Towards efficient microwave-to-optical transduction with erbium-doped LiYF₄ crystals

Microwave-to-optical transduction is crucial for interfacing superconducting qubits through long-distance optical quantum networks, allowing for distributed quantum computing. Ensembles of rare-earth ions offer a promising platform for transduction due to their narrow optical and spin inhomogeneous linewidths, thus a strong magneto-optical nonlinearity. Here, we report a microwave-to-optical transducer based on a ¹⁶⁶Er³⁺:LiYF₄ crystal, chosen for its extremely narrow optical inhomogeneous linewidth and strong coupling to both a loop-gap microwave resonator and a tunable Fabry-Perot optical cavity for enhanced magneto-optical interactions. We perform spectroscopic studies on the erbium spin and optical properties and characterize the overall transduction efficiency and bandwidth in both continuous-wave and pulsed modes of operation at temperatures below 50mK. We also discuss future plans and applications of our system, including interfacing with a compatible transmon qubit and generating hybrid microwave-optical entanglement.