Physicists bring light to a stop
By JOSEPH B. VERRENGIA
AP Science Writer
(AP) - Physicists say they have brought light particles to a
halt, then revved them up again so that they could continue their
journey at a blistering 186,000 miles per 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 L. Walsworth and
Mikhail D. Lukin of the Harvard-Smithsonian Institute for
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
38 mph. ``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 shined a second, probe laser that operated at a
different frequency. When the wave of light particles hit the gas
atoms, the particles slowed dramatically.
To stop the probe light entirely, the researchers waited until
it had entered the vessel, encountered the gas atoms and imprinted
a pattern into the orientation of the spinning atoms.
Then the scientists gradually reduced the intensity of the
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,
according to Hau.
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 percent 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 means that 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
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 vapor of cesium atoms so
it traveled faster than the conventional speed of light.
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