Light breaks up to cloak gaps in time

Method could hide messages without sender's knowledge

A device that manipulates light to open up small gaps in time has crept toward implementation outside the lab. Detailed June 5 in Nature, it could soon improve security over fiber-optic lines or improve data streaming rates.

“It’s exciting to see this exotic manipulation of light and its applications for communications and data processing,” says Alexander Gaeta, a Cornell University physicist who demonstrated the first time cloak two years ago (SN: 8/13/11, p. 12).

The term “cloak” can bring to mind Harry Potter-esque materials that hide an object at a specific point in space. These cloaks, a hot area of research since they were proposed in 2006, manipulate light so that an observer cannot see a stationary object.

Often in physics, what goes for space also holds for time. Last year Gaeta’s team showcased that truism by developing a cloak that hides events during a fixed time interval. A specially designed lens split light beams passing through a fiber into two segments. The trailing segment of light lagged behind the leading one by up to 50 trillionths of a second, creating a gap of total darkness between them. When Gaeta fired a laser at the fiber, the laser shot was undetectable because it had passed through the 50-picosecond interval of invisibility. Finally, Gaeta set up another lens to stitch the light segments back together, ensuring that the light beam emerged from the fiber looking exactly as it did at the start.

After reading Gaeta’s study, Joseph Lukens at Purdue University realized he could improve the technique. He designed an apparatus using off-the-shelf equipment that forced light to interfere and create repeating gaps of darkness at fixed temporal intervals. Each 40-picosecond gap was sandwiched between about 40 picoseconds of light, meaning that the time cloak was on roughly half the time.

Lukens’ study demonstrates how a time-cloaking device could eventually allow law enforcement or the military to prevent a nefarious person from communicating without the person’s realizing it. Just as the time gap in Gaeta’s experiment made the laser undetectable, the gaps created by Lukens’ cloak can conceal digital data. Lukens’ team tried to inject an electrical signal of 1s and 0s into the fiber, a task that would be no problem without a cloak, but the message never got encoded into the light beam. The receiver would assume that no message had been sent.