Ultralow-power spectroscopy of a rare-earth spin ensemble using a superconducting resonator

Interfacing superconducting quantum processors, working in the GHz frequency range, with optical quantum networks and atomic qubits is a challenging task for the implementation of distributed quantum information processing as well as for quantum communication. Using spin ensembles of rare-earth ions provides an excellent opportunity to bridge microwave and optical domains at the quantum level. In this Rapid Communication, we demonstrate the ultralow-power, on-chip, electron-spin-resonance spectroscopy of Er³⁺ spins doped in a Y₂SiO₅ crystal using a high-Q, coplanar, superconducting resonator.