Bridging the gap between quantum-optical and solid-state implementations of quantum information is currently one of the major challenges in the field. Microscopic quantum systems have long coherence times, whereas artificial superconducting atoms can be manipulated and entangled very rapidly and with high fidelity; it is therefore appealing to combine them to form “hybrid” quantum circuits that would take the best of both worlds.
Our hybrid circuit project aims at coupling an ensemble of nitrogen-vacancy (NV) centers in diamond to superconducting circuits. A NV center consists of a substitutional nitrogen atom and an adjacent vacancy having trapped an electron; their electronic ground state has a spin S=1 with the states and separated by 2.87GHz in zero magnetic field.
In a first experiment , we have demonstrated the magnetic coupling of an ensemble of NV centers to a frequency-tunable superconducting resonator. The achievement of strong coupling is manifested by the appearance of a vacuum Rabi splitting in the resonator transmission spectrum when its frequency is tuned through the NV center electron spin resonance.
We have then performed an experiment demonstrating the storage and retrieval of a single microwave photon in the ensemble of NV centers. For that we have coupled a transmon qubit, a high quality factor superconducting resonator, and the NV spin ensemble . We are now implementing refocusing schemes  in order to extend the storage time of the quantum state up to milliseconds .
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