Yu-Huai Li

Turbulence is a complex and chaotic fluid motion state. Atmospheric turbulence presents significant challenges for applications such as remote sensing,astronomical observations, and free-space quantum key distribution (QKD), due to its rapid evolution across temporal and spatial scales. Traditional methods for correcting atmospheric turbulence encounter difficulties, particularly under strong daytime turbulence conditions. In this study, we develop a deep learning-based adaptive method to correct strong atmospheric turbulence in field conditions, facilitating the turbulence correction over 1.4 km and 7 km free-space channels. Experimental results present better correction performance compared to wavefront sensor-based methods, yielding a 2–4 dB Strehl ratio improvement. Additionally, our approach directly estimates phase information from a defocused camera, significantly reducing the implementation cost of adaptive systems. Furthermore, we evaluate the performance of a daytime free-space QKD system incorporating our deep learning–based method, leading to higher key rates and longer propagation distances. Our method provides a practical and efficient solution for daytime QKD applications.

Twin-field quantum key distribution (TF-QKD) improves the secure key rate from the linear scale of channel loss to the square root scale while preserving the security of measurement-device-independent. This scheme is well suited to building the global-scale quantum network that suffers from extremely high channel loss. Since it was proposed, fiber-based demonstrations have been rapidly developed. However, TF-QKD over a free-space channel remains experimentally challenging due to the effect of atmospheric turbulence. Here, we realized the first free-space TF-QKD protocol over two 7.1-km urban atmospheric channels, which exceeds the effective atmospheric thickness. A secure key rate exceeding the repeaterless secret key capacity was demonstrated. By controlling the time and phase of optical pulses through the open channel, our setup avoids the requirement of an additional channel to construct a closed interferometer. Our experiment takes a significant step toward the satellite-based global quantum network with a high level of practical security.

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.

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.