Light That Travels ... Faster Than Light!

On the screen, a small pulse shiftsback and forth – just a little bit. But this seemingly unremarkablephenomenon could have profound technological consequences. Itrepresents the success of Luc Thévenaz and his fellow researchers inthe Nanophotonics and Metrology laboratory at EPFL in controlling thespeed of light in a simple optical fiber. They were able not only toslow light down by a factor of three from its well – established speedc of 300 million meters per second in a vacuum, but they've alsoaccomplished the considerable feat of speeding it up – making light gofaster than the speed of light.

This is not the first time thatscientists have tweaked the speed of a light signal. Even light passingthrough a window or water is slowed down a fraction as it travelsthrough the medium. In fact, in the right conditions, scientists havebeen able to slow light down to the speed of a bicycle, or even stop italtogether. In 2003, a group from the University of Rochester made animportant advance by slowing down a light signal in a room-temperaturesolid. But all these methods depend on special media such as cold gasesor crystalline solids, and they only work at certain well-definedwavelengths. With the publication of their new method, the EPFL team,made up of Luc Thévenaz, Miguel Gonzaléz Herraez and Kwang-Yong Song,has raised the bar higher still. Their all-optical technique to slowlight works in off-the-shelf optical fibers, without requiring costlyexperimental set-ups or special media. They can easily tune the speedof the light signal, thus achieving a wide range of delays.

"Thishas the enormous advantage of being a simple, inexpensive procedurethat works at any wavelength, notably at wavelengths used intelecommunications," explains Thévenaz.

The telecommunicationsindustry transmits vast quantities of data via fiber optics. Lightsignals race down the information superhighway at about 186,000 milesper second. But information cannot be processed at this speed, becausewith current technology light signals cannot be stored, routed orprocessed without first being transformed into electrical signals,which work much more slowly. If the light signal could be controlled bylight, it would be possible to route and process optical data withoutthe costly electrical conversion, opening up the possibility ofprocessing information at the speed of light.

This is exactlywhat the EPFL team has demonstrated. Using their Stimulated BrillouinScattering (SBS) method, the group was able to slow a light signal downby a factor of 3.6, creating a sort of temporary "optical memory." Theywere also able to create extreme conditions in which the light signaltravelled faster than 300 million meters a second. And even though thisseems to violate all sorts of cherished physical assumptions, Einsteinneedn't move over – relativity isn't called into question, because onlya portion of the signal is affected.

Slowing down light isconsidered to be a critical step in our ability to process informationoptically. The US Defense Advanced Research Projects Agency (DARPA)considers it so important that it has been funnelling millions ofdollars into projects such as "Applications of Slow Light in OpticalFibers" and research on all-optical routers. To succeed commercially, adevice that slows down light must be able to work across a range ofwavelengths, be capable of working at high bit-rates and be reasonablycompact and inexpensive.

The EPFL team has brought applicationsof slow light an important step closer to this reality. And Thévenazpoints out that this technology could take us far beyond just improvingon current telecom applications. He suggests that their method could beused to generate high-performance microwave signals that could be usedin next-generation wireless communication networks, or used to improvetransmissions between satellites. We may just be seeing the tip of theoptical iceberg.