  A key challenge in realizing practical quantum networks for long-distance
quantum communication involves robust entanglement between quantum memory nodes
connected via fiber optical infrastructure. Here, we demonstrate a two-node
quantum network composed of multi-qubit registers based on silicon-vacancy
(SiV) centers in nanophotonic diamond cavities integrated with a
telecommunication (telecom) fiber network. Remote entanglement is generated via
the cavity-enhanced interactions between the SiV's electron spin qubits and
optical photons. Serial, heralded spin-photon entangling gate operations with
time-bin qubits are used for robust entanglement of separated nodes. Long-lived
nuclear spin qubits are used to provide second-long entanglement storage and
integrated error detection. By integrating efficient bi-directional quantum
frequency conversion of photonic communication qubits to telecom frequencies
(1350 nm), we demonstrate entanglement of two nuclear spin memories through 40
km spools of low-loss fiber and a 35 km long fiber loop deployed in the Boston
area urban environment, representing an enabling step towards practical quantum
repeaters and large-scale quantum networks.
