamond crystal via a frequency-tunable
superconducting resonator acting as a quantum bus. Using this circuit, we
prepare arbitrary superpositions of the qubit states that we store into
collective excitations of the spin ensemble and retrieve back later on into the
qubit. These results constitute a first proof of concept of spin-ensemble based
quantum memory for superconducting qubits.
