We report the experimental realization of a hybrid quantum circuit combining a superconducting qubit and an ensemble of electronic spins (Fig. a). 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 into the qubit later on (Fig. b) [3]. These results constitute a first proof of concept of spin-ensemble based quantum memory for superconducting qubits. We also report a new method for detecting the magnetic resonance of electronic spins at low temperature with a qubit using the hybrid quantum circuit [4]. [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). [4] Y. Kubo et al., Phys. Rev. B, 86, 064514 (2012).
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