Research Of Quantum Key Distribution In Classical-Quantum Coexistence Systems

With the rapid development of the Internet,the importance of information security cannot be ignored.The growing demand for information security is the catalyst for cryptography research.Classical cryptography ensures the secure transmission of information based on computational complexity.However,the emergence of quantum algorithms and research on quantum computers have threatened the security of classical cryptography.Therefore,quantum key distribution(QKD),which is the most important field in quantum cryptography,was born.QKD,whose security is based on the basic principles of quantum physics,provides an unconditionally secure way for both communication parties to share keys by combining the one-time pad encryption method.Since the first QKD protocol(BB84)was proposed,its practical application process has made significant progress in recent decades,and QKD has become one of the most important and effective means of secure communication in the quantum computing era.One of the goals of practical QKD is to achieve large-scale network deployment on a global scale.However,due to the characteristics of weak quantum signals and easy channel interference,using an independent fiber channel for QKD is more conducive to secure transmission.But it means high resource consumption,high deployment cost and complexity,and low feasibility in classical communication(CC),which have hindered the widespread deployment of QKD.Therefore,with the help of the infrastructure of the existing CC,the classical-quantum coexistence system is one of the important ways to achieve the goal of large-scale deployment of QKD networks.According to the current research,the biggest obstacle to the realization of classicalquantum coexistence is that classical signals will seriously reduce the signal-to-noise ratio of QKD and cannot meet the needs of normal communication.It is mainly reflected in two aspects:on the one hand,the launch power of high-speed CC serving multiple users is very high,and a large amount of nonlinear noise generated in the fiber worsens QKD performance,making it difficult to realize the coexistence of QKD and complex CC with high power and large capacities,such as backbone network and metropolitan area network;on the other hand,to achieve lower transmission loss and meet the needs of higher compatibility between QKD and CC,it is more beneficial to realize the coexistence in the C-band.However,the stronger nonlinear noise caused by the smaller channel spacing further limits QKD performance,and the use of complex noise suppression schemes will reduce the deployment efficiency of the system and increase the cost and complexity of the system.Therefore,simultaneously satisfying QKD performance,increasing classical launch power,improving the compatibility between QKD and CC,and reducing the complexity of the system are the challenges to be solved urgently in coexistence systems.In this paper,we systematically study how to improve the tolerance of QKD to high-power classical signals in the classical-quantum coexistence system by focusing on the core goal of reducing the nonlinear noise of quantum channels.The main research results are summarized as follows:1.By combining the nonlinear noise theory of the classical-quantum coexistence system of wavelength division multiplexing(WDM),the finite-key analysis method for the practical safety of the QKD system,and the power coupling mechanism of multiple core channels,a set of theoretical models applicable to the quantum channel noise of the actual system is constructed,and the accurate characterization of nonlinear noise in coexistence systems is realized.2.Based on the above theoretical tools,the coexistence system of the QKD-Optical Transport Network based on the Faraday-Michelson interferometer is built.Through the band separation,high isolation broadband coupling,and frequency filtering,the classical launch power in the coexistence system based on standard single-mode communication fiber is increased by 1.5 times compared to existing work.The system not only has a high tolerance to high-power CC but also has the advantages of stability and insensitivity to channel polarization disturbance.3.To further improve classical launch power,the classical-quantum coexistence system based on ultra-low-loss fiber is built based on coexistence realized by WDM.Utilizing the lower nonlinear effect characteristics of the ultra-low-loss fiber,the highest classical launch power in the coexistence system has been achieved so far,which is about 2.5 times that of other coexistence work based on WDM.4.To further improve the compatibility and realize the coexistence in the C-band,combined with the nonlinear noise model of multicore crosstalk,the coexistence system based on space division multiplexing(SDM)is achieved by the spatial isolation provided by multicore fiber.The classical launch power is increased by nearly 19 times compared with other coexistence work based on SDM.The system does not need additional narrowband filtering,which reduces resource consumption and improves the deployment efficiency,signal coverage,and practicability of QKD.