How To Cloak A Crime In A Beam Of Light

The 14th member of Danny Ocean's team of thieves might just be a physicist, making use of an "event cloak" dreamed up by Martin McCall's team at Imperial College London.

Unlike invisibility cloaks, which bend light around an object, an event cloak would open up a time gap in the light by controlling its speed through optical fibres, and then seal it again to hide all traces of activity within the gap. A modified version could, in principle, allow a safe-cracker to work while the security camera appears to record an empty room.

McCall's colleague Alberto Favaro compares the way it works to the way a road packed with speeding cars can still allow a pedestrian to cross. Some cars slow down, creating a jaywalker-friendly, vehicle-free gap, before speeding up again to re-establish the seamless flow of traffic.

In the Imperial team's blueprint for their cloak, an optical fibre serves as the road, while the photons passing down the fibre take the place of the cars.

This approach relies on an unusual property of silica optical fibres: their refractive index, the measure of how fast light travels through the silica, changes with the brightness of the light. To open an event cloak in the stream of light passing down the fibre, a control laser injects an additional pulse of light. The increased brightness slows the light down, says team member Paul Kinsler.

As the brighter section moves along the fibre, it falls progressively further behind the dimmer, faster-moving light ahead, creating a gap in illumination. The brighter laser pulse is then filtered out again, leaving all of the light with the same brightness and travelling at the same speed, and so maintaining the size of the gap.

The light signal to be "cloaked" can be injected into this gap and sent down the optical fibre with the rest of the light. After the signal has been read, it can be removed to leave a gap once again.

To seal the gap, a bright laser pulse is injected into the light stream in front of the gap, slowing it down enough to allow the light behind the gap to catch up. Finally, the bright pulse is filtered out again. From then on, an observer monitoring the light would be unaware that it had once contained a gap – or a cloaked signal – apart from a slight delay to the light (see diagram).

To make the changes to the light stream completely undetectable, a more complex arrangement would be needed in which the gaps were created by speeding up some parts of the light stream and slowing down others. This would only be possible using metamaterials.

Because the event cloak opens a gap in the flow of light, anything – or anyone – exploiting it would be working in the dark. But the biggest hindrance to cloaking a safe-cracker is that the longer the event you want to hide, the greater the distance required to manipulate the light. "Light travels so fast that even a very small time corresponds to a very big space," says Favaro.

Using 9km of optical fibre, the elementary set-up McCall and his team envisage could open and close a gap wide enough to equate to about five nanoseconds – long enough to allow a high-priority signal to pass through a data fibre already at capacity.

Alexander Gaeta of Cornell University in Ithaca, New York, has built what he calls a time telescope, which also manipulates the speed of light through fibres. Though he says it would be interesting to try and demonstrate the event cloak, he foresees a possible snag. The intensity of light needed for such substantial changes in the optical fibre's refractive index may also generate scattered photons which could interfere with the control laser beam, Gaeta says.

Image by ViaMoi/Flickr