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Mirror, mirror, you're the wall
鏡子，鏡子，你就是那面墻
Dr Xu and Dr Pan conducted their trial at night, to minimise the amount of background light that might have interfered with the results. Their targets, a dummy of a human being on one experimental run and a giant "H" on another, were concealed behind a barrier in an apartment in a block of flats in Shanghai. Their laser and receiving apparatus were in a second apartment block 1.43km away. The receiving apparatus, an instrument called a single-photon avalanche diode (spad), was, as its name suggests, so sensitive that it could detect and count individual photons, the particles of which light beams are composed. This was just as well, for, of every seven million billion photons fired across the gap by the laser, only a single one returned.
徐飛虎和潘建偉在夜間試驗為的是將可能干擾試驗結果的背景光降至最弱。他們在一次實驗中試圖看到的目標是個假人，在另一次實驗中是一個巨大的字母H，它們都被隱藏在上海一幢公寓樓中某間房屋里的障礙物后面。他們的激光及接收設備位于1.43公里外的第二棟公寓樓內。接收設備是一臺叫作單光子雪崩二極管（SPAD）的儀器，聽名字就足以知道它的靈敏度非常高，可以探測和計數單個光子，即組成光束的粒子。這再好不過了，因為在他們的激光射出的每七千萬億個光子中，只有一個光子返回。
Because each firing of the laser yielded so little information, the researchers had to take many shots to build up an image. To that end, they imagined a grid on the target wall, 64 dots wide and 64 deep. They fired the laser at each dot in turn, and then fed the data from the spad into an algorithm capable of reconstructing, albeit fuzzily, an image of the hidden object.
由于每次激光發射得出的信息太少，研究人員不得不進行大量拍攝以建構一幅圖像。為此，他們在目標墻上想象出一個64個點寬、64個點深的網格。他們依次向每個點發射激光，然后把來自SPAD的數據輸入能夠重建隱藏物體（盡管是模糊的）影像的算法。
The military applications of this technology suggest themselves. They were, after all, why Dstl sponsored Dr Faccio in the first place. But others are also interested. America's space agency, nasa, has paid for such work in the past in the hope of putting a laser on a satellite orbiting a distant world. This would permit the photographing of the otherwise-invisible interiors of caverns on the surfaces of moons and planets. And, in a more practical vein, engineers in the autonomous-vehicle industry would be keen on a technology that let their cars spot other motorists blithely speeding around blind corners.
這項技術在軍事上的應用不言而喻，畢竟這正是Dstl當初資助法喬做這個項目的原因。不過還有其他感興趣的人群。美國國家航空航天局（NASA）在過去就曾為這類研究買單，希望能把激光安裝在繞遙遠天體運行的衛星上，這樣就可以拍攝到衛星和行星表面那些原本不可見的洞穴內部。而且，更實用的是，無人駕駛汽車行業的工程師會熱衷于這種技術，因為這能讓他們的汽車“看到”在盲區彎道飛馳的其他車輛。
For now, such applications remain far in the future. Capturing the experimental data in Shanghai took several hours, which is of little use either on the road or in fastmoving situations like hostage-taking. The amount of light lost between bounces also puts a limit on how far away an object can be from the reflecting wall before the technique stops being useful. The dummy used by Dr Xu and Dr Pan was 75cm from the wall, which is probably near that limit. These caveats aside, however, performing the trick over a distance of almost 1+1/2km is a staggering advance on previous efforts. It would not be surprising, says Dr Faccio, now that it is known what is possible, if that record, too, were broken.
目前這類應用仍遙不可及。在上海捕捉的實驗數據耗時達幾個小時，這要放到道路上或是像劫持人質這樣快速變動的場景下并沒有什么用處。來回反射間損失的光量也限制了這項技術要起作用物體可以距離反射墻面多遠。徐飛虎和潘建偉使用的假人離墻75厘米，可能已經接近極限。然而，撇開這些限制不談，在近1.5公里的距離上演這個戲法，相比之前的成果可謂進步驚人。法喬說，既然現在已經知道了什么是可能的，那么哪天這項記錄也被打破就不足為奇了。
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