Punching a Hole in Time

Ocean's Eleven has nothing on this. A robber breaks into a bank safe and returns home, where he activates a device that conceals his earlier burglary, making it look like he never entered the bank in the first place. Such a "time cloak" is still a long way from reality, but researchers have now made an important first step, demonstrating a cloaking device that can hide for a fraction of a second an event that occurs at a specific point in time.

In recent years, physicists and engineers have developed rudimentary invisibility cloaks that smoothly funnel light around an object so that it cannot be seen. The time cloak, in contrast, essentially opens a gap in a laser beam, so that anything that happens in that gap cannot affect the beam and be detected. Of course, you can create such a gap just by momentarily blocking the beam with your hand. But by stitching together light illuminating the scene before and after the concealed event occurs, Alexander Gaeta, Moti Fridman, , and colleagues at Cornell University stitch the beam together again after the concealed event occurs. So an observer sees a continuous, uninterrupted beam of light and never suspects that part of the action has been edited out.

It's like a chicken crossing a busy road, says Martin McCall, a physicist at Imperial College London, who was not part of the study. When the chicken enters the road, cars slow down to let it pass. But when the chicken reaches the other side, the cars behind accelerate to catch up with those ahead. An observer some distance down the road would not discern the chicken or the cars slowing down but would simply see a continuous stream of passing cars.

To open the gap in the beam, Fridman and his colleagues used a "time lens"—a device that can shift the frequency of the light. At a particular moment, they first shift the frequency of the light higher and then suddenly lower. They then send this "frequency modulated" light through an optical fiber designed so that some wavelengths of light are sped up and travel faster than other wavelengths. As one set of wavelengths races ahead of the other set, a gap opens in the beam. Then, after the light has passed the spot where the hidden event will occur, the experimenters reverse the process. They run the light through a fiber in which the wavelengths that had been sped up are slowed down and those that had been slowed down are sped up, so the gap closes. They then use a second "time lens" to undo the frequency shifting.

The researchers report online this month at arXiv.org that they have hidden an event that lasted 15 trillionths of a second—not nearly long enough to conceal the actions of a safe cracker. However, their experiment has the ability to hide events lasting up to 110 billionths of a second, and time gaps lasting about 100 times longer should be easily achieved, the team notes.

McCall, Paul Kinsler of Imperial College London, and colleagues proposed the idea of a spacetime cloak in the February Journal of Optics. "I am very pleased to see that our concept of the spacetime cloak has been realized experimentally," McCall says. Kinsler notes that the Cornell experiment "is much easier to implement than was our proposed optical experiment and can generate much bigger time gaps."

Of course, there are caveats. As designed, the current experiment works for light of only a single wavelength. And it requires a cooperative spy, one who is willing to shine that laser beam into your device and pay attention to nothing else—such as all that highly visible lab equipment you've got set up for some reason or other. So for now, safe crackers are still going to have to rely on skilled fingertips and a bit of luck.

Posted in Physics