Techniques for Deployed Quantum Networks with Solid-state Defect Centers

Book excerpt: The past decade has seen tremendous progress towards the development of quantum networks, wherein quantum states are transmitted over long distances for applications in distributed quantum computing, quantum-enhanced metrology, and quantum key distribution. In particular, recent results have demonstrated the fundamental building blocks of "quantum repeaters" --- network nodes containing quantum memories that can store, process, and retransmit photonic qubits. Such repeaters are key to deploying scalable quantum networks that can realize the full range of quantum networking applications. However, work in this area has typically been confined to small numbers of low-yield devices, operating in single laboratory environments. Moving from delicate, proof-of-principle physics experiments to robust, practical systems requires advancements on a number of fronts, ranging from fundamental materials science and qubit development to high-level quantum-compatible communications infrastructures. Here, we pursue a full-stack approach towards deployable quantum networks, specifically with solid-state defect centers as quantum memories. We investigate single qubit registers, studying creation techniques and multi-spin architectures that might enhance qubit performance. Next, we propose architectures at the device and repeater levels for improving the ability of a network to take advantage of high-performance qubits. Finally, we develop the classical infrastructure necessary for realizing quantum networks across real-world fiber links, concluding with a demonstration of photon-to-spin quantum state transfer across a 50 km deployed network in the Boston area. Together, these efforts represent a significant step in realizing scalable, memory-enabled quantum networks.