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相比于离散变量量子密钥分发, 连续变量量子密钥分发虽然具备更高的安全码率等优势, 但是在安全传输距离上却略有不足. 尽管量子催化的运用对高斯调制连续变量量子密钥分发协议的性能, 尤其在安全传输距离方面有着显著的提升, 然而能否用来改善离散调制协议的性能却仍然未知. 鉴于上述分析, 本文提出了一种基于量子催化的离散调制协议的方案, 试图在安全密钥率、安全传输距离和最大可容忍过噪声方面进一步提升协议性能. 研究结果表明, 在相同参数下, 当优化量子催化引入的透射率 T, 相比于原始四态调制协议, 所提方案能够有效地提升量子密钥分发的性能. 特别是, 对于可容忍过噪声为0.002, 量子催化可将安全通信距离突破300 km, 密钥率为 10 –8bits/pulse, 而过大的可容忍噪声会抑制量子催化对协议性能的改善效果. 此外, 为了彰显量子催化的优势, 本文给出了点对点量子通信的最终极限Pirandola-Laurenza-Ottaviani-Banchi边界, 仿真结果表明, 虽然原始方案与所提方案都未能突破这种边界, 但是相比于前者, 后者能够在远距离通信上逼近于这种边界, 这为实现全球量子安全通信的最终目标提供理论依据.Compared with discrete variable quantum key distribution (DVQKD), continuous variable (CV) QKD has high security bit rate and other advantages, which, however, are slightly insufficient in secure transmission distance. In addition, the application of quantum catalysis has significantly improved the performance of Gaussian modulated (GM) CVQKD, especially in secure transmission distance. Recently, the application of quantum catalysis has significantly improved the performance of GM-CVQKD. However, whether it can be used to improve the performance of discrete modulated (DM) CVQKD protocol is still ambiguous. Therefore, a scheme of DM CVQKD protocol based on quantum catalysis is proposed in this paper to further improve the performance of the proposed protocol in terms of secure key rate, secure transmission distance and maximum tolerable noise. Our results show that under the same parameters, when the transmittance Tintroduced by quantum catalysis is optimized, the proposed scheme can effectively further improve the performance of QKD system compared with the original four-state modulation CVQKD scheme. In particular, when the tolerable excess noise is 0.002, the use of quantum catalysis can break the safe communication distance of 300 km with a key rate of 10 –8bits/pulse. However, if this noise is too large, the improvement in the effect of quantum catalysis on protocol performance will be restrained. In addition, in order to highlight the advantages of the use of quantum catalysis, the ultimate limit PLOB (Pirandola-Laurenza-Ottaviani-Banchi) bound of point-to-point quantum communication is given in this paper. The simulation results indicate that although neither the original scheme nor the proposed scheme can break the bound, compared with the former, the latter can be close to the boundary in long-distance transmission. These results provide theoretical basis for achieving the ultimate goal of global quantum security communication.
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