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注意：当提到罗彻斯特大学光学工程师时，从名字看有一位是中国哥们（或姐们）。
Physicists in US devise 'time cloak'
PENTAGON-supported physicists say they have devised a "time cloak" that briefly makes an event undetectable.
The laboratory device manipulates the flow of light in such a way that for the merest fraction of a second an event cannot be seen, according to a paper published in the science journal Nature.
It adds to experimental work in creating next-generation camouflage - a so-called invisibility cloak in which specific colours cannot be perceived by the human eye.
"Our results represent a significant step towards obtaining a complete spatio-temporal cloaking device," says the study, headed by Moti Fridman of Cornell University in New York.
The breakthrough exploits the fact that frequencies of light move at fractionally different speeds.
The so-called temporal cloak starts with a beam of green light that is passed down a fibre-optic cable.
The beam goes through a two-way lens that splits it into two frequencies - blueish light that travels relatively fast, and reddish light that is slower.
The tiny difference in speed is then accentuated by placing a transparent obstacle in front of the two beams.
Eventually a time gap opens up between the red and blue beams as they travel through the optical fibre.
The gap is tiny - just 50 picoseconds, or 50 millionths of a millionth of a second.
But it is just long enough to squeeze in a pulse of laser at a different frequency from the light passing through the system.
The red and blue light are then given the reverse treatment.
They go through another obstacle, which this time speeds up the red and slows down the blue, and come to a reverse lens that reconstitutes them as a single green light.
But the 40-picosecond burst of laser is not part of the flow of photons, and thus cannot be detected.
In a commentary, optical engineers Robert Boyd and Zhimin Shi of New York's University of Rochester, likened the experiment to a level crossing on a busy road.
When a train comes, the cars are stopped, and this causes a gap in the traffic.
When the train has passed, the stopped cars speed up until they catch up with the traffic in front of them. To the observer, the flow seems quite normal, and there is no evidence that a train has crossed the intersection.
After proving that the "cloak" is possible, the next step for the researchers is to expand the time gap by orders of magnitude, firstly to microseconds and then to milliseconds, said Boyd and Shi.
The time cloak has a potential use in boosting security in fibre-optic communications because it breaks up optical signals, lets them travel at different speeds and then reassembles them, which makes data hard to intercept.
Last year, scientists reported a step forward in so-called metamaterials that act as a cloaking of space, as opposed to time.
Metamaterials are novel compounds whose surface interacts with light at specific frequencies thanks to a tiny, nano-level structure. As a result, light flows around the object - rather like water that bends around a rock in a stream - as opposed to being absorbed by it.
Fridman's work was part-supported by the Defense Advanced Research Project Agency, or DARPA, a Pentagon unit that develops futuristic technology that can have a military use. Its achievements include DARPANet, a predecessor of the internet.
科学家研发出时间斗篷可使物体隐形 据悉，这种被称为“时间斗篷”的技术由纽约康奈尔大学的科学家们研发，研究人员利用“不同频率的光飞行速度不同”原理，先将绿色光束穿过光纤电缆，让它穿过双向镜，将光束分拆成蓝光(飞行较快)和红光(飞行较慢)，将速度差异通过一件透明障碍物扩大，产生了50皮秒(1皮秒等于1万亿分之1秒)的“时间空隙”，科学家们便乘机将一串不同频率的激光束“挤进”。 纽约罗彻斯特大学光学工程师对此作出了形象的比喻：就像在十字路口，一列火车驶过，汽车停下让路，火车经过后汽车要加速追上前面的汽车，造成的结果就像从没有火车通过十字路口一样。 分享到：
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