Storage and retrieval of microwave fields at the single-photon level in a spin ensemble

We report the storage of microwave pulses at the single-photon level in a spin-ensemble memory consisting of 10¹⁰ nitrogen-vacancy centers in a diamond crystal coupled to a superconducting $LC$ resonator. The energy of the signal, retrieved 1000.28em0exμs later by spin-echo techniques, reaches $0.3%$ of the energy absorbed by the spins. This $0.3%$ storage efficiency is quantitatively accounted for by simulations. This figure of merit is sufficient to envision first implementations of a quantum memory for superconducting qubits.