Yuan Cao

Measurement-device-independent quantum key distribution (MDI-QKD), based on two-photon interference, is immune to all attacks against the detection system and allows a QKD network with untrusted relays. Since the MDI-QKD protocol was proposed, fiber-based implementations aimed at longer distance, higher key rates and network verification have been rapidly developed. However, owing to the effect of atmospheric turbulence, MDI-QKD over free-space channel remains experimentally challenging. Herein, by developing a robust adaptive optics system, high-precision time synchronization and frequency locking between independent photon sources located far apart, we realized the first free-space MDI-QKD over a 19.2-km urban atmospheric channel, which well exceeds the effective atmospheric thickness. Our experiment takes the first step towards satellite-based MDI-QKD. Moreover, the technology developed herein opens the way to quantum experiments in free space involving long-distance interference of independent single photons.

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.

Fiber-based quantum key distribution (QKD) is a cornerstone technology for ensuring secure communication. Leveraging the spontaneous parametric down-conversion (SPDC) of periodically poled lithium niobate waveguide, we have generated a high-brightness and broad-band polarization-entangled photon source. By integrating this source with wavelength division multiplexing technology, we demonstrated an entanglement-based QKD experiment spanning over 400 km of optical fiber, marking a substantial advancement in extending distribution distances. This experiment represents a significant stride towards realizing future entanglement-based quantum networks.

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.

Satellite-based quantum communication is a promising approach for realizing globalscale quantum networks. However, due to atmospheric turbulence, achieving a highly efficient and stable spatial single-mode receiver, which is very important for daylight free-space quantum key distribution and more complex quantum information tasks involving quantum interference, is difficult. Here, we develop a spatial single-mode receiver with an adaptive optics (AO) system based on a modal version of the stochastic parallel gradient descent algorithm (M-SPGD) and conduct a field test of its performance over an 8 km urban terrestrial free-space channel. Our experimental results demonstrate the AO technology based on M-SPGD has a great boosting for single-mode receiver and is useful for large-scale quantum communication.