January 19, 2001
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Scientists bend universal laws by 'stopping' light
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(AP) - Physicists said they have brought light particles to a screeching halt, then revved them up again so they could continue their journey at a blistering 300,000 kilometres a second.

The results are the latest in a growing number of experiments that manipulate light, the fastest and most ephemeral form of energy in the universe.

Eventually, researchers hope to harness its speedy properties in the development of more powerful computers and other technologies that store information in light particles, rather than electrons.

The experiments were conducted in separate laboratories in Cambridge, Mass., by groups led by Lene Vestergaard Hau of Harvard and the Rowland Institute of Science and Ronald Walsworth and Mikhail Lukin of the Harvard-Smithsonian Institute for Astrophysics.

The results will be published in upcoming issues of the journals Nature and American Physical Letters.

Physicists who did not participate in the experiments said the two research papers make an important contribution to understanding the properties of light. However, any practical applications are far off, they said.

''It's a real first,'' said Stanford physicist Stephen Harris, who collaborated on a 1999 experiment with Hau that slowed light to 61 km/h.

''These experiments are beautiful science.''

In the latest experiments, researchers took steps to not only slow light to a virtual crawl but to stop it completely.

To do so, they created a trap in which atoms of gas were chilled magnetically to within a few-millionths of a degree of absolute zero and a consistency they described as ''optical molasses.'' Hau's group used sodium atoms, while Waldsworth's group used rubidium, an alkaline metal.

Normally, the gas atoms would absorb any light directed into the trap. The researchers solved this problem by aiming a ''control'' laser beam into the gas, which transformed it from opaque to a state known as electromagnetic ally-induced transparency, or EIT.

Then they shone a second, probe laser that operated at a different frequency. When the wave of light particles hit the gas atoms, the particles slowed dramatically.

Then the scientists gradually reduced the intensity of the control beam.

As a result, the probe light dimmed and then vanished. But information in the light particles still was imprinted on the atoms of sodium and rubidium, effectively freezing or storing it, Hau said.

Then the scientists gradually restored the control beam. The light that had been stored in the spinning atoms was reconstituted and continued its journey through the vessel.

''It's as if you stretched a silk thread across a railroad track and a train vanishes into it,'' said University of Colorado physicist Eric Cornell, who reviewed the Hau study for Nature.

''You wait and then - bam! - the train reappears and goes zooming down the track,'' Cornell said.

''It's not at all what you would expect from a pulse of light.''

About 50 per cent of the light - and its information - was retrieved in the regenerated light pulse, scientists said. That might not be good enough for a practical computing system but it demonstrates how such a system might store and ship data.

''Nothing is ready to be picked up by the optical communications industry,'' Harris said.

''It needs further invention.''

Whether either group actually stopped the light completely is open to some interpretation. The probe laser actually is a bundle of light waves that form a single wave. This is known to physicists as the group velocity; it is the light that your eye sees and a camera uses to record an image.

Does stopping the group velocity mean the individual light waves themselves were stopped? That's a deeper quantum question, physicists said but they considered the Cambridge groups' claims to be valid.

''It is a real effect,'' said Ben Stein of the American Physical Society.

Manipulating light's properties is a subject of intensely competitive research. In July, physicists in Princeton, N.J., apparently pushed a laser pulse through a vapour of cesium atoms so it travelled faster than the conventional speed of light.