February 8, 2007

Wizardry at Harvard: Physicists Move Light

It’s like three-card monte. Now you see it. Now you don’t. Then you  
see it — over there.

In a quantum mechanical sleight of hand, Harvard physicists have  
shown that they can not only bring a pulse of light, the fleetest of  
nature’s particles, to a complete halt, but also resuscitate the  
light at a different location and let it continue on its way.

That ability to catch, store, move and release light could be used in  
future computers to process information encoded in the light pulses.

“It’s been a wonderful problem to try to wrap your brain around,”  
said Lene Vestergaard Hau, a professor of physics at Harvard and  
senior author of a paper describing the experiment that appears today  
in the journal Nature. “There are so many doors that open up.”

In 1999, Dr. Hau headed a team of scientists that slowed light, which  
travels a brisk 186,282 miles a second when unimpeded, to a leisurely  
38 miles an hour by shining it into an exotic, ultracooled cloud of  
sodium atoms. At temperatures a fraction of a degree above absolute  
zero, the atoms coalesce into a single quantum mechanical entity  
known as a Bose-Einstein condensate. Shining a laser on the cloud  
tunes its optical properties so that it becomes molasses when a  
second light pulse enters it.

Then, in 2001, Dr. Hau and a second team of physicists, this one from  
the Harvard-Smithsonian Center for Astrophysics, brought light to a  
complete halt by slowly turning off the laser. The Bose-Einstein  
cloud turned opaque, trapping the light pulse inside. When the laser  
was turned back on, the trapped light pulse flew out.

The latest results add an additional twist: transporting the pulse to  
a second Bose-Einstein cloud and regenerating the light there.  
“That’s the sort of stuff we find really sexy in this business,” said  
Eric A. Cornell, a senior scientist at the National Institute of  
Standards and Technology.

In the new Harvard experiment, when the initial pulse slammed into  
the first Bose-Einstein cloud, the collision caused 50,000 to 100,000  
of the sodium atoms to start spinning, almost like small tops, and  
pushed this small clump forward at less than a mile an hour.

Dr. Hau described the clump of atoms as a “metacopy” of the light  
pulse. Although it consisted of sodium atoms instead of particles of  
light, it exactly captured the characteristics of the light pulse.

The clump floated out from the rest of the cloud, traveled about two- 
tenths of a millimeter and burrowed into a second Bose-Einstein  
cloud. When a laser was shined on the second cloud, the atom clump  
transformed into a new pulse of light identical to the original pulse.

It was refinements to the 2001 experimental technique that extended  
the time the particles maintain quantum collective behavior. This  
allowed the clump to reach the second cloud.

Transforming a light signal into a clump of atoms could be a way of  
storing information. (“You could put it on the shelf for a while,”  
Dr. Hau said.) It could also enable a way of performing calculations  
in future optical computers that employ quantum algorithms to speed  
through certain types of calculations.

But one hurdle to building a computer that calculates with light is  
that it is difficult to grab onto and manipulate a quick-moving light  
pulse. Performing calculations with atomic clumps would be much  
easier with the result changed back into light and then sped to the  
next step.

“That has been a missing link,” Dr. Hau said.

The advance could also find applications in quantum cryptography,  
which can hide messages in codes that cannot be broken.

Dr. Hau said the current apparatus was just a proof of the concept  
and far from anything that could be used practically for any  

But that has not stopped other physicists from starting to ponder  
what the applications might be, just as her earlier experiments have  
spurred physicists and engineers in a new active field of research,  
looking for ways to harness slow light for use in optical networks.

Currently, optical signals need to be changed into electronic ones  
for processing and then changed back into light. All-optical devices  
could save on costs and power use.


Copyright 2007 The New York Times Company

s. e. anderson (author of "The Black Holocaust for Beginners" -  
Writers + Readers) +