Made with optical collusion, the cloak is a stitch in time (Image: Science Photo Library/Getty)

A “time cloak” that conceals events rather than objects can hide secret messages through a trick of light, making information invisible to all but the intended recipient.

Like an invisibility cloak that makes something disappear in plain sight, a time cloak makes an event disappear in time. It works by manipulating light traveling along an optical fibre.

Imagine a row of cars speeding along a road slowing down in concert to create brief paths for pedestrians to safely cross. When the cars that let the pedestrians cross ahead of them speed up and re-join the rest of the traffic, no one can tell there was ever a gap in the flow – the pedestrians’ presence has been cloaked.


In the same way, photons’ paths can be tweaked to create brief gaps where information can safely hide.

Last year, a team at Purdue University in Indiana built a cloak that could transfer hidden data at 1.5 gigabits a second, fast enough to make it theoretically useful for real communication. The only thing was, the message was hidden so well that no one could actually read it. That problem has now been solved.

Tricks of the light

“With this new device, we don’t just limit ourselves to thinking about cloaks as a way of preventing somebody from getting information, but also as a way to enable communication,” says Joseph Lukens, an electrical engineer at Purdue. “One guy sees nothing, the other guy sees everything.”

Lukens and his colleagues created two different communications channels using lasers tuned to two different frequencies. One is a regular frequency and the other is a time-cloaked channel that remains hidden unless you know it’s there. Photons from each laser traveled along the same fibre, but the intended recipient just needs to tune in to the right channel to reveal the secret information.

Not only could the cloak deliver the messages, it also successfully fended off outside attempts to scramble the information.

A similar device could one day improve current communication systems, says Moti Fridman at Bar-Ilan University in Israel.

“As we get closer and closer to the limits of current data transfer systems, we need to consider creative ideas for increasing the bandwidth in current systems with minimal changes,” says Fridman. “This is a beautiful example for using cloaking.”

Journal reference: Optica, DOI: 10.1364/OPTICA.1.000372