Coherent coupling of a superconducting flux-qubit to an electron spin   ensemble in diamond
  Electron-spin nitrogen-vacancy color centers in diamond are a natural candidate to act as a quantum memory for superconducting qubits because of their large collective coupling and long coherence times. We report here the first demonstration of strong coupling and coherent exchange of a single quantum of energy between a flux-qubit and an ensemble of nitrogen-vacancy color centers. 
Xiaobo Zhu, Shiro Saito, Alexander Kemp, Kosuke Kakuyanagi, Shin-ichi   Karimoto, Hayato Nakano, William J. Munro, Yasuhiro ...
quant-ph cond-mat.mes-hall cond-mat.supr-con