SCI: slow light

Ken Kittlitz (
Thu, 18 Feb 1999 10:47:32 -0700

>From: Jason McVean <>
>To: "''" <>
>Subject: dicklit: slow light
>Date: Thu, 18 Feb 1999 10:33:12 -0700
>X-Mailer: Internet Mail Service (5.0.1460.8)
>From the NY Times on the web:
>February 18, 1999
> In a Major Breakthrough, Danish Physicist Slows the
> Speed of Light
> 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.
> 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.
> 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.
> "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.
> The achievement, by Dr. Hau, two Harvard graduate students and Dr.
>Steve Harris of Stanford
> University, is being reported on Thursday in the journal Nature.
>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.
> 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.
> 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.
> 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.
> 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.
> 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.
> 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.
> 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.
> 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.
> 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.
> 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.
> 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.
> 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.
> 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
> 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.
> Slow light could also be exploited in filtering noise from optical
>communications systems, Dr. Hau
> said.
> 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."

Ken Kittlitz			Administrator, Foresight Exchange
AudeSi Technologies Inc.		personal: