- When light travels through empty space,
it zips along at a speed of 186,171 miles a second -- the highest speed
anything can attain, even in principle. A moonbeam takes only a little
over one second to reach Earth.
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- But a Danish physicist and her team of
collaborators have found a way to slow light down to about 38 miles an
hour, a speed exceeded by a strong bicyclist.
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- The physics team, headed by Dr. Lene
Vestergaard Hau, who works concurrently at the Rowland Institute for Science
in Cambridge, Mass., and at Harvard University, expects soon to slow the
pace of light still further, to a glacial 120 feet an hour -- about the
speed of a tortoise.
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- "We're getting the speed of light
so low we can almost send a beam into the system, go for a cup of coffee
and return in time to see the light come out," Dr. Hau said in an
interview.
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- The achievement, by Dr. Hau, two Harvard
graduate students and Dr. Steve Harris of Stanford University, is being
reported on Thursday in the journal <onsiteNature. Physicists said it
had many potential uses, not only as a tool for studying a very peculiar
state of matter but also in optical computers, high-speed switches, communications
systems, television displays and night-vision devices.
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- One of the most desirable features of
the apparatus that the researchers built for their work is that it does
not transfer heat energy from the laser light it uses to the ultracold
medium on which the light shines. This could have an important stabilizing
effect on the functioning of optical computers, which operate using photons
of light instead of conventional electrons. A switch using the system could
be made so sensitive that it could be turned on or off by a single photon
of light, Dr. Hau said.
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- The medium Dr. Hau and her colleagues
used in slowing light by a factor of 20 million was a cluster of atoms
called a "Bose-Einstein condensate" chilled to a temperature
of only fifty-billionths of a degree above absolute zero. (Absolute zero
is the temperature at which nothing can be colder. It is minus 273.15 degrees
on the Celsius scale, minus 459.67 on the Fahrenheit scale and zero on
the Kelvin scale.
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- Dr. Hau's group reached an ultralow temperature
in stages, using lasers to slow the atoms in a confined gas and then evaporating
away the warmest remaining atoms. The temperature they attained, one of
the lowest ever reached in a laboratory, was far colder than anything in
nature, including the depths of space.
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- Bose-Einstein condensates (named for
the theorists who predicted their existence, Satyendra Nath Bose and Albert
Einstein) were first prepared in a laboratory four years ago and became
the objects of intense research in the United States and Europe. They owe
their existence to some of the rules of quantum mechanics.
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- One of these is Werner Heisenberg's uncertainty
principle, which states that the more accurately a particle's position
is known, the less accurately its momentum can be determined, and vice
versa.
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- In the case of a Bose-Einstein condensate,
atoms chilled nearly to absolute zero can barely move at all, and their
momentum therefore approaches zero. But because zero is a very precise
measure of momentum, the uncertainty principle makes the positions of these
atoms very uncertain. In a condensate, as a result, such atoms are forced
to overlap each other and merge into superatoms sharing the same quantum
mechanical "wave function," or collection of properties.
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- It was such a superatom, made of a gas
of superpositioned sodium atoms, that provided Dr. Hau and her associates
with the optical molasses they needed to slow light down.
-
- Beginning their project last spring,
the group tuned a "coupling" laser to the resonance of the atoms
in their condensate, shot the laser into the cold cluster of atoms and
thereby created a quantum mechanical system of which both the laser light
and the condensate of atoms were components. At this stage, the system
was no more transparent than a block of lead, Dr. Hau said.
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- The next step was to send a brief pulse
of tuned laser light from a "probe" into the condensate, at a
right angle to the coupling laser, in such a way that the laser-condensate
system interacted with the probe laser. Under these conditions about 25
percent of the probe laser light passed through the "laser-dressed
condensate," but at an astonishingly slow speed.
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- The light that emerged from the apparatus,
not visible to the naked eye, was only 25 percent as strong as the light
that entered, but detectors found that it had roughly the same color.
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- The speed of light is reduced in any
transparent medium, including water, plastic and diamond. Glass prisms
and lenses, for example, slow light by differing amounts that depend on
the thickness of the glass. The slowing of light causes the bending by
which lenses focus images.
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- But the reduction of light speed in a
laser-coupled Bose-Einstein condensate works in an entirely different,
quantum-mechanical way. Not only is the speed brought to a crawl, but the
refractive index of the condensate becomes gigantic.
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- Refractive index is a measure of the
degree to which a medium bends light. The refractive index of the condensate
created by Dr. Hau's group was about 100 trillion times greater than that
of a glass optical fiber.
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- Although Dr. Hau said it might take 10
years before major applications were developed, the huge refractive index
of the condensate, which can be precisely controlled, may make it a basis
for "up shifting" devices that increase the frequencies of light
beams from the infrared end of the spectrum up through visible light to
ultraviolet. Possible applications include ultrasensitive night-vision
glasses and laser light projectors that could create very bright projected
images.
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- Laser-condensate combinations may also
lead to ultrafast optical switching systems useful in computers that would
operate using one light beam to control another light beam. Such a system
could function as an optically switched logic gate, replacing the electronic
logic gates computers now use.
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- Slow light could also be exploited in
filtering noise from optical communications systems, Dr. Hau said.
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- Dr. Jene Golovchenko, a physics professor
at Harvard familiar with Dr. Hau's work, commented, "She has worked
long and hard on this, and now she's really hit a home run."
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- Scientists Slow Down The Speed Of Light
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- The London Telegraph 2-19-99
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- SCIENTISTS have managed to slow down the speed of light
so that it can be overtaken by a bicycle. By passing it through an illuminated
atomic cloud, they have cut the speed of a pulse of yellow laser light
from 186,000 miles per second to 0.01 mile per second and plan to reduce
it further to a crawl of about half an inch a second.
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- This puts the speed of light in the shade when compared
with the world-record cyclist Bruce Bursford, who has clocked up 207 miles
per hour or 0.057 miles per second.
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- The feat is reported today by Dr Lene Hau of the Rowland
Institute for Science, and Harvard University, Cambridge, Massachusetts,
and colleagues. Light is already known to slow down a little when it enters
a piece of glass, because glass has a refractive index which is larger
than that of free space.
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- Dr Hau said: "We are using a much more interesting
mechanism to slow light down by a factor of 20 million." The trick
is to use one light beam to alter the refractive index of an unusual medium
- a cloud of sodium atoms cooled to ultra-low temperatures known as a "Bose-Einstein
condensate" - in such a way that it can slow down a second pulse of
light.
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- The trick has a number of applications, Dr Hau said,
such as converting infra-red light into blue light. "In the future,
this could be of importance for laser light projectors - it is hard to
generate blue light otherwise."
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- Another possible use is in night vision. She said: "This
technique can be used to convert infrared light to the visible spectrum
(so we can see it) at low power cost." The technology could also help
to reduce the noise in communications, and create switches that can control
light. These may be useful in computers that work on light rather than
electricity.
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- Dr Hau said: "These possible applications are of
course for the future - perhaps 10 years down the line if we get to work
on it. Right now we have an experimental set-up where we are pushing technology
to the outermost limit. We'll have to figure out how to make this into
a practical instrument."
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