Light stands still in atom clouds.

Two independent teams of physicists have overcome the restless nature of light and stopped laser pulses in their tracks. A proposed method, now in tests by a third team, may even make light pulses creep backwards.

It's not normal for photons to stop, let alone to back up. Those quintessential particles of light usually zip straight through a vacuum at 300,000 kilometers per second and traverse other materials at slower but still dazzling speeds.

In a sense, stopping light is a trivial feat, comments Michael Fleischhauer of Kaiserslautern University in Germany, who helped develop the theory behind the new stopping techniques. "Take a black piece of paper and you can stop light very easily," he notes. However, the photons absorbed by that paper are destroyed.

What's remarkable about the new experiments is that the researchers stop a light pulse without destroying the photons, he explains. In each experiment, a so-called coupling laser illuminates a gaseous group of atoms in a glass chamber or magnetic trap while a pulse from another laser penetrates the gas.

When the scientists turn off the coupling laser, the electromagnetic energy from the other laser vanishes, but the information that defines its light, such as amplitude and wave properties, transfers onto the gas atoms' quantum state known as spin. By turning the coupling beam back on, the researchers reconstitute the original pulse from the stored information.

"We park the light pulse in the atom cloud. When we feel like it, we turn the coupling laser back on ... and out comes [the original pulse]," says Lene V. Hau of the Rowland Institute for Science and Harvard University, both in Cambridge, Mass.

Reporting in the Jan. 25 NATURE, she and her team describe how they shone a coupling laser on a cloud of ultracold sodium atoms, causing pulses from another laser to slow and then stop. A couple of years ago, she and Stephen E. Harris of Stanford University and their colleagues made headlines by slowing light to a bicyclist's speed (SN: 3/27/99, p. 207). In later experiments, Hau's group and others further reduced the speed.

Using rubidium gas warmed to around 80[degrees]C, Ronald L. Walsworth, Mikhail D. Lukin, and their colleagues at the Harvard-Smithsonian Center for Astrophysics in Cambridge also stopped light dead. A report of the work is scheduled to appear in the Jan. 29 PHYSICAL REVIEW LETTERS (PRL).

The transfer of light-defining information between light and atoms may ease the development of powerful computers (SN: 8/26/00, p. 132) and exceptionally secure forms of communication that exploit the quantum properties of particles, says Lukin.

Whereas atoms confined in one place may handle quantum computations, photons seem better suited for transporting quantum information, Lukin explains. The new experiments suggest that data transfers between the two operations--computation and data transport--may be possible. To become practical, however, the technique will probably have to work in solids, not just in atomic clouds, he adds.

The expansion of light-slowing techniques from gases to solids, a more complex medium, is already under way. In an unpublished experiment, Philip R. Hemmer of Hanscom Air Force Base near Boston and his colleagues have for the first time dramatically slowed light in a solid, Hemmer told SCIENCE NEWS. Pulse speeds fell to about 45 meters per second in an exotic type of crystal.

Methods of slowing and stopping light have evolved from a technique for making opaque material transparent to selected light frequencies (SN: 6/1/91, p. 340). In that technique, scientists shine on the material a coupling laser tuned to a frequency slightly different from that of the laser pulse entering the material. Like two audio tones generating a low-frequency warbling or beating sound when they're close in frequency, the two light signals interfere to create a relatively low-frequency beam. Instead of being absorbed by the atoms, as each of the two parent waves would have been, this signal passes right through them, Fleischhauer explains.

However, the combined signal's photons still interact with the material's atoms by repeatedly imprinting their defining features onto the atoms' spins and then retrieving that information from the atoms. Those interactions bog down the pulse. Shutting off the coupling beam freezes that give-and-take between the photons and atoms, stopping the pulse.

So far, Hau and her team report the longest storage time for pulses--about a millisecond. By then, random atomic motion had washed out most pulse information, the researchers suspect. The Harvard-Smithsonian team reports that its pulses' information is erased partly because atoms escape from the region lit by the coupling laser.

It also may be possible to halt a light pulse in a warm gas without turning off the coupling laser, scientists at Texas A & M University in College Station now propose. In the Jan. 22 PRL, they suggest tuning the lasers so the pulse's photons interact only with atoms whose random thermal motion opposes the pulse's direction.

If that backward velocity equals the light pulse's forward velocity, the pulse will stand still. Study coauthor Yuri Rostovtsev says that if the atoms' velocity is greater than the light pulse's velocity, excited atoms should "drag the light" backwards.