Light Packets Slow to Jet Speed
Credit: NSF
Credit: NSF

The speed limit for light is 186,000 miles per second, but that doesn't mean it can't travel slower than that.  Light moves through glass at about 60 percent of its maximum.

By bundling up light waves into special packets, physicists have proposed a stable way to slow light signals to one-millionth of the speed limit, which is about as fast as a jet aircraft.

Light has been made to go slower than this, even made to stand still.  But most light packets will lose their shape when their speed is decreased -- a fact that hurts their application in the telecommunication industry. 

The new packets, however, belong to a type of wave pattern, called a soliton, which has a robust shape that does not easily decay.

"Solitons were discovered in the 1800s as water waves that propagate without losing their height for miles and miles," said Lu Deng of the National Institute of Standards and Technology.

Optical solitons generally are light waves that travel close to the speed of light.  But Deng and his colleague, Ying Wu, have devised a way to make optical solitons that travel much slower, giving them more applicability in data transfer applications.

Currently, when an optical signal traveling down a fiber needs to be routed, it is converted to an electrical signal, so that it can be stored in a buffer, while the address is read.  Once its destination is known, the signal is converted from electrical back to optical and sent on its way.

But Deng said that these conversions waste resources. It would be favorable to instead simply slow down the main signal while the address is read.  

This is possible in tiny cells filled with gas atoms.  By shining a laser into the cell, the speed of light can be tuned to whatever the researcher wants.

The problem, though, with these cells, or "optical buffers" as they are called, is that slowing down a wave can cause it to break up -- thereby losing the signal you are trying to send. 

"People have been working for years on an optical buffer," Deng said. "Unfortunately, they all have significant loss and terrible distortion."

Deng compared the signal to an ice cream scoop sliding along a table.  If it moves too slowly, the ice cream melts before it arrives at its destination. 

But if the signal can be converted into a soliton it should maintain its shape.   Deng and Wu have shown, in a recent issue of Physical Review Letters, how this soliton transformation can be done theoretically.  They are now gearing up to prove their calculations in an experiment.

Continuing with the ice cream analogy, Deng said that a slow-moving soliton wave would be like a scoop with a metal shield.

"Analogies are never perfect," he admitted.  "The point is that [the non-soliton] degrades, but the soliton does not."