Anisotropic rare-earth spin ensemble strongly coupled to a   superconducting resonator
  Interfacing photonic and solid-state qubits within a hybrid quantum architecture offers a promising route towards large scale distributed quantum computing. Ideal candidates for coherent qubit interconversion are optically active spins magnetically coupled to a superconducting resonator. We report on a cavity QED experiment with magnetically anisotropic Er3+:Y2SiO5 crystals and demonstrate strong coupling of rare-earth spins to a lumped element resonator. In addition, the electron spin resonance and relaxation dynamics of the erbium spins are detected via direct microwave absorption, without aid of a cavity. 
S. Probst, H. Rotzinger, S. W\"unsch, P. Jung, M. Jerger, M. Siegel,   A. V. Ustinov, and P. A. Bushev
quant-ph cond-mat.supr-con