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HARVARD GAZETTE ARCHIVES
http://www.news.harvard.edu/gazette/1999/02.18/light.html
Physicists Slow Speed of Light

By William J. Cromie

Gazette Staff


Lene Hau has shed new light on a new form of matter. Photo by MaryAnn
Nilsson.

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 of laser
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."

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."


Lene Hau, Zachary Dutton, and Cyrus Behroozi (from left to right) stand by
the equipment they used to create the ultra-high vacuum and super-low
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. Photo by
MaryAnn Nilsson.

Members of Hau'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 Bose, 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 until 1995,
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. (Warning: Don't try this at home.)

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 of light 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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