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quote: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 Cambridge.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 — Dr. David Phillips, Annet Fleischhauer and Dr. Alois Mair, all at Harvard- Smithsonian — 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 experiments.Those experiments take the next step, stopping the light's propagation completely."We've been able to hold it there and just let it go, and what comes out is the same as what we sent in," Dr. Walsworth said. "So it's like a freeze frame."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 passes through.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 — done very convincingly, and beautifully."
quote:In the past few years, scientists have found ways to make light go both faster and slower than its usual speed limit, but now researchers at the University of Rochester have published a paper today in Science on how they've gone one step further: pushing light into reverse. As if to defy common sense, the backward-moving pulse of light travels faster than light.Confused? You're not alone."I've had some of the world's experts scratching their heads over this one," says Robert Boyd, the M. Parker Givens Professor of Optics at the University of Rochester. "Theory predicted that we could send light backwards, but nobody knew if the theory would hold up or even if it could be observed in laboratory conditions."Boyd recently showed how he can slow down a pulse of light to slower than an airplane, or speed it up faster than its breakneck pace, using exotic techniques and materials. But he's now taken what was once just a mathematical oddity—negative speed—and shown it working in the real world."It's weird stuff," says Boyd. "We sent a pulse through an optical fiber, and before its peak even entered the fiber, it was exiting the other end. Through experiments we were able to see that the pulse inside the fiber was actually moving backward, linking the input and output pulses."So, wouldn't Einstein shake a finger at all these strange goings-on? After all, this seems to violate Einstein's sacred tenet that nothing can travel faster than the speed of light."Einstein said information can't travel faster than light, and in this case, as with all fast-light experiments, no information is truly moving faster than light," says Boyd. "The pulse of light is shaped like a hump with a peak and long leading and trailing edges. The leading edge carries with it all the information about the pulse and enters the fiber first. By the time the peak enters the fiber, the leading edge is already well ahead, exiting. From the information in that leading edge, the fiber essentially 'reconstructs' the pulse at the far end, sending one version out the fiber, and another backward toward the beginning of the fiber."Boyd is already working on ways to see what will happen if he can design a pulse without a leading edge. Einstein says the entire faster-than-light and reverse-light phenomena will disappear. Boyd is eager to put Einstein to the test.So How Does Light Go Backwards?Boyd, along with Rochester graduate students George M. Gehring and Aaron Schweinsberg, and undergraduates Christopher Barsi of Manhattan College and Natalie Kostinski of the University of Michigan, sent a burst of laser light through an optical fiber that had been laced with the element erbium. As the pulse exited the laser, it was split into two. One pulse went into the erbium fiber and the second traveled along undisturbed as a reference. The peak of the pulse emerged from the other end of the fiber before the peak entered the front of the fiber, and well ahead of the peak of the reference pulse.But to find out if the pulse was truly traveling backward within the fiber, Boyd and his students had to cut back the fiber every few inches and re-measure the pulse peaks when they exited each pared-back section of the fiber. By arranging that data and playing it back in a time sequence, Boyd was able to depict, for the first time, that the pulse of light was moving backward within the fiber.To understand how light's speed can be manipulated, think of a funhouse mirror that makes you look fatter. As you first walk by the mirror, you look normal, but as you pass the curved portion in the center, your reflection stretches, with the far edge seeming to leap ahead of you (the reference walker) for a moment. In the same way, a pulse of light fired through special materials moves at normal speed until it hits the substance, where it is stretched out to reach and exit the material's other side Conversely, if the funhouse mirror were the kind that made you look skinny, your reflection would appear to suddenly squish together, with the leading edge of your reflection slowing as you passed the curved section. Similarly, a light pulse can be made to contract and slow inside a material, exiting the other side much later than it naturally would .To visualize Boyd's reverse-traveling light pulse, replace the mirror with a big-screen TV and video camera. As you may have noticed when passing such a display in an electronics store window, as you walk past the camera, your on-screen image appears on the far side of the TV. It walks toward you, passes you in the middle, and continues moving in the opposite direction until it exits the other side of the screen.A negative-speed pulse of light acts much the same way. As the pulse enters the material, a second pulse appears on the far end of the fiber and flows backward. The reversed pulse not only propagates backward, but it releases a forward pulse out the far end of the fiber. In this way, the pulse that enters the front of the fiber appears out the end almost instantly, apparently traveling faster than the regular speed of light. To use the TV analogy again—it's as if you walked by the shop window, saw your image stepping toward you from the opposite edge of the TV screen, and that TV image of you created a clone at that far edge, walking in the same direction as you, several paces ahead ."I know this all sounds weird, but this is the way the world works," says Boyd.