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Params: post, 304176, a:24:{i:0;s:99:"近日，中国科学技术大学潘建伟团队与清华大学王向斌团队再度强强联合，";i:1;a:4:{s:3:"tag";s:1:"b";s:6:"option";N;s:8:"original";a:2:{i:0;s:3:"[B]";i:1;s:4:"[/B]";}s:8:"children";a:1:{i:0;s:203:"突破了远距离独立激光相位干涉技术，实现 500 公里量级真实环境光纤的双场量子密钥分发（TF-QKD），从而再次刷新了量子密钥分发传输距离的世界纪录。";}}i:2;s:408:"
当地时间 2020 年 2 月 20 日，上述团队名为 Sending-or-Not-Sending with Independent Lasers: Secure Twin-Field Quantum Key Distribution over 509 km（"发送-不发送"独立激光器：509 公里安全双场量子密钥分发）的研究成果发表在世界著名物理学顶级学术周刊《物理学评论快报》（Physical Review Letters）上，并入选为“编辑推荐”文章。
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“发送—不发送”的双场量子密钥分发方案
在密码学中，密钥（key）指用来完成加密、解密、完整性验证等密码学应用的秘密信息。实际上，自人类使用语言以来，通过密钥给信息加密的技术就在不断发展。
1984 年，物理学家 Bennett 和密码学家 Brassard 提出了基于量子力学测量原理的世界首个量子密钥分发协议（BB84 协议），旨在增加安全通信距离、提高安全性。依据该协议提出的量子密钥分发（QKD，即 quantum key distribution）可以为分隔两地的用户提供无条件的安全共享密钥，在根本上保证了密钥的安全性。
不过，由于受到通信光纤损耗、探测器噪声等限制，量子密钥分发系统通常只能在 100 公里内获得较高的成码率——在无量子中继（中继指两个交换中心之间的一条传输通路）的情形下，安全成码率受线性界限的约束。
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【抗黑客攻击量子密钥分发示意图】
基于此，一种新型量子密钥分发方案——““发送—不发送”的双场量子密钥分发方案”应运而生，该方案由清华大学物理系教授、清华大学密码理论中心副主任、济南量子技术研究院院长、该论文作者之一王向斌提出。
据悉，这种方案巧妙地利用单光子干涉的特性，大大提高了对相位噪声的容忍能力和安全性，让量子密钥分发的成码率在长距离的情况下也维持较高水平。
简单来讲，双场量子密钥就像是一对双胞胎携手，甚至可以在无中继的情形下轻松突破成码率线性界限。
2019 年，中国“量子之父”、中科院院士潘建伟及其团队便在 300 公里真实环境的光纤中完成了双场量子密钥分发实验。
不过雷锋网了解到，双场量子密钥分发的技术要求相当苛刻：
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[*]
要求两个远程独立激光器的单光子级干涉；
[*]
需要通过单光子探测结果实现长距离光纤链路相对相位快速漂移的精准估计；
[*]
需要同时满足高计数率、高效率及超低暗计数的单光子探测器。
";}}i:8;s:320:"
量子密钥分发传输距离世界新纪录
此次，研究团队正是应用了上述方案，在 509 公里长的超低损耗光纤上实验证明了一种安全的密钥分发，突破了无中继量子密钥的绝对密钥速率限制，成功创造了地基量子密钥分发最远距离新的世界纪录。
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具体来讲，潘建伟团队基于王向斌提出的“发送-不发送”双场量子密钥分发方案，发展时频传输技术和激光注入锁定技术，将两个独立的远程激光器的波长锁定为相同，并利用附加相位参考光来估计光纤的相对相位快速漂移。同时，团队结合了中科院上海微系统所研制的高计数率低噪声单光子探测器，最终将量子密钥分发的安全成码距离推至 500 公里量级。
下图为实验装置示意图，Alice 和 Bob 为远程锁频稳定连续波激光器，由相位调制器（PM）和三个强度调制器（IM1、IM2、IM3）调制，用于相位随机化、编码和诱饵强度调制。
另外，图中 ATT 为衰减器，PC 为偏振控制器，PBS 为偏振分束器，DWDM 为密集波分复用器，CIR 为循环器，BS 为分束器，FM 为法拉第镜，PD 为光电二极管。
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根据论文，509 公里处的光纤成码率比传统无中继量子密钥分发的相对界限高 7 倍以上，即超过了理想的探测装置（探测器效率为 100%）下的无中继量子密钥分发成码极限。
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此外，如果将系统重复频率升级至京沪干线等远距离量子通信网络中采用的 1GHz，300 公里处成码率可达 5kbps，从而大量减少骨干光纤量子通信网络中的可信中继数量，大幅提升光纤量子保密通信网络的安全性。
实际上，当地时间 2020 年 3 月 2 日，国际学术期刊《自然》子刊《自然·光子学》（Nature Photonics） 刊登了潘建伟院士等人的另一篇研究成果，名为 Implementation of quantum key distribution surpassing the linear rate-transmittance bound（超过线性速率-透射率界限的量子密钥分发的实施情况）。
雷锋网注意到，该研究是以清华大学教授马雄峰提出的相位匹配量子密钥分发（PM-QKD）协议为基础，进一步推进了远程量子通信的发展，为方兴未艾的量子互联网打下基础。
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2019 年 9 月 10 日，在中国科学院与德国国立科学院（Leopoldina）联合举办的第一届双边研讨会上，潘建伟院士曾表示：
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在人类实现远距离安全量子通信的征途上有两大挑战，分别是现实条件下的安全性问题和远距离传输问题。
";}}i:18;s:144:"
可见，此次研究对量子通信来说意义不凡，在此雷锋网也期待我国科研人员带来的更多突破！
参考资料：
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