Shihai Sun

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.

Quantum key distribution (QKD) provides a way to share unconditional secure key between two remote parties, but the deviation between theory and practice will break the security of generated key. In this article, we evaluate the security of QKD with weak basis-choice flaw, in which the random bit used by Alice and Bob to determine their bases are weakly controlled by Eve. In fact, a tight and analytical bound is obtained to estimate the phase error for both single photon source and weak coherent source, then the key rate can be rapidly improved. And the key rate of QKD with biased bases is also considered, in which Alice and Bob distill key from two bases independently. Furthermore, by evaluating the security of QKD under wavelength attack, the performance of a commercial beam splitter is measured, and then the key rate is estimated, which is just slightly reduced with our method.