A photonic integrated long-distance quantum communication network

Quantum networks provide a revolution for cryptographic and communication applications. Worldwide quantum key distribution (QKD) networks have been demonstrated in the field. The measurement-device-independent protocols have closed critical side channels in physical implementations and offer a trusted-node-free network structure. In particular, the twin-field (TF)-QKD can greatly extend the transmission distances with currently available technologies. Here we propose and demonstrate a photonic integrated TF-QKD network over long distances. We develop a hybrid photonic transmitter chip that integrates a self-injection locking laser featuring a high-Q silicon nitride microring resonator and a thin-film lithium niobate circuit consisting of multiple high-speed intensity and phase modulators. Furthermore, we propose a scalable quantum spine-leaf network structure, which has high capacity and flexible configuration. To prove the feasibility, we experimentally demonstrate a four-user quantum spine-leaf network based on the TF-QKD protocol and show the full connections among all users. We report the network over up to 540-km optical fibres at secret key rates higher than the repeaterless channel capacity. Furthermore, we show that our network can reliably support up to 50 users for high-quality video calls at metropolitan distances. The results show that the photonic integrated quantum spine-leaf network, benefiting from miniaturization, CMOS compatibility and long distance, paves the way for the large-scale deployment of quantum communications. A four-user quantum spine-leaf network over up to 540-km fibre is demonstrated using a twin-field quantum key distribution protocol. It allows the full connections between any two user pairs and is operated at a 1.25 GHz clock rate with a secret key rate of 9.38 × 10−9 bit per pulse.