A quantum memory at microwave frequencies, able to store the state of
multiple superconducting qubits for long times, is a key element for quantum
information processing. Electronic and nuclear spins are natural candidates for
the storage medium as their coherence time can be well above one second.
Benefiting from these long coherence times requires to apply the refocusing
techniques used in magnetic resonance, a major challenge in the context of
hybrid quantum circuits. Here we report the first implementation of such a
scheme, using ensembles of nitrogen-vacancy (NV) centres in diamond coupled to
a superconducting resonator, in a setup compatible with superconducting qubit
technology. We implement the active reset of the NV spins into their ground
state by optical pumping and their refocusing by Hahn echo sequences. This
enables the storage of multiple microwave pulses at the picoWatt level and
their retrieval after up to 35μs, a three orders of magnitude improvement
compared to previous experiments.Comment: 8 pages, 5 figures + Supplementary information (text and 6 figures
