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Physicists Slow Speed of Light
Lene Hau has shed new light on a new form of matter.
Light, which normally travels the 240,000 miles from the Moon to Earth in less than two
seconds, has been slowed to the speed of a minivan in rush-hour traffic --38 miles an
hour.
An entirely new state of matter, first observed four years ago, has made this possible.
When atoms become packed super-closely together at super-low temperatures and
super-high vacuum, they lose their identity as individual particles and act like a single
super- atom with characteristics similar to a laser.
Such an exotic medium can be engineered to slow a light beam 20 million-fold from
186,282 miles a second to a pokey 38 miles an hour.
"In this odd state of matter, light takes on a more human dimension; you can almost touch
it," says Lene Hau, a Harvard University physicist.
Hau led a team of scientists who did this experiment at the Rowland Institute for Science,
a private, nonprofit research facility in Cambridge, Mass., endowed by Edwin Land, the
inventor of instant photography.
In the future, slowing light could have a number of practical consequences, including the
potential to send data, sound, and pictures in less space and with less power. Also, the
results obtained by Hau's experiment might be used to create new types oflaser projection
systems and night vision cameras with power requirements a million times less than what
is presently possible.
But that's not why Hau, a research scientist at both Harvard and the Rowland Institute,
originally set out to do the experiments. "We did them because we are curious about this
new state of matter, " she says. "We wanted to understand it, to discover all the things that
can be done with it."
Lene Hau, Zachary Dutton, and Chris Behroozi (from left to right) stand by the equipment
they used to create the ultra-high vacuum and super-Iow temperatures with which they
slowed down pulses of light. The process also compresses the pulses from 2,500 feet to
0.002 inches in length.
It took Hau and three colleagues several years to make a container of the new matter.
Then followed a series of 27-hour-long trial runs to get all the parts and parameters
working together.
"So many things have to go right," Hau comments. "But the results finally exceeded our
expectations. It's fascinating to see a beam of light almost come to a standstill. II
Members ofHau's team included Harvard graduate students Zachary Dutton and Cyrus
Behroozi. Steve Harris from Stanford University served as a long-distance collaborator.
Making a Super-atomic Cloud
The idea of this new kind of matter was first proposed in 1924 by Albert Einstein and
Satyendra Nath Hose, an Indian physicist. According to their theory, atoms crowded close
enough in ultra-low temperatures would lock together to form what Hau calls "a single
glob of solid matter which can produce waves that behave like radio waves."
This so-called Bose-Einstein condensate was not actually made unti11995, because the
right technological pot to cook it up in did not exist. Vacuums hundreds of trillions of
times lower than the pressure of air at Earth's surface, and temperatures almost a billion
times colder that that in interstellar space, are needed to produce the condensate.
Temperatures must be lowered to within a few billionths of a degree of absolute zero
(minus 459.7 degrees F), where atoms have the least possible energy and all but cease to
move around.
Hau and her group started with a beam of sodium atoms injected into a vacuum chamber
and moving at speeds of more than a thousand miles an hour. These hot atoms have an
orange glow, like sodium highway and street lights.
Laser beams moving at the normal speed of light collide with the atoms. As the atoms
absorb particles of light (photons), they slow down. The laser light also orders their
random movement so they move in only one direction.
When the atoms are slowed to a modest 100 miles an hour or so, the experimenters load
the atoms into what they call "optical molasses," a web of more laser beams. Each time an atom collides with a photon it is knocked back in the direction from which it came, further
slowing it down, or cooling it.
The atoms are now densely packed in a cigar-shaped clump kept floating free of the walls
of their container by powerful magnetic fields.
"It's nifty to look into the chamber and see the clump of cold atoms floating there, " Hau
remarks.
In the final stage, known as "evaporative cooling," atoms still too hot or energetic are
kicked out of the magnetic field.
The stage is now set for slowing light. One laser is shot across the width of the cloud of
condensate. This controls the speed of a second pulsed laser beam shot along the length of
the cloud. The first laser sets up a "quantum interference" such that the moving light
beams of the second laser interfere with each other. When everything is set up just right,
the light can be slowed by a factor of 20 million.
The process is described in detail in the Feb. 18 issue of the scientific journal Nature.
Relativity and the Internet
Slowing light this way doesn't violate any principle of physics. Einstein's theory of
relativity places an upper, but not lower, limit on the speed of light.
According to relativity theory, an astronaut traveling at close to the speed oflight will not
get old as fast as those she leaves behind on Earth. But driving at 38 miles an hour, as
everyone knows, will not affect anyone's rate of aging.
"However, slowing light can certainly help our understanding of the bizarre state of matter
of a Bose-Einstein condensate," Hau points out.
And a system that changes light speed by a factor of 20 million might be used to improve
communication. It can be used to greatly reduce noise, which allows all types of
information to be transmitted more efficiently. Also, optical switches controlled by low
intensity light could cut power requirements a million-fold compared to switches now
operating everything from telephone equipment to supercomputers.
But what about the cost and exotic equipment needed for such improvements?
"Technologies that push past old limits are always expensive and impractical to begin with;
then they become cheaper and more manageable," Hau says matter-of-factly. She sees the possibility that slow light will lead to "significant advances in communications ten years
from now, if we get to work on it right away. "
What will she do next?
Hau sweeps her hand over a roomful of equipment and explains how things are already
being set up to slow light speed even more, to one centimeter (less than a half-inch) a
second. That's a leisurely 120 feet an hour .
Hau will give a lecture on her experiments at 4:30 p.m. on Monday, Feb. 22, at Room
250, Jefferson Laboratories.
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