Molecules of Light

Light wave lab. A laser, mirrors, and other tools for making molecules
of light.

Like atoms binding to make a molecule, pulses of light can bind to make a "light molecule," according to a new study. Researchers say these light molecules could improve digital communication by adding a third digit to the binary code.

Light usually isn't very compact. The shortest blip will inevitably spread out in both space and in time unless it's constrained. But researchers can use tricks to keep a light pulse compact. For example, in some types of cable, bright light moves more slowly than dim light. If the fastest frequencies of a light pulse are made bright enough, they will slow down and keep pace with the slower frequencies, and the whole pulse will hang together over long distances. This type of light pulse is called a soliton.

In the 30 September issue of Physical Review Letters, physicist Martin Stratmann and colleagues at the University of Rostock in Germany describe how they linked two solitons together and then sent them traveling down a fiber optic cable. First, they shot a laser pulse at a mirror that reflects 50% of the light that hits it and transmits the other half. Using mirrors, the researchers sent the newborn pair of pulses along separate, precisely measured paths designed to recombine them with their phases exactly out of step. If the two pulses were trapeze artists, they'd swing in opposite directions and only cross in the middle. Where the edges of the pulses crossed, they canceled out. And when the pair was launched into a fiber, the result was a pair of bright solitons separated by a dark spot. This dark spot stays exactly the same size, and, just like two atoms in a molecule, the two solitons prefer to stay a certain distance apart. If they are stretched apart, they snap back; if they get too close, they quickly bounce away.

Fedor Mitschke, a researcher on the team, says some telecom companies already transmit binary information with solitons, using the pulses to represent ones and the spaces between them for zeroes. Light molecules, they say, could add a '2' to the data stream, enabling computers to communicate more succinctly and transmit more information along the same amount of cable.

"I think this is a nice result from a scientific standpoint," says Joseph Kahn, an electrical engineer at Stanford University in Palo Alto, California. "I just don't think it has the engineering value that they claim." He says even methods using simple solitons are not commercially viable, and there are more-promising technologies on the horizon.

Related sites
Soliton links
An example of a telecom company's soliton system