Towards a spin-ensemble quantum memory for superconducting qubits

A multi-mode quantum memory able to store coherently large numbers of qubit states is a desirable resource for quantum information. We report progress towards this direction, using an ensemble of electronic spins (NV centers in diamond) coupled to a superconducting transmon qubit via a tunable resonator. We demonstrate the reversible coherent storage and retrieval of a single microwave photon from the qubit into the spin ensemble [1]. In this experiment the storage time was however limited by inhomogeneous broadening of the ensemble of spins. We propose a realistic protocol that should extend the ensemble storage time by several orders of magnitude, based on spin-echo like pulse sequences; first experimental results will be presented. \\[4pt] [1] Y. Kubo et al., PRL 107, 220501 (2011).