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cloaking

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Original author: 
John Timmer

FirasMT

In the past few years, there have been a regular series of announcements about devices that cloak something in space. These typically bend light around the cloak so that it comes out behind the object looking as if it had never shifted at all. In contrast, there's just been a single description of a temporal cloaking device, something that hides an event in time. The device works because in some media different frequencies of light move at different speeds. With the right combination of frequency shifts, it's possible to create and then re-seal a break in a light beam.

But that particular cloak could only create breaks in the light beam that lasted picoseconds. Basically, you couldn't hide all that much using it. Now, researchers have taken the same general approach and used it to hide signals in a beam of light sent through an optical fiber. When the cloak is in operation, the signals largely disappear. In this case the cloak can hide nearly half of the total bandwidth of the light, resulting in a hidden transmission rate of 12.7 Gigabits per second.

The work started with the Talbot effect in mind, in which a diffraction grating causes repeated images of the grating to appear at set distances away from it. The cloaking device relies on the converse of this. At other distances, the light intensity drops to zero. The key trick is to convert the Talbot effect from something that happens in space to something that happens in time.

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After years of research, "perfect" invisibility cloaks are finally a reality— at least so long as you are a tiny cylinder.

In 2006, the development of metamaterials resulted in a working example of a cloaking device, an essential accoutrement for young wizards and evil Klingon generals alike. Practical complexities, however, meant the material offered no more than a "simplifying approximation" of the desired functionality.

Now, however, researchers Nathan Landy and David R. Smith have described a "perfect" implementation in A full-parameter unidirectional metamaterial cloak for microwaves, a new study published by Nature: "Here, we design and experimentally characterize a two-dimensional, unidirectional cloak that makes no approximations to the underlying transformation optics formulation, yet is capable of reducing the scattering of an object ten wavelengths in size. We demonstrate that this approximation-free design regains the performance characteristics promised by transformation optics."

In other words, the cloaked object is completely invisible, unlike previous attempts in which reflections were visible: good enough for the Predator's interstellar hunting trips, but not for the Center for Metamaterials and Integrated Plasmonics in Durham, N.C.

While this is the first successful demonstration of the original 2006 paper's claims, that's not to say that there'll be practical implementations any time soon. The effect still only worked when viewed from one direction, and on a perfectly cylindrical object.

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