Invisibility 'time cloak' developed

Optical fibres Scientists were able to hide data sent by optical transmission

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An "invisibility" time cloak which is able to hide events in a continuous stream of light has been developed by scientists.

The cloak works by manipulating the speed of light in optical fibres and means any interaction which takes place during this "hole in time" is not detected.

That is, a beam of light can be manipulated along its path.

The study is published in the journal Nature.

The research builds upon a time cloak described last year that was only able to hide single brief events of time in an optical beam.

Hidden data

This work is different to other "invisibility cloaks" in that it hides events in time, rather than spatial objects - which similar efforts have looked into.

The team from the Purdue University in Indiana has shown it can hide events in the path of a continuous light beam by having several "holes in time".

The researchers were able to cloak nearly half the data put in the beam's path, which they would otherwise be able to detect.

Cloaking, just as it sounds, is where an object or event is hidden from vision. This can apply to frequencies of light or sound. For example, stealth war planes can be difficult to detect on enemy radar.

"We were able to push the light forward and back using commercial telecoms components, that are controlled by electrical signals," said Andrew Weiner, who co-authored the paper.

"When one sends high-speed data over an optical fibre in the existing infrastructure, in many cases it's just 1s and 0s (binary code).

Bendy light

Other work on cloaking

optical camouflage, keio university
  • Optical camouflage technology: A modified background image is projected onto a cloak of retro-reflective material (the kind used to make projector screens); the wearer becomes invisible to anyone standing at the projection source
  • The "mirage effect": Electric current is passed through submerged carbon nanotubes to create very high local temperatures, this causes light to bounce off them, hiding objects behind
  • Adaptive heat cloaking: A camera records background temperatures, these are displayed by sheets of hexagonal pixels which change temperature very quickly, camouflaging even moving vehicles from heat-sensitive cameras
  • Calcite crystal prism: Calcite crystals send the two polarisations of light in different directions. By gluing prism-shaped crystals together in a specific geometry, polarised light can be directed around small objects, effectively cloaking them

"In our system, we can hide the 1s and 0s. There can also be other kinds of disturbances in the light but this cloak provides a zone where one doesn't see how the light is being changed," Prof Weiner told BBC News.

He compared how a stream of light is manipulated to a flowing river.

"Think about taking a region of that river and pushing some of it forward, and some backwards so there are holes where there isn't any water. Maybe there's a dam, and we can pop the dam on and off very quickly, to somehow disturb or divert the water.

"If we part the water so it doesn't see the dam popping up and down, it isn't disturbed, and afterwards we can put the water back together so it looks like a nice calm river again.

"That's how we control the flow of the light. We're pushing it forward and backwards in time, so it avoids events that would otherwise disturb it," Prof Weiner explained.

Though called a time cloak, it's actually "not a manipulation of time, it's a manipulation of light" explained Greg Gbur, who specialises in optical physics at the University of North Carolina at Charlotte.

The researcher, who was not involved in the study, said it showed a huge advance in the work on the time cloak.

"In the first time cloak paper, they discussed hiding events of a few billionths of a second once in a while. Here, they are talking about being able to hide data 46% of the time. This really suggests that this has gone from a curiosity to something that could be used in optical communications and data processing," added Dr Gbur.

Ortwin Hess, a physicist at Imperial College London, said the study was a "remarkable extension of the previously demonstrated time lens principle".

'Undesirable communication'

"An important part of the present paper exploits the principle of space-time duality, which means that like in the original concept of a temporal cloak, one of the directions of spatial cloaking had been replaced by time.

"It shows how beautiful the space-time principles are that can be used in optics. While previous cloaks are interesting as well, in the sense that they change optics in space, now we can change the way light, and thus information, behaves in space and time," Prof Hess told BBC News.

The research has several possible applications, he added. It could make data more tamper proof, could be used to monitor "undesirable communication" and could be used by governments or large firms that handle sensitive or confidential information.

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