雅思阅读精读：中国的量子网络

　　Quantum cryptography in space,The early bird，The world’s first quantum-cryptographic satellite network is likely to be Chinese

　　IN THE never-ending arms race between encryptors and eavesdroppers, many of those on the side that is trying to keep messages secret are betting on quantum mechanics, a description of how subatomic particles behave, to come to their aid. In particular, they think a phenomenon called quantum entanglement may provide an unsubvertable way of determining whether or not a message has been intercepted by a third party. Such interception, quantum theory suggests, will necessarily alter the intercepted message in a recognizable way, meaning that the receiver will know it is insecure. This phenomenon depends on the fact, surprising but true, that particles with identical properties which are created simultaneously are entangled in a way that means one cannot have its properties altered without also altering the other, no matter how far apart they are.

　　加密者和窃听者之间的军备竞赛永无止境。在努力为信息保密这一方，如今有许多人押注量子力学能够帮助他们——这一理论主要描述微观粒子的运动规律。他们尤其认为一种叫做量子纠缠的现象或许可以提供一条绝对可靠的途径，用以判断信息是否曾遭第三方拦截。根据量子理论，这类拦截必定会以可识别的方式改变被拦截的信息，从而令信息的接收端认识到信息是不安全的。

　　这种现象基于一个令人吃惊但确切的事实：无论相距多远，同时生成、属性完全相同的粒子会形成纠缠态，其中一方状态的改变必将导致另一方状态相应改变。

　　Researchers in several countries have experimented with the idea of quantum encryption, with some success. They have sent quantum-entangled messages through optical fibres, and also through the air, as packets of light. This approach, though, suffers from the fact that the signal is absorbed by the medium through which it is passing. The farthest that a quantum signal can be sent through an optical fibre, for example, is about 100km. Sending one farther than that would require the invention of quantum repeaters, devices that could receive, store and re-transmit quantum information securely. Such repeaters are theoretically possible, but so technologically complex that they remain impossible in practice.

　　已经有几个国家的研究人员尝试了量子加密的构想并取得了一些成果。

　　An alternative is to beam entangled photons through the vacuum of space, where there is nothing to absorb them. This would mean transmitting them via satellite. Whether that can be done while preserving entanglement was, for a long time, unclear. But it is clear now. Experiments conducted recently, by Pan Jianwei, a physicist at the University of Science and Technology of China, in Hefei, have shown that it can.

　　一个替代方案是在太空真空环境中传送纠缠光子，那里没有任何东西会吸收它们。这就需要通过卫星来传送了。很长时间里，人们并不清楚是否能做到这一点而仍然保持光子的纠缠态。

　　The keys to the high castle高堡密钥

　　Such tests have been made possible by the launch, in August 2016, of Micius, the world’s first quantum-communication satellite. Micius(named after a Chinese philosopher of the 5th century BC, who studied optics) now orbits Earth at an altitude of 500km. Using it, Dr Pan and his colleagues have been testing the protocols that a global quantum-communications network will need to work.

　　2016年8月，世界首颗量子通信卫星“墨子号”成功发射，令上述实验成为可能。“墨子号”(以公元前五世纪研究光学的中国哲学家墨子命名)如今在距地球500公里的太空轨道上运行。潘博士及其同事用这颗卫星来测试创建一个全球量子通信网络所需的协议。

　　Their first study, published in June, showed that entangled photons sent by the satellite to pairs of ground stations remain entangled, even when those stations are as much as 1,200km apart. Following that success, they attempted to use entanglement to “teleport” information from the ground to orbit. Information teleporting, so called because it happens without anything physical passing from one place to another, involves the sender changing a quantum aspect of one photon of an entangled pair that he has control over, and the receiver observing the same change in the other member of the pair, over which he has control. A series of such changes on successively transmitted photons can carry information, provided a code has been agreed on in advance.

　　To minimize the amount of atmosphere in the way, and thus the risk of signal disruption, Dr Pan and his team put their ground station for this experiment in Ngari, a region of south-western Tibet that has an altitude of 5,100 metres. They beamed one of an entangled pair of photons to Micius and kept the other on the ground. They then entangled the grounded photon with a third photon, and measured how this altered its polarization and the polarization of the photon on the satellite. The result, reported in July, was that the two do, indeed, change in lockstep. The team had thus succeeded in teleporting information from the ground to the satellite.

　　他们于今年6月发表的首个研究显示，从“墨子号”发送至两个地面站的纠缠光子仍保持纠缠态，即便两个地面站相隔1200公里之遥。这项实验取得成功后，他们尝试利用量子纠缠态从地面向太空轨道“隐形传输”信息。之所以称之为信息的“隐形传输”，是因为并没有任何有形的东西从一处传到另一处。在这个过程中，发送端改变了一对纠缠光子中由它控制的那个光子的量子态，接收端随即观察到自己控制的另一个光子发生了同样的改变。只要传输的两端事先商定一套信息编码，在连续传输的光子上发生一系列这样的变化就能传输信息了。

　　为尽可能地减少传输途中接触到的大气，从而减小信号中断的风险，潘博士和他的团队将这项实验中的地面站设在了西藏西南部海拔5100米的阿里地区。他们将一对纠缠光子中的一个发射到“墨子号”上，将另一个留在地面。而后用第三个光子来和地面上的那个光子制备出纠缠态，检测这如何改变了地面光子的偏振，以及卫星上光子的偏振。于7月发表的实验结果显示，两个光子确实都相继改变了。这样，研究团队成功地将信息从地面隐形传输到了卫星上。

　　In a third study, also published in July, Dr Pan showed that Micius is able to transmit useful information, in the form of quantum-encryption keys, to a ground station in Xinglong, near Beijing. The transmission of such keys is crucial to quantum cryptography. Quantum-encryption keys are the quantum states of long strings of photons. Using one, a receiver can decrypt a message which has been encrypted with the key in question.

　　The security of quantum cryptography relies on the fact that eavesdropping breaks the entanglement by observing what is going on. It is a real-life example of the thought experiment known as Schrdinger’s cat, in which a cat in a box remains both dead and alive until someone opens the box to look—at which point it becomes one or the other. Though entanglement-breaking will not be noticed by the receiver of a single photon, doing it to a series of photons will be statistically detectable, alerting him that the line is insecure.

　　在同样发表于7月的第三项研究中，潘博士演示了“墨子号”能将量子密钥这种有用信息发送到位于北京附近的兴隆地面站。这类密钥的传送对量子密码技术至关重要。量子密钥是长串光子的量子态。信息接收端可以使用一个量子密钥来解密用该密钥加密的信息。

　　量子加密的安全性基于这样一个事实：窃听者在观察传送的信息时会破坏量子纠缠态。它是思想实验“薛定谔的猫”的现实版：箱子中的猫既是死的也是活的，直到有人打开箱子一探究竟，它才变成或生或死的确定状态。虽然单个光子的接收端不会注意到纠缠态被破坏，对一系列光子状态的影响将在统计数据上显现出来，提醒接收端传输线路不安全。

　　This third demonstration of Micius’s capabilities paved the way for a subsequent, successful, attempt to share a secure key between Xinglong and a station 2,500km away in Nanshan, a town in Xinjiang, China’s westernmost province. To do so, Micius sent one half of a stream of entangled photon pairs to Xinglong when it passed over the place, and held the other half on board for two hours until it passed over Nanshan on its succeeding orbit.

　　这第三次对“墨子号”能力的展示为接下来又一项成功的尝试铺平了道路。研究团队在兴隆站和距其2500公里、位于中国最西部省份新疆的南山地面站之间分享了一个密钥。“墨子号”在经过兴隆上空时向其发送一串成对纠缠的光子中的一半，而后继续搭载剩余的另一半沿轨道飞行，直至两小时后经过南山的上空。

　　The next stage, scheduled to happen in about five years’ time, will be to launch a quantum-communications satellite in a higher orbit than Micius’s. The altitude Dr Pan has in mind is 20,000km, which will permit the satellite to communicate simultaneously with a much bigger part of Earth’s surface and allow him to test the feasibility of building a practical quantum-communications network. He is also hoping to put an experimental quantum-communications payload on board China’s space station, which is scheduled for completion by 2022. Having this device on board the station will mean it can be maintained and upgraded by human operators—a rare example of space-station crew doing something that could not easily be accomplished by robots. If all this goes well, the ultimate goal is a world-spanning ring of satellites in geostationary orbits.

　　下个阶段的计划是发射一颗量子通信卫星到比“墨子号”更高的轨道上，将在五年左右实现。潘博士设想的高度是两万公里，这将使卫星能同时和地球表面大得多的区域展开通信，让他能测试打造一个实用的量子通信网络的可行性。他还希望能在将于2022年建成的中国太空站上装设实验性的量子通信设备。在空间站装载这套设备将意味着它能由人类操作员维护和升级，这会成为一个罕见的例子——由空间站的工作人员来执行一些无法由机器人轻易完成的任务。假如这一切进展顺利，最终目标是发射一系列卫星到地球同步轨道上，环绕覆盖全世界。

　　One question Dr Pan and his colleagues particularly want to answer with their next experiments is whether entanglement is affected by a changing gravitational field. They could do this by comparing photons that stay in the weaker gravitational environment of orbit with their entangled partners sent to Earth. He also has other questions about the basic physics underlying entanglement—in particular, how it is that an entangled particle “knows” the result of changes made to its far-distant partner? That would be Nobel-prizeworthy stuff. Albert Einstein, famously, called the phenomenon of quantum entanglement “spooky actions at a distance”. Dr Pan’s work is helping to exorcise those particular ghosts.

　　潘博士和他的同事们尤其想通过日后的实验解答一个问题：量子纠缠是否受引力场的变化影响?这可以通过将处于轨道弱重力环境中的光子，与它被送到地球上的量子纠缠对象做比对来实现。他也想探究量子纠缠背后的物理原理，尤其是一个纠缠态粒子是如何“得知”其遥远的纠缠对象改变后的结果的?