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 [1,2]. 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 [3] with a fidelity of about 10%. These results constitute a proof of concept of spin-ensemble based quantum memory for superconducting qubits.

[1] Y. Kubo et al., Phys. Rev. Lett. 105, 140502 (2010)
[2] Y. Kubo et al., Phys. Rev. A 85, 012333 (2012)
[3] Y. Kubo et al.,Phys. Rev. Lett. 107, 220501 (2011)