Ye Chen

Quantum key distribution (QKD) enables information-theoretically secure communication, even in the era of quantum information. In all QKD systems, clock synchronization between two remote users---commonly referred to as Alice and Bob---is a fundamental requirement. This is typically achieved by transmitting an additional reference clock signal from Alice to Bob. In such a scheme, additional synchronization devices are required, increasing system complexity and introducing external noise. To address these issues, a novel synchronization technology, called the qubit-based synchronization method, was proposed. This method directly synchronizes two users using quantum signals, thereby dramatically reducing system complexity. However, previous qubit-based synchronization methods are not applicable to time-bin phase-encoding QKD systems, as multiple time slides introduce disturbances to time recovery. In this paper, we propose a machine-learning-enhanced qubit-based synchronization method. By introducing a K-nearest neighbor model, this method can efficiently classify each time slide in time-bin phase-encoding QKD, thereby enabling successful time recovery. We demonstrate our method using a time-bin phase-encoding reference-frame-independent (RFI)-QKD and successfully distribute secure key bits over up to 200 km of fiber spools. Our work simplifies the complexity of QKD system and significantly advances the practical application of QKD.

Integrated photonics provides a promising platform for quantum key distribution (QKD) system in terms of miniaturization, robustness and scalability. Tremendous QKD works based on integrated photonics have been reported. Nonetheless, most current chip-based QKD implementations require additional off-chip hardware to demodulate quantum states or perform auxiliary tasks such as time synchronization and polarization basis tracking. Here, we report a demonstration of resource-efficient chip-based BB84 QKD with a silicon-based encoder and decoder. In our scheme, the time synchronization and polarization compensation are implemented relying on the preparation and measurement of the quantum states generated by on-chip devices, thus no need additional hardware. The experimental tests show that our scheme is highly stable with a low intrinsic QBER of 0.50 ± 0.02% in a 6-h continuous run. Furthermore, over a commercial fiber channel up to 150 km, the system enables realizing secure key distribution at a rate of 866 bps. Our demonstration paves the way for low-cost, wafer-scale manufactured QKD system.

Quantum key distribution (QKD) is a method that enables two remote parties to share a secure key string. Clock synchronization between two parties is a crucial step in the normal operation of QKD. Qubit-based synchronization can achieve clock synchronization by transmitting quantum states between two remote parties, eliminating the necessity for hardware synchronization and thereby greatly reducing the hardware requirements of a QKD system. Nonetheless, classical qubitbased synchronization exhibits poor performance in continuous and high-loss systems, hindering its wide applicability in various scenarios. Here, we propose a qubit-based distributed frame synchronization method that can achieve time recovery in a continuously running system and resist higher losses. Experimental results show that the proposed method outperforms the advanced qubit-based synchronization method Qubit4Sync in a continuously running system. Particularly, the results demonstrate that our method surpasses all previous works in key parameters, including frequency and the synchronization length. We believe our method is applicable to a broad range of QKD scenarios, including drone-based QKD and quantum network construction.