Strong coupling of an Er3+ doped YAlO3 crystal to a superconducting   resonator
  Quantum memories are integral parts of both quantum computers and quantum communication networks. Naturally, such a memory is embedded into a hybrid quantum architecture, which has to meet the requirements of fast gates, long coherence times and long distance communication. Erbium doped crystals are well suited as a microwave quantum memory for superconducting circuits with additional access to the optical telecom C-band around 1.55 {\mu}m. Here, we report on circuit QED experiments with an Er3+:YAlO3 crystal and demonstrate strong coupling to a superconducting lumped element resonator. The low magnetic anisotropy of the host crystal allows for attaining the strong coupling regime at relatively low magnetic fields, which are compatible with superconducting circuits. In addition, Ce3+ impurities were detected in the crystal, which showed strong coupling as well. 
A. Tkalcec, S. Probst, D. Rieger, H. Rotzinger, S. W\"unsch, N.   Kukharchyk, A. D. Wieck, M. Siegel, A. V. Ustinov, and P. ...
cond-mat.mes-hall cond-mat.mtrl-sci cond-mat.supr-con quant-ph