  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.
