LIGHT, it turns out, can be coaxed into racing off at top speed, even in a substance where it usually moves slowly. The result hints at a possible way to speed up computers and communication networks.
Scientists generally assumed that in any given medium, a light signal cannot travel faster than the 鈥済roup velocity鈥� 鈥� the speed of a broad pulse of light. In glass, for example, the group velocity is about 0.67 times c, the speed of light in a vacuum. But experiments by Daniel Gauthier from Duke University in Durham, North Carolina, and his colleagues at the University of Arizona in Tucson suggest this is not the limit.
In their experiments, the team took a tube of rarefied potassium gas and shone a carefully tuned laser through it. This generates a peculiar state where the gas and laser light are tightly coupled together. When light pulses from a second laser move through this medium, they slow down dramatically (New Scientist, 22 May, page 32). In this case the light slowed to 0.01c.
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At least, most pulses did. Halfway through sending an ordinary broad pulse, the team suddenly boosted the brightness. This little 鈥済litch鈥� zoomed off and emerged at the other end of the tube long before most of the light in the pulse. The team has not yet had a chance to fully analyse the results, but they say the glitch moved at a speed of at least 0.6c, and possibly more than 0.95c.
Gauthier announced the findings last week at a meeting of the American Physical Society鈥檚 Division of Atomic, Molecular and Optical Physics in Tucson. 鈥淧eople in the physics community are having hard time accepting this,鈥� he says.
While theory suggests a pulse in a slow-light medium can have a weak 鈥減recursor鈥� signal that races off at almost c, physicists believed it would fade away and rapidly become too weak to detect. Gauthier thinks the sudden step-up in brightness somehow made the precursor signal persist, and he hopes to find ways of making it even stronger and faster.
鈥淚 can see that this would be a surprise to a lot of people,鈥� says slow-light expert Lene Hau of Harvard University. But she says she is cautiously positive about Gauthier鈥檚 new results. 鈥淭here are some people in this field making wild, quick and dirty claims, but he is a very careful scientist,鈥� she says. Hau is reluctant to make further comment, however, until the group reports its work in a paper.
Gauthier鈥檚 team plans to look for the same effect in a less exotic material, optical fibre. It is possible that communications networks could exploit this kind of signal, though the reduction in communication time would be slight.
The idea could have more practical importance in chip design. Modern computer chips are coming up against a barrier: they process data so fast that one side of the chip can barely keep in touch with the other, even though electrical signals travel across the chip at about 0.66c. It is possible that by changing pulse shapes so that signals move across the chip at nearly c, designers could push up the processing power of computers even more.