Min-Yan Wang

Twin-field quantum key distribution (TF-QKD) allows a secure key rate to break the repeaterless bound, which is known as the Pirandola-Laurenza-Ottaviani-Bianchi (PLOB) bound. Together with the security of measurement device independence, it is important in the future global quantum network. TF-QKD requires single photon interference between two independent optical fields transmitted through different channels. In free-space channels, atmospheric turbulence strongly disturbs the laser beam's wavefront, leading to a significant intensity fluctuation of received photons. This random fluctuation causes intensity distinguishability between two beams, thus reducing the visibility of interference and the secure key rate. Here, we proposed a scheme to increase the secure key rate under such unstable channels. The characteristics, especially the intensity fluctuation, of free-space channels are presented. Numerical analysis is performed to demonstrate the improvement of the secure key rate with our strategy. The result shows that, under a typical atmospheric condition of 14 km distance, the secure key rate of TF-QKD can be increased to 3.75 times. Our method can be a general tool widely used in the future long-distance horizontal or satellite-based free-space quantum key distribution.

One of the most significant vulnerabilities in the source unit of quantum key distribution~(QKD) is the correlation between quantum states after modulation, which shall be characterized and evaluated for its practical security performance. In this work, we propose a methodology to characterize the intensity correlation according to the single-photon detection results in the measurement unit without modifying the configuration of the QKD system. In contrast to the previous research that employs extra classical optical detector to measure the correlation, our method can directly analyse the detection data generated during the raw key exchange, enabling to characterize the feature of correlation in real-time system operation. The basic method is applied to a BB84 QKD system and the characterized correlation significantly decreases the secure key rate shown by the security proof. Furthermore, the method is extended and applied to characterize the correlation from the result of Bell-state measurement, which demonstrates its applicability to a running full-scheme MDI QKD system. This study provides an approach for standard certification of a QKD system.