Rare-earth ions in solids are a promising platform for quantum technologies because they combine atom-like optical and spin transitions with the practical advantages of a solid-state host compatible with optical and microwave resonators. This thesis studies cavity-coupled 171Yb3+ ensembles in oxide crystals as a platform for collective cavity-QED and many-body physics, motivated by the goal of using rare-earth ions as hybrid quantum interconnects for quantum computation and networking. The central theme is that coupling many rare-earth ions to a shared resonator mode creates a rich setting for studying fundamental collective and many-body physics, while also providing a practical route to microwave-to-optical conversion, protected spin storage, and interfaces between superconducting circuits and optical photons. In 171Yb3+:YVO4, a nanophotonic cavity coupled to an inhomogeneously broadened ion ensemble reveals collective cavity QED in a solid. This work led to the discovery of collectively induced transparency, a cavity-QED phenomenon arising from collective interference in a driven, disordered ensemble. The same system exhibits optical superradiance and subradiance, and supports an interacting microwave spin system in which dipolar exchange competes with disorder, allowing studies of quantum thermalization. With Floquet control, these spin dynamics can be modified to reveal discrete time-crystal signatures. In 171Yb3+:CaWO4, related experiments demonstrate microwave superradiance, one-axis twisting, and many-body gap protection in a solid-state spin ensemble. These physics results are developed alongside quantum-technology applications. The 171Yb3+:YVO4 platform further enables low-noise microwave-to-optical transduction with percent-level on-chip efficiency and added noise near the single-photon level, establishing rare-earth ensembles as a promising approach to optical interconnects for superconducting quantum systems. In 171Yb3+:CaWO4, cavity-mediated gap protection extends Ramsey coherence and supports the development of a spin-based microwave quantum memory, including a classical-regime demonstration of storage and optical readout. The final part develops the superconducting qubit architecture needed to drive a rare-earth transducer with single microwave excitations, including qubit readout, tunable-coupler SWAP control, and cable-mode characterization. Together, these results establish cavity-coupled rare-earth ensembles as a versatile platform for studying fundamental cavity QED and many-body physics and for developing quantum technologies.
Rikuto Fukumori (Thu,) studied this question.