Light clocked going faster than light – sort of

Researchers at the National Institute of Standards and Technology (NIST) have worked out a way to produce light pulses that – in a way –  travel faster than the speed of light.

This isn’t a case of those pesky CERN neutrinos again, but rather a technique called four-wave mixing, which reshapes parts of light pulses as they travel through a vacuum and shunts them forward from their natural position.

It could be used, say the researchers, to improve the timing of communications signals and to investigate the propagation of quantum correlations.

Einstein’s special theory of relativity gives us a sort of cosmic speed limit, with no information able to travel faster than light in a vacuum. But there’s a loophole, says the team. A short burst of light arrives as a generally-symmetric curve, like a bell curve in statistics.

And while the leading edge of that curve can’t exceed the speed of light, the main hump – the peak of the pulse – can be shifted forward to arrive sooner than it normally would.

The NIST team isn’t the first to try and put this idea into practice by amplifying the leading edge of the pulse and cutting off the back. But until now, it hasn’t been terribly productive: a great deal of noise has been introduced, with no great increase in the apparent speed.

However, says NIST, four-wave mixing produces cleaner, less noisy pulses with a greater increase in speed.

It involves sending 200-nanosecond-long ‘seed’ pulses of laser light into a heated cell containing atomic rubidium vapor along with a separate ‘pump’ beam at a different frequency from the seed pulses. The vapor amplifies the seed pulse and shifts its peak forward so that it becomes superluminal.

At the same time, photons from the inserted beams interact with the vapor to generate a second pulse, called the ‘conjugate’ – and thispeak, too, can travel faster or slower depending on how the laser is tuned and the conditions inside the laser.

In the experiment, the pulses’ peaks arrived 50 nanoseconds faster than light traveling through a vacuum. This ‘fast light’, says the team, could be useful for the transmission and processing of quantum information.