A Cloak in Time Could Secure Networks


Making an object invisible in space? That’s been done. What about making one invisible in time?

At Cornell University, Alexander L. Gaeta and Moti Fridman led a team that experimented with making events invisible in time. Even though they only managed to mask a picosecond-scale event, the work points the way to making not only true cloaking devices, but better security over fiber-optic lines as well. Gaeta and Fridman first demonstrated the proof of concept in July, and now their work will be published in the journal Nature.

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To understand what they did first requires a look at what “ordinary” cloaking is like. In the last several years, scientists and engineers have been looking at how to make objects invisible. One method is to use refraction, the same property that causes prisms to split sunlight into the different colors of the rainbow. But instead of using an ordinary transparent material found in a prism to bend the light waves, scientists are using another class of materials, called metamaterials.


These materials are engineered to have properties that may not be found in nature. For example, they can have a negative index of refraction, which makes light rays bend in a totally different way. A number of scientists have used these materials to bend light rays away from objects, essentially making them invisible. Because the bottom line is, if the light waves don't hit an object, it can't be seen.

But cloaking time is a whole nother thing. Instead of bending light waves around an object, the Cornell team slowed the light down on one side of an object, using a “time lens” that slowed wavelengths of light (think: red, blue, indigo, violet, etc.) and caused them to arrive at their destination at different times.

By passing the light through the time lens and then through a medium that dispersed it, they were able to create a gap in the light — a time when no wavelength was visible. First one would see the shorter wavelengths (violet to blue and green), a gap, then the longer wavelengths (yellow to orange and red).  

Next, by sending the light through another medium that reversed the dispersion and then another time lens, the beam looked just as it did before. The gap would be stitched together, with no way to know that anything had happened.

It’s like breaking up a line of cars at a traffic signal, and then allowing the ones that were stopped to catch up with the rest and re-form a continuous line. To test their idea, the scientists sent a light pulse through the gap, at a different frequency from the reference beam. That pulse was nearly invisible.

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There isn’t a practical technology yet to take advantage of this phenomenon on a large scale -– the Cornell team created a gap of only 50 picoseconds. But combining this technique with the spatial invisibility could make it easier to come up with a true cloaking device.

Gaeta’s team used a fiber-optic cable as the dispersive medium. Such cables carry beams of light to transmit data. Using a setup like this one could break up a beam of light, creating gaps that should be invisible to an observer. If anyone tampers with the beam, though, the gaps would become visible, as it would disrupt the cloaking effect.

Image: Paul Aresu/Getty Images

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