Lene Hau has already shaken scientists' beliefs about the nature of
things. Albert Einstein and just about every other physicist insisted
that light travels 186,000 miles a second in free space, and that it
can't be speeded-up or slowed down. But in 1998, Hau, for the first
time in history, slowed light to 38 miles an hour, about the speed of
rush-hour traffic.
Two years later, she brought light to a complete halt in a cloud of
ultracold atoms. Next, she restarted the stalled light without changing
any of its characteristics, and sent it on its way. These highly
successful experiments brought her a tenured professorship at Harvard
University and a $500,000 MacArthur Foundation award to spend as she
pleased.
Now Mallinckrodt Professor of Physics and of Applied Physics, Hau
has done it again. She and her team made a light pulse disappear from
one cold cloud then retrieved it from another cloud nearby. In the
process, light was converted into matter then back into light. For the
first time in history, this gives science a way to control light with
matter and vice versa.
It's a thing that most scientists never thought was possible. Some
colleagues had asked Hau, "Why try that experiment? It can't be done."
In the experiment, a light pulse was slowed to bicycle speed by
beaming it into a cold cloud of atoms. The light made a "fingerprint"
of itself in the atoms before the experimenters turned it off. Then Hau
and her assistants guided that fingerprint into a second clump of cold
atoms. And get this - the clumps were not touching and no light passed
between them.
"The two atom clouds were separated and had never seen each other
before," Hau notes. They were eight-thousandths of an inch apart, a
relatively huge distance on the scale of atoms.
The experimenters then nudged the second cloud of atoms with a laser
beam, and the atomic imprint was revived as a light pulse. The revived
light had all the characteristics present when it entered the first
cloud of atomic matter, the same shape and wavelength. The restored
light exited the cloud slowly then quickly sped up to its normal
186,000 miles a second.
Communicating by light
Light carries information, so think of information being
manipulated in ways that have never before been possible. That
information can be stored - put on a shelf, so to speak - retrieved at
will, and converted back to light. The retrieved light would contain
the same information as the original light, without so much as a period
being lost.
Or the information could be changed. "The light waves can be
sculpted," is the way Hau puts it. "Then it can be passed on. We have
already observed such re-sculpted light in our lab."
A weird thing happens to the light as it enters the cold atomic
cloud, called a Bose-Einstein condensate. It becomes squeezed into a
space 50 million times smaller. Imagine a light beam 3,200 feet (one
kilometer) long, loaded with information, that now is only a hair width
in length but still encodes as much information.
From there it becomes easier to imagine new types of computers and
communications systems - smaller, faster, more reliable, and
tamper-proof.
Atoms at room temperature move in a random, chaotic way. But when
chilled in a vacuum to about 460 degrees below zero Fahrenheit, under
certain conditions millions of atoms lock together and behave as a
single mass. When a laser beam enters such a condensate, the light
leaves an imprint on a portion of the atoms. That imprint moves like a
wave through the cloud and exits at a speed of about 700 feet per hour.
This wave of matter will keep going and enter another nearby ultracold
condensate. That's how light moves darkly from one cloud to another in
Hau's laboratory.
This invisible wave of matter keeps going unless it's stopped in
the second cloud with another laser beam, after which it can be revived
as light again.
Atoms in matter waves exist in slightly different energy levels and
states than atoms in the clouds they move through. These energy states
match the shape and phase of the original light pulse. To make a long
story short, information in this form can be made absolutely tamper
proof. Personal information would be perfectly safe.
Such a light-to-matter, matter-to-light system "is a wonderful thing to wrap your brain around," Hau muses.
Details of the experiments appear as the cover story of the Feb. 8
issue of Nature. Authors of the report include graduate student Naomi
Ginsberg, postdoctoral fellow Sean Garner, and Hau.
In a practical manner
You won't see a light-matter converter flashing away in a factory,
business, or mall anytime soon. Despite all the intriguing
possibilities, "there are no immediate practical uses," Hau admits.
However, she has no doubt that practical systems will come. And
when they do, they will look completely different from anything we are
familiar with today. They won't need a lot of wires and electronics.
"Instead of light shining through optical fibers into boxes full of
wires and semiconductor chips, intact data, messages, and images will
be read directly from the light," Hau imagines.
Creating those ultracold atomic clouds in a factory, office, or
recreation room will be a problem, but one she believes can be solved.
"The atomic clouds we use in our lab are only a tenth of a millimeter
(0.004 inch) long," she points out. "Such atom clouds can be kept in
small containers, not all of the equipment has to be so cold. Most
likely, a practical system designed by engineers will look totally
unlike the setup we have in our lab today."
There are no "maybes" in Hau's voice. She is coolly confident that
light-to-matter communication networks, codes, clocks, and guidance
systems can be made part of daily life. If you doubt her, remember she
is the person who stopped light, converted it to matter, carried it
around, and transformed it back to light.
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