  We report the storage of microwave pulses at the single-photon level in a
spin-ensemble memory consisting of $10^{10}$ NV centers in a diamond crystal
coupled to a superconducting LC resonator. The energy of the signal, retrieved
$100\, \mu \mathrm{s}$ later by spin-echo techniques, reaches $0.3\%$ of the
energy absorbed by the spins, and this 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.
