Entanglement of Nanophotonic Quantum Memory Nodes in a Metropolitan Telecom Network

Long-range quantum networks have the potential to enable provably secure communications and distributed quantum computing, but a key obstacle thus far has been the difficulty in extending lab-scale entanglement to deployed city-scale fiber networks. Here, we present a two-node quantum network based on the silicon-vacancy center (SiV) in diamond in nanophotonic cavities, with each node containing an electronic spin communication qubit and a nuclear spin memory qubit. Cavity-enhanced interactions enable heralded entangling gates between the spin qubits and a time-bin photonic qubit, allowing us to generate remote entanglement between two nodes located in separate cryostats, while utilizing the long-lived nuclear spins to achieve second-long entanglement storage with integrated error detection. By integrating bi-directional quantum frequency conversion to the low-loss telecom band at 1350 nm, we demonstrate compatible operation of our quantum network with existing commercial fibers and entangle two nuclear spin qubits through a 35 km long deployed fiber loop in the Boston metropolitan area. This represents a significant step towards the practical deployment of quantum networks and enables applications in blind quantum computing and nonlocal sensing.