雅思閱讀精讀：中國(guó)的量子網(wǎng)絡(luò)

　　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.

　　加密者和竊聽者之間的軍備競(jìng)賽永無止境。在努力為信息保密這一方，如今有許多人押注量子力學(xué)能夠幫助他們——這一理論主要描述微觀粒子的運(yùn)動(dòng)規(guī)律。他們尤其認(rèn)為一種叫做量子糾纏的現(xiàn)象或許可以提供一條絕對(duì)可靠的途徑，用以判斷信息是否曾遭第三方攔截。根據(jù)量子理論，這類攔截必定會(huì)以可識(shí)別的方式改變被攔截的信息，從而令信息的接收端認(rèn)識(shí)到信息是不安全的。

　　這種現(xiàn)象基于一個(gè)令人吃驚但確切的事實(shí)：無論相距多遠(yuǎn)，同時(shí)生成、屬性完全相同的粒子會(huì)形成糾纏態(tài)，其中一方狀態(tài)的改變必將導(dǎo)致另一方狀態(tài)相應(yīng)改變。

　　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.

　　已經(jīng)有幾個(gè)國(guó)家的研究人員嘗試了量子加密的構(gòu)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.

　　一個(gè)替代方案是在太空真空環(huán)境中傳送糾纏光子，那里沒有任何東西會(huì)吸收它們。這就需要通過衛(wèi)星來傳送了。很長(zhǎng)時(shí)間里，人們并不清楚是否能做到這一點(diǎn)而仍然保持光子的糾纏態(tài)。

　　The keys to the high castle高堡密鑰

　　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，世界首顆量子通信衛(wèi)星“墨子號(hào)”成功發(fā)射，令上述實(shí)驗(yàn)成為可能。“墨子號(hào)”(以公元前五世紀(jì)研究光學(xué)的中國(guó)哲學(xué)家墨子命名)如今在距地球500公里的太空軌道上運(yùn)行。潘博士及其同事用這顆衛(wèi)星來測(cè)試創(chuàng)建一個(gè)全球量子通信網(wǎng)絡(luò)所需的協(xié)議。

　　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.

　　他們于今年6月發(fā)表的首個(gè)研究顯示，從“墨子號(hào)”發(fā)送至兩個(gè)地面站的糾纏光子仍保持糾纏態(tài)，即便兩個(gè)地面站相隔1200公里之遙。這項(xiàng)實(shí)驗(yàn)取得成功后，他們嘗試?yán)�用量子糾纏態(tài)從地面向太空軌道“隱形傳輸”信息。之所以稱之為信息的“隱形傳輸”，是因?yàn)椴�沒有任何有形的東西從一處傳到另一處。在這個(gè)過程中，發(fā)送端改變了一對(duì)糾纏光子中由它控制的那個(gè)光子的量子態(tài)，接收端隨即觀察到自己控制的另一個(gè)光子發(fā)生了同樣的改變。只要傳輸?shù)膬啥耸孪壬潭ㄒ惶仔畔⒕幋a，在連續(xù)傳輸?shù)墓庾由习l(fā)生一系列這樣的變化就能傳輸信息了。

　　為盡可能地減少傳輸途中接觸到的大氣，從而減小信號(hào)中斷的風(fēng)險(xiǎn)，潘博士和他的團(tuán)隊(duì)將這項(xiàng)實(shí)驗(yàn)中的地面站設(shè)在了西藏西南部海拔5100米的阿里地區(qū)。他們將一對(duì)糾纏光子中的一個(gè)發(fā)射到“墨子號(hào)”上，將另一個(gè)留在地面。而后用第三個(gè)光子來和地面上的那個(gè)光子制備出糾纏態(tài)，檢測(cè)這如何改變了地面光子的偏振，以及衛(wèi)星上光子的偏振。于7月發(fā)表的實(shí)驗(yàn)結(jié)果顯示，兩個(gè)光子確實(shí)都相繼改變了。這樣，研究團(tuán)隊(duì)成功地將信息從地面隱形傳輸?shù)搅诵l(wèi)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.

　　在同樣發(fā)表于7月的第三項(xiàng)研究中，潘博士演示了“墨子號(hào)”能將量子密鑰這種有用信息發(fā)送到位于北京附近的興隆地面站。這類密鑰的傳送對(duì)量子密碼技術(shù)至關(guān)重要。量子密鑰是長(zhǎng)串光子的量子態(tài)。信息接收端可以使用一個(gè)量子密鑰來解密用該密鑰加密的信息。

　　量子加密的安全性基于這樣一個(gè)事實(shí)：竊聽者在觀察傳送的信息時(shí)會(huì)破壞量子糾纏態(tài)。它是思想實(shí)驗(yàn)“薛定諤的貓”的現(xiàn)實(shí)版：箱子中的貓既是死的也是活的，直到有人打開箱子一探究竟，它才變成或生或死的確定狀態(tài)。雖然單個(gè)光子的接收端不會(huì)注意到糾纏態(tài)被破壞，對(duì)一系列光子狀態(tài)的影響將在統(tǒng)計(jì)數(shù)據(jù)上顯現(xiàn)出來，提醒接收端傳輸線路不安全。

　　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.

　　這第三次對(duì)“墨子號(hào)”能力的展示為接下來又一項(xiàng)成功的嘗試鋪平了道路。研究團(tuán)隊(duì)在興隆站和距其2500公里、位于中國(guó)最西部省份新疆的南山地面站之間分享了一個(gè)密鑰。“墨子號(hào)”在經(jīng)過興隆上空時(shí)向其發(fā)送一串成對(duì)糾纏的光子中的一半，而后繼續(xù)搭載剩余的另一半沿軌道飛行，直至兩小時(shí)后經(jīng)過南山的上空。

　　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.

　　下個(gè)階段的計(jì)劃是發(fā)射一顆量子通信衛(wèi)星到比“墨子號(hào)”更高的軌道上，將在五年左右實(shí)現(xiàn)。潘博士設(shè)想的高度是兩萬公里，這將使衛(wèi)星能同時(shí)和地球表面大得多的區(qū)域展開通信，讓他能測(cè)試打造一個(gè)實(shí)用的量子通信網(wǎng)絡(luò)的可行性。他還希望能在將于2022年建成的中國(guó)太空站上裝設(shè)實(shí)驗(yàn)性的量子通信設(shè)備。在空間站裝載這套設(shè)備將意味著它能由人類操作員維護(hù)和升級(jí)，這會(huì)成為一個(gè)罕見的例子——由空間站的工作人員來執(zhí)行一些無法由機(jī)器人輕易完成的任務(wù)。假如這一切進(jìn)展順利，最終目標(biāo)是發(fā)射一系列衛(wèi)星到地球同步軌道上，環(huán)繞覆蓋全世界。

　　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.

　　潘博士和他的同事們尤其想通過日后的實(shí)驗(yàn)解答一個(gè)問題：量子糾纏是否受引力場(chǎng)的變化影響?這可以通過將處于軌道弱重力環(huán)境中的光子，與它被送到地球上的量子糾纏對(duì)象做比對(duì)來實(shí)現(xiàn)。他也想探究量子糾纏背后的物理原理，尤其是一個(gè)糾纏態(tài)粒子是如何“得知”其遙遠(yuǎn)的糾纏對(duì)象改變后的結(jié)果的?