Invisible, but not for Long Enough to Steal a Painting

Physicists are coming up with new ways to manipulate light that render objects effectively invisible. This has already been done by bending light around objects, so-called "spatial cloaking." Now Moti Fridman and other physicists from Cornell have developed a new trick called "temporal cloaking," which works by splitting up a pulse of light into two packets with a gap between, doing something during the gap, and then reassembling the pulse, so that it arrives intact at its destination. The event that took place in the gap is effectively invisible. The gap, alas, was only 50 trillionths of a second long, and it is not likely to get much longer.

I think these quantum tricks are interesting as physics, and maybe they will one day have applications in the microscopic world of computer chips and nanowires. But they are not scalable; there is no conceivable way to make them work for large objects at human time scales.

A more technical explanation of the experiment:

In their experiment, previewed this summer in an online archive and reported in detail in Nature on Tuesday, Fridman and his collaborators sent a laser beam of light down a fiber-optic cable. At the starting end of the cable, they pulsed the beam with a second laser that changed the light from a single wavelength to a range of wavelengths, essentially different colors.

The beam then entered a section of cable that had the property of carrying light of different wavelengths at different speeds, specifically blue light faster than red. As a consequence, the two colors separated until there was a space between them where there was no light at all. This blip of total darkness — one centimeter long and lasting 50 picoseconds — is what the researchers called a “time gap” or “time hole.” The beam was then reassembled by reversing the steps, sending it through glass with the opposite effect on blue and red light, and then through another laser that restored the light to the original single wavelength.

To show that an event occurring in the “time gap” was undetected, the researchers pulsed a ray of light through it. Normally, that would perturb the first beam in a way that was obvious when the light came out the far end of the cable. But when the ray went through the time gap and then the beam was reassembled successfully, the detector at the end of the cable perceived no change.