In the past few years, there has been a regular series of announcements about devices that cloak something in space. These typically bend light around the cloak so that it comes out behind the object looking as if it had never shifted at all. In contrast, there's just been a single description of a temporal cloaking device, something that hides an event in time. The device works because in some media different frequencies of light move at different speeds. With the right combination of frequency shifts, it's possible to create and then re-seal a break in a light beam.

But that particular cloak could only create breaks in the light beam that lasted picoseconds. Basically, you couldn't hide all that much using it. Now, researchers have taken the same general approach and used it to hide signals in a beam of light sent through an optical fiber. When the cloak is in operation, the signals largely disappear. In this case the cloak can hide nearly half of the total bandwidth of the light, resulting in a hidden transmission rate of 12.7 Gigabits per second.

The work started with the Talbot effect in mind, in which a diffraction grating causes repeated images of the grating to appear at set distances away from it. The cloaking device relies on the converse of this. At other distances, the light intensity drops to zero. The key trick is to convert the Talbot effect from something that happens in space to something that happens in time.

With the right combination of phase modulators and diffraction hardware, the authors could start with a beam that had light distributed across a range of wavelengths from 1,541nm to 1,543nm and convert it into a single, high-intensity pulse right in the middle at 1,542nm. When the light hit a set of hardware that was oriented in the opposite direction, the original spread of the light was restored. But since this Talbot effect is limited in time (rather than space), the authors note that they could have just let the light travel for long enough and the original light would be restored.

So how can you use this to hide a signal? It's possible to encode a signal in the ratio of light at different frequencies. If there's more at the high frequencies, the beam will deposit more energy into a photodetector than it would if more of the light was present at the low frequencies. But when the cloaking device was in operation, the original configuration wouldn't matter. All the light would simply look like it's at the frequency at the midpoint (1,542nm in this case).

The authors created a sinusoidal pattern, with light shifting from the lower to higher frequencies. With the cloaking device off, this registered as a sine wave running between lower and higher energies deposited in the detector. Switch it on, and the dramatic waves turned into a series of very small bumps and troughs. The authors then encoded bits with high energy being 1 and low energy being zero. These were easy to read when the cloaking device was off but vanished into a bit of noise once it was switched on.

The authors calculated that 46 percent of the total bandwidth of the light could be transmitted as cloaked data, which gets them the 12.7 Gbps figure. But they also suggest that, by putting three phase modulators in series, the total available to be cloaked could be roughly doubled, reaching 90 percent of the total bandwidth.

The other neat thing about this work is that most of the hardware seems to be standard off-the-shelf fiber optics equipment (the frequency modulators were driven by Agilent hardware and the optical cables came from Corning). There's no obvious reason that the work can't be put to use fairly quickly. The authors realize this, writing, "This potential to cloak real-world messages introduces temporal cloaking into the sphere of practical application, with immediate ramifications in secure communications."

Nature, 2013. DOI: 10.1038/nature12224 (About DOIs).