Cavity-QED Tools for Quantum Interconnects

Much like robust data interconnects in classical networks, photonic quantum interconnects will be essential for high-fidelity transmission of quantum information between spatially-separated nodes in a quantum network. For matter-based qubits, this requires efficient light-matter interactions to extract quantum information from qubits in the form of light and convert it to telecom wavelengths for low-loss transmission through fibers. Cavities, by modifying the electromagnetic environment of emitters and enhancing interaction strengths, provide a powerful tool for enabling such interfaces. Leveraging techniques from atomic physics, nonlinear optics and geometric optics, this thesis presents cavity-based schemes for frequency and spatial mode conversion of light, as well as new cavity architectures with intracavity optics. The first part of the thesis demonstrates transduction of millimeter-wave photons to near-infrared optical photons using an atomic ensemble coupled to a simultaneous optical-millimeter-wave cavity, achieving an internal conversion efficiency of 58(11)%, a conversion bandwidth of 360(20) kHz and added thermal noise of 0.6 photons. Extending the idea of dual-cavity enhancement for frequency conversion, the thesis introduces an optical cavity with an intracavity nonlinear crystal that enables strong coupling between cavity modes for optical-to-optical frequency and spatial mode conversion. Additionally, by integrating macroscopic intracavity optics into moderate- to high-finesse cavities, this work demonstrates a large-scale cavity array with approximately 30 x 30 cavities, high-bandwidth modulation of a high-finesse cavity using an intracavity nonlinear crystal, cavities with lenses supporting finesses over 10,000, and cavities with patterned end-mirrors. These techniques provide pathways for efficient extraction and transfer of quantum information in the form of light, supporting applications in quantum networking, scalable modular architectures for fault-tolerant quantum computing, and hybrid quantum systems. Beyond quantum information science, they also address technical challenges such as aberration correction in degenerate cavities and high-bandwidth cavity stabilization, while offering unique platforms for sensing and simulation.