放送今日份的雅思閱讀文章，標題為：量子網絡 , 太空量子密碼學 , 中國很可能建成世界首個量子保密衛星網絡。加密者和竊聽者之間的軍備競賽永無止境。已經有幾個國傢的研究人員嘗試瞭量子加密的構想並取得瞭一些成果，而中國的科學傢團隊已經取得瞭一定的成果。本篇雅思閱讀文章建議精讀時間：25 分鐘，閱讀難度偏大，有不少專業詞匯。
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賽永無止境。在努力為信息保密這一方，如今有許多人押註量子力學能夠幫助他們——這一理論主要描述微觀粒子的運動規律。他們尤其認為一種叫做量子糾纏的現象或許可以提供一條絕對可靠的途徑，用以判斷信息是否曾遭第三方攔截。根據量子理論，這類攔截必定會以可識別的方式改變被攔截的信息，從而令信息的接收端認識到信息是不安全的。
這種現象基於一個令人吃驚但確切的事實：無論相距多遠，同時生成、屬性完全相同的粒子會形成糾纏態，其中一方狀態的改變必將導致另一方狀態相應改變。
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.
已經有幾個國傢的研究人員嘗試瞭量子加密的構想並取得瞭一些成果。
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.
一個替代方案是在太空真空環境中傳送糾纏光子，那裡沒有任何東西會吸收它們。這就需要通過衛星來傳送瞭。很長時間裡，人們並不清楚是否能做到這一點而仍然保持光子的糾纏態。
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星 " 墨子號 " 成功發射，令上述實驗成為可能。" 墨子號 " ( 以公元前五世紀研究光學的中國哲學傢墨子命名 ) 如今在距地球 500 公裡的太空軌道上運行。潘博士及其同事用這顆衛星來測試創建一個全球量子通信網絡所需的協議。
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.
他們於今年 6 月發表的首個研究顯示，從 " 墨子號 " 發送至兩個地面站的糾纏光子仍保持糾纏態，即便兩個地面站相隔 1200 公裡之遙。這項實驗取得成功後，他們嘗試利用量子糾纏態從地面向太空軌道 " 隱形傳輸 " 信息。之所以稱之為信息的 " 隱形傳輸 "，是因為並沒有任何有形的東西從一處傳到另一處。在這個過程中，發送端改變瞭一對糾纏光子中由它控制的那個光子的量子態，接收端隨即觀察到自己控制的另一個光子發生瞭同樣的改變。隻要傳輸的兩端事先商定一套信息編碼，在連續傳輸的光子上發生一系列這樣的變化就能傳輸信息瞭。
為盡可能地減少傳輸途中接觸到的大氣，從而減小信號中斷的風險，潘博士和他的團隊將這項實驗中的地面站設在瞭西藏西南部海拔 5100 米的阿裡地區。他們將一對糾纏光子中的一個發射到 " 墨子號 " 上，將另一個留在地面。而後用第三個光子來和地面上的那個光子制備出糾纏態，檢測這如何改變瞭地面光子的偏振，以及衛星上光子的偏振。於 7 月發表的實驗結果顯示，兩個光子確實都相繼改變瞭。這樣，研究團隊成功地將信息從地面隱形傳輸到瞭衛星上。
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.
在同樣發表於 7 月的第三項研究中，潘博士演示瞭 " 墨子號 " 能將量子密鑰這種有用信息發送到位於北京附近的興隆地面站。這類密鑰的傳送對量子密碼技術至關重要。量子密鑰是長串光子的量子態。信息接收端可以使用一個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於這樣一個事實：竊聽者在觀察傳送的信息時會破壞量子糾纏態。它是思想實驗 " 薛定諤的貓 " 的現實版：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。雖然單個光子的接收端不會註意到糾纏態被破壞，對一系列光子狀態的影響將在統計數據上顯現出來，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 " 墨子號 " 能力的展示為接下來又一項成功的嘗試鋪平瞭道路。研究團隊在興隆站和距其 2500 公裡、位於中國最西部省份新疆的南山地面站之間分享瞭一個密鑰。" 墨子號 " 在經過興隆上空時向其發送一串成對糾纏的光子中的一半，而後繼續搭載剩餘的另一半沿軌道飛行，直至兩小時後經過南山的上空。
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.
下個階段的計劃是發射一顆量子通信衛星到比 " 墨子號 " 更高的軌道上，將在五年左右實現。潘博士設想的高度是兩萬公裡，這將使衛星能同時和地球表面大得多的區域展開通信，讓他能測試打造一個實用的量子通信網絡的可行性。他還希望能在將於 2022 年建成的中國太空站上裝設實驗性的量子通信設備。在空間站裝載這套設備將意味著它能由人類操作員維護和升級，這會成為一個罕見的例子——由空間站的工作人員來執行一些無法由機器人輕易完成的任務。假如這一切進展順利，最終目標是發射一系列衛星到地球同步軌道上，環繞覆蓋全世界。
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後的實驗解答一個問題：量子糾纏是否受引力場的變化影響 ? 這可以通過將處於軌道弱重力環境中的光子，與它被送到地球上的量子糾纏對象做比對來實現。他也想探究量子糾纏背後的物理原理，尤其是一個糾纏態粒子是如何 " 得知 " 其遙遠的糾纏對象改變後的結果的 ?
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