Rare-earth ions (REIs) in solid hosts have important applications in the storage and conversion of quantum information at optical and microwave frequencies. Yttrium orthosilicate is a widely used host for REIs, with excellent optical properties, but its compatibility with superconducting resonators and circuits for quantum technology is not known. This study uses Y
2SiO
5 directly as a substrate for superconducting devices, revealing dielectric losses comparable to those of sapphire. This enables fabrication of high-quality microwave devices coupled to optically accessible rare-earth spins, pointing to applications in quantum storage and microwave-to-optical conversion.
Most investigations of rare-earth ions in solids for quantum information have used crystals where the rare-earth ion is a dopant. Here, we analyze 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. These concentrated crystals are attractive for frequency conversion because of their large ion densities and small linewidths. We show that conversion with both high efficiency and large bandwidth is possible in these crystals. 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 modes in the crystal.