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A team of physicists at Cornell University have learned how to hide an event by using time. This process is called “temporal cloaking”, which is built on experiments researchers have previously conducted where they verified that it is possible to hide objects from view. Scientists have already accomplished “spatial cloaking”. It involves bending light around an object in a way that makes it seem invisible to the eye, while temporal cloaking involves interrupting light to make a seeming gap in time where an event can be concealed. So far, the time gap scientists made is around 50 trillionths of a second, so practical implications of the process are a long way coming.
Alexander Gaeta, the leader of the team, said that researchers are interested in attempting to make the amount of time a beam’s gap stays open longer. Essentially, the team turned off a laser beam for a brief amount of time in such a way that instruments receiving the beam did not have the ability to detect it. An observer of this would have no idea that the beam had blinked and would possess no evidence that anything had happened to the beam in the time frame of 50 trillionths of a second.
Zhimin Shi and Robert Boyd, physicists at the University of Rochester, were not a part of Dr. Gaeta’s team, but they compared the occurrence to cars at a railroad crossing. The crossing gate falls and interrupts the flow of traffic (the beam) as the train passes. From the train’s perspective, there are no cars, and it can pass freely (the hidden event). After the gate climbs back up once the train has passed, traffic begins flowing again. If one were to see this from one or two miles away, the traffic flow would not show any sign of interruption and no evidence that the train had been there.
To accomplish the feat of opening up a gap in the laser beam, researchers took advantage of how fast different colors travel at different speeds when light goes through a material. The researchers employed a laser-based mechanism called a time lens to change colors in a section of the laser beam. Typical glass lenses will bend light and alter its distribution in three-dimensional space, but time lenses “do really funny things” to light by changing its traits for a certain period of time, said Gaeta.
In the case of the team’s experiment, the modified time lenses gave two adjoining parts of the green beam a red hue and a blue hue for a short period of time. When these parts traveled through a specially designed length of optical fiber, the blue light went faster, while the red light grew slower. This difference opened up a gap in the beam- meaning there was no light- and lasted for around 50 trillionths of a second. The researchers reversed the process once it came out the other side and slowed the blue hue and sped up the red. They then passed the colors through another time lens and returned the beam to its previous green hue with almost no evidence of its temporary alteration.
For now, a lot of work will center on gaining a better understanding of the physics involved in the process and how to better take advantage of them, said Dr. Shi of Rochester.
Image Courtesey of Cornell University
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