Light is fast and that is handy, but engineers building computers that use light rather than electrons need to slow it down on occasion. It wouldn’t do any good, for instance, to add two numbers before one of them had even arrived. In November two physicists revealed a way of creating a sort of optical molasses that can slow light in an optical fiber down to a crawl.
Dick Slusher and Ben Eggleton of Bell Laboratories in Murray Hill, New Jersey zapped a two-inch stretch of optical fiber with ultraviolet light at 150,000 evenly spaced points. The radiation damaged the fiber slightly and increased its index of refraction at each point, creating a grating made of, as Eggleton puts it, 150,000 tiny mirrors. Each mirror reflected only a small portion of light coming through the fiber, but in combination they showed some interesting results.
In particular, when the light’s wavelength was a bit more or less than twice the distance between the mirrors, a small portion of the light passed through the grating, but it took almost twice the amount of time to make the journey than it would have through plain optical fiber. The photons still moved as fast as ever, but they were bouncing around inside the grating as if in some endless house of mirrors, which meant they had to travel farther to make it through.
Slowing light down is just one of many things researchers will need to do before they can build useful optical computers, which are still a long way away. But some day, says Eggleton, this may have some really important applications.