Scientific Method / Science & Exploration

Optical setup helps researchers hide an event from time

Researchers have managed to perform a bit of optical trickery to ensure that, …

Cloaking devices are one of the inventions of science fiction that have made a few tentative steps towards the real world in recent years. Now, researchers have moved the concept into the fourth dimension, creating a setup that hides a specific point in time from being perceived by observers. But if you want to make an event disappear, you have to act fast: right now, we can only hide a few picoseconds worth of time.

The cloaking devices we've made all work based on a similar principle: light that enters the device is bent in such a way that when it exits, its location and direction make it appear that the device itself, and anything within it, were not present. In other words, while within the device, light travels as if it were present. It's just that, once it exits the other side, there's no evidence that anything unusual has taken place. The same general idea governs the action of a temporal cloaking device.

The basic idea is that, when it's not in operation, a light beam can pass through the cloaking device unhindered. When it's switched on, a short temporal gap is opened up in the beam, then sealed back up on its way out of the hardware. One way to think of this is to view the light beam as a bit of old-fashioned magnetic tape. You can cut the tape so that a single instant of a recording can be physically separated. While separated, you can pass anything you want through the gap, but when you glue the tape back together, the recording is seamless. There's only a before and after while the tape is cut and separated.

It's easy to do that with tape, but a bit harder to do it with a beam of light. The key to the process is what's being termed a split time lens, which is matched with a dispersive medium. When activated, the lens takes the light that comes before the point of cloaking and shifts it to bluer wavelengths, which travel faster through the dispersive medium than the base speed of the light in the same medium. At the cloak point, the lens switches, shifting the light beam to longer, redder wavelengths. These travel through the dispersive medium more slowly.

The end result is that a gap develops in the beam of light as the blue light races ahead and the red light slows down. In this cloaking device, that gap maxes out at about 50 picoseconds, long enough for a very brief event to occur (in this case, the event was simply an interaction with a pulse from another laser). The light can then be sent through a dispersive medium with the opposite effect, slowing the bluish light back down while accelerating the red. 

 By the time it exits the device, the gap has been sealed back up and the effects of the wavelength shifts reversed. Any events that took place during the 50 picosecond gap (as long as they occur in both the right place and time) never happened, at least as far as the light beam that exits is concerned.

The authors timed things so that a picosecond laser pulse should interact with the light beam at the right time and place. When the cloaking device was off, a clear signal was apparent in the output. Once it was switched on, however, the signal dropped to background levels. By sampling many events, it was still possible to pick out a signal with the expected frequency, but it was over ten times smaller than the one present when the cloaking device was switched off.

Fifty picoseconds isn't a lot to work with. With the device as it now stands, the authors think they can stretch out the temporal gap to about 110 picoseconds; changing the optics a bit would push it up against the limits of optical cabling, which maxes out at about 50km. Using the full length of the cables would net you a gap of a few nanoseconds.

As with the spatial cloaking devices, it's not obvious that the temporal cloaking device will ever have any sort of practical utility. Still, it's an impressive display of what we can do with a bit of finely tuned optics.

Nature, 2012. DOI: 10.1038/nature10695  (About DOIs).

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