Twin-field quantum key distribution with optical injection locking and phase encoding on-chip

Twin-field (TF) quantum key distribution (QKD) has been proved to overcome the linear rate-distance limit of repeaterless quantum communications and has the capability to generate high key rates over long distances compared with other QKD protocols. Photonic integration has proven a promising technique for the large-scale deployment of QKD. While it has been adopted to implement a range of linearly scaling QKD protocols, there is to date no demonstration of TF QKD using photonic integrated chips. One important challenge resides in establishing high coherence between remote laser chips. In this study, we exploit optical injection locking to implement phase dissemination to independent lasers integrated onto photonic circuits and further demonstrate complete phase control on the same chip using electro-optic phase modulators. We achieve high laser coherence between remote transmitter nodes, which lets us demonstrate a quadratic key-rate improvement of the secure key rate beyond 80 dB of channel attenuation. The experimental results are in excellent agreement with the theory in both finite size and asymptotic regimes, in which the fundamental rate-distance limit is overcome. Our work paves the way for the deployment of large-scale TF QKD networks enabled by compact, low-cost, and high-yield chip-based devices.