雅思阅读材料盘点：中国的量子网络

　　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.