Stopping Light in Its Tracks

Stop and go. In "electromagnetically induced transparency," a control laser makes a sodium gas transparent to another laser beam and causes it to slow down. Shutting off the control beam causes the light to come to a halt, while turning

Last year, physicists made headlines by slowing light down to the speed of a leisurely bicycle ride. Now, they have slammed on the brakes. In papers in Physical Review Letters and Nature, scientists report that they have used atomic gases to grab light pulses, squeeze them into a smaller space, imprint them on atoms, and read them out again after a delay. The researchers speculate that such sleight-of-light tricks might one day be useful in the still theoretical field of quantum information processing.

Light travels at about 300 million meters per second, a fact hammered into the heads of beginning science students. But that's in a vacuum, and it is only half the story. In fact, light has two different velocities--the phase velocity and the group velocity--and they can be very different. Phase velocity is the speed of a theoretical, pure light wave of a single frequency. Group velocity, by contrast, measures how fast a real signal moves through real stuff.

Last year, researchers led by Lene Hau of Harvard University slowed light to a crawl by exploiting the way special kinds of atomic matter can play around with the group velocity of a laser pulse. They started with a gas of sodium atoms, chilling it down to nanokelvin temperatures and making it transparent by tweaking the atomic energy levels with another laser. This "electromagnetically induced transparency" (EIT) slows down the light pulses by factors of millions and shrinks them by seven orders of magnitude.

In their new experiments, Hau's group added a twist: When the pulse had fully entered the atomic soup, they turned off the beam that makes the vapor transparent. "The light pulse comes to a grinding halt," Hau says. "All the information in the pulse gets stored in the atoms, and we can park it there for a while." When they switched the coupling laser on again, the vapor became transparent again, and the atomic spins regenerated a perfect copy of the original laser pulse.

Meanwhile, a group at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, has stored and reemitted light by very different methods. Rather than supercold sodium atoms, Ron Walsworth, Mikhail Lukin, and their colleagues used warm rubidium vapor to catch a laser pulse and then read it out again. In contrast to Hau's tailor-made equipment, the Harvard-Smithsonian group cobbled theirs together from components they were using in other experiments. "We basically did this with atomic clock technology," Lukin says. Their simpler apparatus stored light for up to half a millisecond before releasing it.

"This is very brilliant stuff," says Steve Harris of Stanford University, a physicist who first measured the optical slowing effect 5 years ago. But both sets of researchers stress that they haven't actually trapped photons like butterflies in a jar. The information contained in the laser pulse, they point out, is converted into atomic states that sit around until the control beam tells the light to emerge. Then energy from the control beam is converted into an outgoing pulse identical to the input pulse.

Many researchers are excited by the prospect of using the technique as a kind of coherent optical storage device--a sort of quantum hologram. Or it might lead to a quantum Internet, with light beams coherently ferrying information from atom cloud to atom cloud. But there is a long way to go before anyone will be saving e-mail in frozen light.

Related sites
Lene Hau's home page
The Walsworth group at the Harvard-Smithsonian Center for Astrophysics

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