Guo Hong's team at Peking University makes a breakthrough in classically compatible digital quantum key distribution!

On March 24, 2023, a joint Peking University-Beijing University of Posts and Telecommunications research team led by Prof. Hong Guo made a breakthrough in the research of continuous-variable quantum key distribution and completed the security proof of digital signal processing technology applied to continuous-variable quantum key distribution, and the result provides a strong support for the construction of digital quantum key distribution compatible with classical coherent optical communication signal processing algorithms.

The related results were published online in the international academic journal "npj quantum information" under Nature Publishing [1].

Quantum key distribution (QKD) can provide users with theoretically information-theoretically provably secure symmetric keys. Over the past three decades, QKD technology has been rapidly developing and maturing, and has gradually started to enter the stage of commercial network construction.

Among QKD technologies, the Continuous Variable Quantum Key Distribution (CVQKD) technology system has more evolutionary potential in terms of integration because its system construction can utilize existing commercial optical communication devices, and can provide a cost-effective and more realistic solution for metro QKD network construction.

Since the proposal of the local local oscillation CVQKD system scheme in 2015, researchers have gradually introduced advanced digital processing (DSP) techniques for classical coherent optical communications to enhance CVQKD system performance, leading to the third phase of CVQKD system development: the digital CVQKD phase. The introduction of advanced DSP techniques unleashed the potential of CVQKD systems to reach tens of GHz bandwidth, but also introduced the difficulty of proving security.

The digital CVQKD system introduces digital signal processing after coherent detection at the receiver side, and the difficulty of its security analysis is that the spectral spreading phenomenon introduced by ultra-high-speed modulation pushes the actual quantum state from the single-mode assumption to the continuous-mode optical field case, so that the single-mode optical field assumption of the traditional CVQKD security analysis is no longer satisfied, and the DSP algorithm further corrects the time domain and frequency domain of the detection results. characteristics of the detection results, leading to the complexity of the definition of the regular component detection.

Digital Receiver Model

In this work, Prof. Hong Guo's team proposes a general approach based on continuous-mode quantum optics theory to analyze the system time domain and frequency domain characteristics as well as the security of linear DSP algorithms. By constructing a reasonable normalized scattered particle noise unit to define the corresponding continuous-mode optical field, the security analysis of the continuous-mode system can be equivalently approximated to the case of a single-mode optical field, which can return to the applicability of the current security analysis theory and complete the security proof.

In addition, in terms of system performance, this work also shows the direction for designing and optimizing DSP algorithms in terms of how to improve system efficiency. This work lays an important foundation for further design and analysis of digital CVQKD systems compatible with classical optical communication.

The research work was done under the leadership of Prof. Hong Guo at Peking University, and Dr. Ziyang Chen, an assistant researcher at Peking University, was the first author of the paper. Co-investigators also include Prof. Song Yu and Associate Prof. Xiangyu Wang from Beijing University of Posts and Telecommunications and Dr. Zhengyu Li from Huawei Technologies Co. This research was supported by the National Natural Science Foundation of China, the China Postdoctoral Science Foundation, and the State Key Laboratory of Information Photonics and Optical Communication Foundation.

Reference Links:

https://www.nature.com/articles/s41534-023-00695-8