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>Date: Thu, 18 Jan 2001 09:28:29 EST
>Subject: pho: NYT: Stop Light
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>Quantum computing in your lifetime. Fiber (or other photonic conduits?)
>January 18, 2001
>Scientists Bring Light to Full Stop, Hold It, Then Send It on Its Way
>By JAMES GLANZ
>Researchers say they have slowed light to a dead stop, stored it and then
>released it as if it were an ordinary material particle.
>The achievement is a landmark feat that, by reining in nature's swiftest and
>most ethereal form of energy for the first time, could help realize what are
>now theoretical concepts for vastly increasing the speed of computers and the
>security of communications.
>Two independent teams of physicists have achieved the result, one led by Dr.
>Lene Vestergaard Hau of Harvard University and the Rowland Institute for
>Science in Cambridge, Mass., and the other by Dr. Ronald L. Walsworth and Dr.
>Mikhail D. Lukin of the Harvard-Smithsonian Center for Astrophysics, also in
>Light normally moves through space at 186,000 miles a second. Ordinary
>transparent media like water, glass and crystal slow light slightly, an
>effect that causes the bending of light rays that allows lenses to focus
>images and prisms to produce spectra.
>Using a distantly related but much more powerful effect, the Walsworth-Lukin
>team first slowed and then stopped the light in a medium that consisted of
>specially prepared containers of gas. In this medium, the light became
>fainter and fainter as it slowed and then stopped. By flashing a second light
>through the gas, the team could essentially revive the original beam.
>The beam then left the chamber carrying nearly the same shape, intensity and
>other properties it had when it entered. The experiments led by Dr. Hau
>achieved similar results with closely related techniques.
>"Essentially, the light becomes stuck in the medium, and it can't get out
>until the experimenters say so," said Dr. Seth Lloyd, an associate professor
>of mechanical engineering at the Massachusetts Institute of Technology who is
>familiar with the work.
>Dr. Lloyd added, "Who ever thought that you could make light stand still?"
>He said the work's biggest impact could come in futuristic technologies
>called quantum computing and quantum communication. Both concepts rely
>heavily on the ability of light to carry so-called quantum information,
>involving particles that can exist in many places or states at once.
>Quantum computers could crank through certain operations vastly faster than
>existing machines; quantum commmunications could never be eavesdropped upon.
>For both these systems, light is needed to form large networks of computers.
>But those connections are difficult without temporary storage of light, a
>problem that the new work could help solve.
>A paper by Dr. Walsworth, Dr. Lukin and three collaborators ’Äî Dr. David
>Phillips, Annet Fleischhauer and Dr. Alois Mair, all at Harvard- Smithsonian
>’Äî is scheduled to appear in the Jan. 29 issue of Physical Review Letters.
>Citing restrictions imposed by the journal Nature, where her report is to
>appear, Dr. Hau refused to discuss her work in detail.
>Two years ago, however, Nature published Dr. Hau's description of work in
>which she slowed light to about 38 miles an hour in a system involving beams
>of light shone through a chilled sodium gas.
>Dr. Walsworth and Dr. Lukin mentioned Dr. Hau's new work in their paper,
>saying she achieved her latest results using a similarly chilled gas. Dr.
>Lukin cited her earlier work, which Dr. Hau produced in collaboration with
>Dr. Stephen Harris of Stanford University, as the inspiration for the new
>Those experiments take the next step, stopping the light's propagation
>"We've been able to hold it there and just let it go, and what comes out is t
>he same as what we sent in," Dr. Walsworth said. "So it's like a freeze
>Dr. Walsworth, Dr. Lukin and their team slowed light in a gas form of
>rubidium, an alkaline metal element.
>The deceleration of the light in the rubidium differed in several ways from
>how light slows through an ordinary lens. For one thing, the light dimmed as
>it slowed through the rubidium.
>Another change involved the behavior of atoms in the gas, which developed a
>sort of impression of the slowing wave.
>This impression, actually consisting of patterns in a property of the atoms
>called their spin, was a kind of record of the light's passing and was enough
>to allow the experimenters to revive or reconstitute the original beam.
>Both Dr. Hau's original experiments on slowing light, and the new ones on
>stopping it, rely on a complex phenomenon in certain gases called
>electromagnetically induced transparency, or E.I.T.
>This property allows certain gases, like rubidium, that are normally opaque
>to become transparent when specially treated.
>For example, rubidium would normally absorb the dark red laser light used by
>Dr. Walsworth and his colleagues, because rubidium atoms are easily excited
>by the frequency of that light.
>But by shining a second laser, with a slightly different frequency, through
>the gas, the researchers rendered it transparent.
>The reason is that the two lasers create the sort of "beat frequency" that
>occurs when two tuning forks simultaneously sound slightly different notes.
>The gas does not easily absorb that frequency, so it allows the light to pass
>through it; that is, the gas becomes transparent.
>But another property of the atoms, called their spin, is still sensitive to
>the new frequency. Atoms do not actually spin but the property is a
>quantum-mechanical effect analagous to a tiny bar magnet that can be twisted
>by the light.
>As the light passes through, it alters those spins, in effect flipping them.
>Though the gas remains transparent, the interaction serves as a friction or
>weight on the light, slowing it.
>Using that technique, Dr. Hau and Dr. Harris in the earlier experiment slowed
>light to a crawl. But they could not stop it, because the transparent
>"window" in the gas became increasingly narrower, and more difficult to pass
>through, as the light moved slower and slower.
>In a recent theoretical advance, Dr. Lukin, with Dr. Suzanne Yelin of
>Harvard-Smithsonian and Dr. Michael Fleischhauer of the University of
>Kaiserslautern in Germany, discovered a way around this constraint.
>They suggested waiting for the beam to enter the gas container, then smoothly
>reducing the intensity of the second beam.
>The three physicists calculated that this procedure would narrow the window,
>slowing the first beam, but also "tune" the system so that the beam always
>The first beam, they theorized, should slow to an infinitesimally slow speed,
>finally present only as an imprint on the spins, with no visible light
>remaining. Turning the second beam back on, they speculated, should
>reconstitute the first beam.
>The new experiments bore those ideas out.
>"The light is actually brought to a stop and stored completely in the atoms,"
>Dr. Harris said. "There's no other way to do that. It's been done ’Äî done
>convincingly, and beautifully."
>Copyright 2001 The New York Times Company
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