  Most investigations of rare earth ions in solids for quantum information have
used rare earth ion doped crystals. Here we analyse the conversion of quantum
information from microwave photons to optical frequencies using crystals where
the rare earth ions, rather than being dopants, are part of the host crystal.
The potential of large ion densities and small linewidths makes such systems
very attractive in this application. We show that, as well as high efficiency,
large bandwidth conversion is possible. In fact, the collective coupling
between the rare earth ions and the optical and microwave cavities is large
enough that the limitation on the bandwidth of the devices will instead be the
spacing between magnon mode modes in the crystal.
