Coupling of an erbium spin ensemble to a superconducting resonator
  A quantum coherent interface between optical and microwave photons can be used as a basic building block within a future quantum information network. The interface is envisioned as an ensemble of rare-earth ions coupled to a superconducting resonator, allowing for coherent transfer between optical and microwave photons. Towards this end, we have realized a hybrid device coupling a Er$^{3+}$ doped Y$_2$SiO$_5$ crystal in a superconducting coplanar waveguide cavity. We observe a collective spin coupling of 4 MHz and a spin linewdith of down to 75 MHz. 
Matthias U. Staudt, Io-Chun Hoi, Philip Krantz, Martin Sandberg,   Michael Simoen, Pavel Bushev, Nicolas Sangouard, Mikael A...
quant-ph cond-mat.mes-hall