Scalable On-Chip Platforms for Quantum Microwave-Optical Interface with Solid-State Ensembles

Superconducting quantum circuits based on Josephson junctions are one of the most promising platforms for future quantum information processing. Tens of superconducting quantum bits have been integrated on a single chip with performances exceeding the most advanced classical computers. However, these new quantum machines operate at microwave frequencies, which have enormous thermal noise and photon loss at room temperature. This fundamentally limits the future application of this technology in distributed quantum computing and quantum networks. Conversely, optical photons are an ideal information carrier as the photon loss is extremely small in fibers and the thermal noise is negligible at room temperature. Therefore, a quantum transducer that converts between microwave and optical frequencies at the single-photon level is of great importance. This thesis is centered on building such chip-scale interfaces with rare-earth ion (REI) doped crystals. First, we focus on developing a theoretical understanding of microwave-to-optical transducers. Based on coupled mode theories, we derive a clean theoretical result of the on-resonance transduction model. This allows us to condense the relevant material properties for transduction into a single parameter, effective χ⁽²⁾, describing the strength of the non-linearities provided by the rare-earth ion materials. Next, we designed, fabricated, and measured the chip under cryogenic temperatures, where percent-level efficiency and single-photon level of added noise referred to the input is achieved. To further demonstrate the unique advantage of atom-based platforms, we perform two transducer interference experiments, showing the scalability and capacity towards transducer-assisted remote entanglement of superconducting quantum bits. Lastly, with large microwave cooperativities achieved, we observe novel quantum electrodynamics enabled by controllable initialization of the excited-state spin system. By initializing the spins into spin-down and spin-up states, we observe collectively induced transparency and periodic superradiant emissions, respectively. Simulations are developed to explain the experimental results. These results establish REI doped crystals as a highly competitive platform for microwave-optical quantum interfaces and pave the way toward remote transducer-assisted entanglement of superconducting quantum machines.