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以某商用单光子探测器为例,正弦门控频率为1.25 GHz,探测效率25%,所设计后脉冲概率测量系统如图3所示。
图 3 单光子探测器后脉冲测量系统框图
Figure 3. Block diagram of afterpulse measurement system for single-photon detector
脉冲光源接收信号发生器的低频周期驱动(625 kHz)产生低频信号光,经可调光衰减器衰减至单光子水平(每脉冲平均光子数μ=0.1)注入被测单光子探测器。单光子探测器产生探测脉冲输出至TDC进行探测脉冲到达时间测数量,上位机(PC)读取TDC数据并开展数据分析。采用“Start-Stop型”时间间隔测量方式的TDC对探测信号进行测量(时间分辨率50 ps)。Start信号连接和光信号驱动同源同频的周期信号(625 kHz);Stop信号连接探测脉冲,由于探测效率和注入光子数限制,探测脉冲是随机信号。通过Start-Stop信号的时间测量结果归一化可以找到探测脉冲的主峰位置,探测脉冲时间测量结果归一化如图4所示。
探测脉冲时间测量结果经归一化后,可以方便获取探测脉冲的计数分布,探测脉冲计数分布(舍弃时长为100 ns)如图5所示。横轴是归一化时间间隔(单位是50 ps,最长是1.6 μs),纵轴是单位时间内的探测脉冲计数。A区域是光信号对应的主峰位置,B区域是主要的后脉冲区域;可以看出,后脉冲主要分布在光脉冲主峰后的1 μs (横轴20000)时间内并且呈现指数下降分布,1 μs之后的后脉冲计数接近暗计数水平。值得注意的是,在B区域中,光脉冲主峰后存在一定长度内(等于舍弃时长)探测计数凹陷区,并且凹陷区内的探测计数也呈现指数下降分布趋势。这是因为APD内部的雪崩信号的幅度是随机的,只有超过甄别阈值的雪崩信号才被识别为探测脉冲。
凹陷区的脉冲计数产生机理如图6所示,主峰位置的部分光子虽然其雪崩信号幅度没有超过甄别阈值,但该雪崩信号同样会产生初级或者次级后脉冲,其后脉冲有可能会超过雪崩阈值而被识别为探测脉冲。因此,在归一化操作条件下,主峰位置未通过甄别域的雪崩脉冲所引发的后脉冲计数会显示在主峰之后的舍弃时长范围内,其计数值也呈现指数分布下降规律且很快接近暗计数水平。
图 6 未超过甄别阈雪崩信号引发的后脉冲示意
Figure 6. Afterpulses triggered by avalanche signals that do not pass the discrimination threshold
根据探测脉冲计数分布情况可以计算出后脉冲概率(Pap),其计算如公式(1)所示:
$$ P_{{\rm{a p}}}=\frac{\left(R-R_{{\rm{p h}}}\right)-R_{\rm{d}}}{R_{{\rm{p h}}}} $$ (1) 式中:Pap为后脉冲概率,R为总探测计数,包含光信号计数和噪声计数;Rph为对应位置的光信号主峰计数;Rd为暗计数。
单光子探测器不同舍弃时长的后脉冲概率变化如图7所示。
图 7 不同舍弃时长的单光子探测器后脉冲概率变化
Figure 7. Afterpulse probability changes with different discard times of single-photon detector
可以看出,有效探测事件之后的舍弃时长越长则后脉冲概率越低。当舍弃时长为0 ns时,后脉冲概率为5.59%;当舍弃时长为500 ns时,后脉冲概率降低到2.46%;当舍弃时长达到5000 ns时,后脉冲概率降低到1.97%。一般来说,QKD系统可容忍的错误率约为3%~5%,后脉冲概率所引入的错误率约为后脉冲概率的一半,因此,当舍弃时长达到500 ns及以上时,后脉冲所引入的错误率处于QKD系统可容忍错误率范围内,可以满足QKD系统的使用要求。
更进一步地,对于门控型探测,基于时间测量方式区分探测脉冲的到达时间,不仅可用于降低单光子探测的后脉冲概率,还可以根据探测脉冲的到达时间识别雪崩过渡区攻击事件、门外攻击事件,从而丢弃被攻击区域的探测脉冲来提升QKD系统抗量子黑客攻击的能力。不同到达时间的探测脉冲示意如图8所示。
门控型探测器的门内是有效探测区域时间窗口,其他区域(门外区域、雪崩过渡区)是无效探测时间区域。根据探测脉冲到达时间进行探测事件有效性划分的工作流程:首先获取每个探测脉冲时间信息,然后根据处理策略判断该脉冲属于有效区域还是无效区域;当识别到处于有效区域的探测脉冲时,正常输出该探测脉冲;当识别到处于无效区域的探测脉冲时,产生告警信息进行并舍弃该脉冲,从而提高探测事件的输出质量。
Afterpulse suppression scheme of InGaAs/InP high speed sinusoidal gated single photon detector
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摘要: 基于量子力学基本原理的量子密钥分发(QKD)系统具有信息论安全的水平,单光子探测器是QKD系统的重要组成,其后脉冲对QKD系统的安全成码率和安全成码距离均有重要影响。文中根据InGaAs/InP高速正弦门控探测器的后脉冲概率随时间呈现指数递减分布的规律,提出了一种基于时间测量的后脉冲抑制方案,采用“Start-Stop型”时间间隔测量方式对探测脉冲进行测量,通过对探测事件进行时间标记并舍弃一段时长内探测脉冲的方法降低了后脉冲概率。实际验证了某型号单光子探测器的后脉冲概率随舍弃时长的关系,在500 ns舍弃时长条件下,后脉冲概率2.46%,增加舍弃时长至5 μs可降低后脉冲至2%以下。同时,分析了100 ns舍弃时长条件下的典型探测脉冲计数分布,指出了主要后脉冲分布区域以及未过甄别阈值的雪崩脉冲引起的后脉冲机理。进一步地,指出基于时间测量方式区分探测脉冲的到达时间不仅可用于降低单光子探测的后脉冲概率,还可以根据探测脉冲的到达时间识别雪崩过渡区攻击事件、门外攻击事件,从而丢弃被攻击区域的探测脉冲来提升QKD系统抗量子黑客攻击的能力,可支撑高速正弦门控单光子探测器应用于实用化的QKD系统。
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关键词:
- 量子密钥分发 /
- 正弦门控单光子探测器 /
- 后脉冲概率 /
- 时间测量
Abstract:Objective Quantum key distribution (QKD) system based on the basic principles of quantum mechanics can reach the level of information theory security. Single photon detector is an important component of QKD system, and the afterpulse probability has an important effect on the performance of QKD system. In this paper, an InGaAs/InP high sinusoidal gated detector afterpulse suppression scheme is designed to meet the requirements of QKD system. Methods In this paper, according to the law that the probability of the afterpulse of the InGaAs/InP high-speed sinusoidal gated detector shows an exponential decreasing distribution with time, the detection pulse is measured by using the "Start-Stop" time interval measurement method. Each detection pulse is time-marked separately, and the probability of the afterpulse of the detector is reduced by discarding the detection pulse within a certain period of time (Fig.2). Results and Discussions This paper actually verifies the relationship between the afterpulse probability and the discard time of a single photon detector (Fig.5). The main afterpulse distribution area is pointed out and the reason for the depression in the range of discard time is explained (Fig.6). The afterpulse probability under different discard time conditions was calculated.The afterpulse probability is 2.46% when the discard time is 500 ns, and 1.97% when the discard time is 5 μs. Furthermore, it is pointed out that distinguishing the arrival time of detection pulse by time measurement can also improve the ability of QKD system to resist quantum hacker attacks such as avalanche transition region attack and behind door attack. Conclusions According to the law that the afterpulse probability of the InGaAs/InP high-speed sinusoidal gated detector is exponentially decreasing with time, a afterpulse suppression scheme based on time measurement is proposed in this paper. The afterpulse probability of the detector is reduced by marking the detection event with time and discarding the detection pulse in a period of time. The proposed afterpulse probability suppression scheme has the characteristics of clear principle and easy engineering implementation, and has no direct influence on the working process of single photon detector, but the detector saturation count rate will decrease with the increase of discard time. At the same time, distinguishing the arrival time of detection pulse by time measurement can also improve the ability of QKD system to resist quantum hacker attacks, and can support the application of high-speed sinusoidal gated single photon detector in QKD system. -
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