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
“In this odd state of matter, light takes on a more human
dimension; you can almost touch it,” says Lene Hau, a Harvard
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
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.”
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
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
“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
The process is described in detail in the Feb. 18 issue of the
scientific journal Nature. (Warning: Don’t try this at
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
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.