Hybrid quantum circuit with a superconducting qubit coupled to a spin ensemble

We report the experimental realization of a hybrid quantum circuit combining a superconducting qubit and an ensemble of electronic spins. The qubit, of the transmon type, is coherently coupled to the spin ensemble consisting of nitrogen-vacancy (NV) centers in a diamond crystal via a frequency-tunable superconducting resonator acting as a quantum bus [1,2]. Using this circuit, we prepare arbitrary superpositions of the qubit states that we store into collective excitations of the spin ensemble and retrieve back later on into the qubit [3]. These results constitute a first proof of concept of spin-ensemble based quantum memory for superconducting qubits.\\[4pt] [1] Y. Kubo \textit{et al.}, Phys. Rev. Lett. \textbf{105}, 140502 (2010).\\[0pt] [2] Y. Kubo \textit{et al.}, arXiv: 1109.3960.\\[0pt] [3] Y. Kubo \textit{et al.}, arXiv: 1110.2978.