Long-range quantum entanglement is essential for building large-scale quantum networks and unconditionally secure cryptographic systems based on quantum key distribution (QKD). While photonic integrated circuits offer a highly scalable platform, the fragility of phase coherence between spatial modes has prevented the distribution of path-encoded entanglement over long distances. Here, we report chip-to-chip distribution of path-encoded entangled states over 80 km between fully integrated silicon photonic transmitter and receiver chips. Telecom-band entangled photon pairs are generated via spontaneous four-wave mixing in on-chip spiral waveguides and distributed between chips over a dual-core, actively stabilized fiber link. Upon distribution, we measure a Bell state fidelity of
85.7±0.2%. Implementing the BBM92 protocol with the same source, we obtain a secure key rate of 2.03 bit/s in the infinite-key regime. These results establish silicon photonic chips as a viable platform for long-distance path-encoded entanglement-based quantum key distribution, paving the way toward scalable, device-independent quantum networks.
Quantum key distribution (QKD) is one of the most commercially-advanced quantum optical technologies operating in the single-photon regime. The commercial success of this disruptive technology relies on customer trust. Network device manufacturers have to meet stringent standards in order to ensure the operational security of their devices. The National Physical Laboratory (NPL) and the University of Bristol (Bristol) are working to produce a suite of tests to determine the operating characteristics and implementation security of chip-scale quantum devices designed for security purposes. These tests will inform and provide assurance to potential customers of such devices. Results from initial measurements performed on the Bristol chip-scale transmitter and receiver are presented, with the aim of informing the development of the system.