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A cloak in time

Physicists hide lab events for trillionths of a second

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In his final battle against evil on the big screen, Harry Potter could have used a newfound type of cloak: one that hides not objects in space, but events in time.

Like filmmakers cutting together a movie, physicists have found a way to temporarily tear a hole in a beam of light. Events that occur during a brief period of time remain unseen, as does the hole itself.

Moti Fridman and colleagues at Cornell University report the first experimental demonstration of such “temporal cloaking” online July 11 at arXiv.org.

While this trick won't be hiding bank robbers from security cameras anytime soon, it could find its way into optical and electronic devices.

Previous invisibility cloaks hid objects from view by bending light. Just as water flows around a rock in the middle of a river, light waves curve around a cloak and rejoin perfectly on the other side, leaving no trace of their detour.

A time cloak conceals an event by changing light's speed, not its direction. With the speed of light capped at 299,792,458 meters per second, this trick works only when light travels slower than it would in a vacuum — such as through fiber-optic cables.

The Cornell team, who declined an interview pending the paper's publication, manipulated light in a fiber-optic cable using a time lens, a silicon device originally developed to speed up data transfer. Some of the light passing through this lens speeds up, and some slows down. The waves divide, Moses-style, creating a gap of darkness. A second lens farther along the cable then stitches the light back together so that it arrives at its destination intact, with no record of a hole — or anything that happened during this brief window of opportunity.

This hole lasted for 15 trillionths of a second, long enough to conceal pulses of light created inside the cloak, the researchers write. A longer cable could, in theory, increase this time gap to more than a microsecond. Longer than that, imperfections in the technique would grow large enough to reveal the presence of the gap.

“This is a much bigger time gap than we were thinking would be possible,” says Paul Kinsler, a physicist at Imperial College London.

Kinsler and his colleagues first described the idea of a time cloak in a paper published in the February Journal of Optics. Their perfectly undetectable time cloak required exotic metamaterials, manmade structures used in traditional invisibility cloaks.

“You would need metamaterials that change their properties in time as well as space,” says team member Martin McCall, also at Imperial College London. “It's currently beyond metamaterial technology to produce that ideal situation.”

The imperfect Cornell cloak, which is not made of metamaterials, may be useful for signal processing. It could, in theory, interrupt one data stream, allow another to be processed and then reconstitute the original signal for a detector that would be unaware of the interruption.

Larger time gaps on everyday scales, though, are unlikely. Even with a theoretically perfect metamaterial cloak, a mere eight-minute gap would require a device the size of the solar system, McCall estimates.

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