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1550nm单模光纤中的量子密钥分配
时间:2015-02-15 浏览次数:808次 无忧论文网
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1550nm单模光纤中的量子密钥分配# 桂有珍*,莫小范,韩正甫,郭光灿 (中国科学院,中国科学技术大学量子信息重点实验室,合肥,230026)摘要:本文从实验上实现了红外通讯波段的远程量子密钥分配。系统采用相位编码方式,其中编码和解码均为不等臂M-Z干涉仪。工作波长选定在光纤通讯窗口的1550nm。以强衰减半导体激光模拟理想单光子光源,衰减值要求达到平均约0.1光子/光脉冲。每个M-Z干涉仪的长-短臂长度差为3m,使得前后两个脉冲的时差ΔT=15ns。干线单模光纤的衰减约0.2dB/km。为了有效抑制暗计数和去除两边峰的干扰,红外单光子探测器工作在门模式下,其中的雪崩二极管工作在-50℃。结合上述实验系统,本文分析了量子密钥分配中传输距离与误码率之间相互关系。给出了理论上理想不等臂M-Z系统与实际系统的最低误码率与传输距离间的关系,以及系统实测传输距离与误码率的关系。实验结果显示,本密钥分配系统已非常接近理想实验系统的性能。理论分析还表明:在一定误码率限制的安全条件下,制约传输距离的关键因素有四个,它们分别是:探测器的暗计数和量子效率、系统的条纹干涉度、接收系统的损耗以及干线光缆的损耗。实验所用的探测器实测暗计数为2′10-4/脉冲,量子效率10%;接收系统附加损耗1dB;条纹干涉度达98%以上。因为干线光缆损耗通常无法改进,就我们现有的系统而言,接收系统附加损耗和条纹干涉度的改进余地已经很小,增加密钥分配距离的关键是更高量子效率和更低暗计数的单光子探测器。原则上在一定的量子效率下,暗计数每降低一个量级密钥极限分配距离可以增加50公里,使用暗计数低一个量级的单光子探测器,超过100公里的密码分配距离是可期望的。关键词:量子密码,量子密钥分配,误码率 Quantum Key Distribution in Optical Fiber at Wavelength of 1550nm Gui Youzhen, Mo Xiaofan, Han Zhengfu, Guo Guangcan (Key Laboratory of Quantum Information, University of Science and Technology of China, Chinese Academy of Sciences, Hefei,230026) Abstract: The experiment of long-distance quantum key distribution was preformed. Here two unequal M-Z interferometers were used for phase-coding scheme. The wavelength of laser is at 1550nm. For approaching ideal single-photon source, a pulse diode laser was strongly attenuated to 0.1photon/pulse. The length difference of long-short arms of each M-Z interferometers is 3m, which results in the light travel time delay ΔT=15ns. The attenuation of the fiber in the trunk is 0.2dB/km. In order to depress the dark count of detectors and eliminate the disturbance of those two sides pulses, single-photon detector worked in gated-mode, and the APD is cooled to -50℃. In this letter, the relationship between transmission length (L) and bit error rate (BER) was analyzed both in theory and experiment. It were shown that the relationship of the minimum BER and transmission length in theory with ideal M-Z interferometers and that with a real M-Z interferometers, and also that of this experiment system. This result demonstrates that our system is very close to the performance of ideal experimental system. By analysis, it was point out that there are four factors confine the transmission length in the restricted BER. They are the dark count and efficiency of detector, fringe visibility, attenuation of the receiving system and the attenuation of trunk fiber. In this system, the detector has 2*10-4/gate dark counts and 10% quantum efficiency, the inherent attenuation of the receiving system is only 1dB, and fringe visibility can reach above 98%. What we need is the lower dark counts single photon detectors for over 100km quantum cryptography distribution, because the trunk fiber cannot be easily improved. Key words: quantum cryptography, quantum key distribution, bit error rate
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