High-Cooperativity Coupling of a Rare-Earth Spin Ensemble to a Superconducting Resonator Using Yttrium Orthosilicate as a Substrate

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${}_{2}$SiO${}_{5}$ 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.