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Scientists apply brakes to speed of light

February 19, 1999
Web posted at: 3:23 p.m. EST (2023 GMT)

BOSTON (AP) -- There's no practical application yet, but scientists say they've been able to slow the speed of light to a leisurely 38 mph, a pace that would get a highway motorist pulled over for driving too slow.

Light normally moves through a vacuum at about 186,000 miles per second. Nothing in the universe moves faster, and Albert Einstein theorized that nothing ever will. But a Danish physicist and her collaborators trimmed that speed by a factor of 20 million.

"We have really created an optical medium with crazy, bizarre properties," said Lene Vestergaard Hau, whose team accomplished the feat in slowness by shooting a laser through extremely cold sodium atoms, which worked like "optical molasses" to slow the light.

While slow-speed light now is just a laboratory plaything for top physicists, Hau believes practical applications are not too far in the future. She envisions improved communications technology, television displays, even night-vision devices.

The research, conducted at the Rowland Institute for Science in Cambridge and Harvard University and described in Thursday's issue of the journal Nature, isn't something that can be replicated in a home workshop.

The laggard laser moves through a high density group of atoms called a Bose-Einstein condensate, created when matter is cooled almost to absolute zero, the lowest temperature theoretically possible. That is 459.67 degrees below zero.

Now that the scientists have reduced the speed of light to 38 mph, they believe it's possible to slow it 1,000 times further -- to a crawl.

"A human could move faster than that," said Stanford University's Steve Harris, who participated in the project. "But a human couldn't move through a Bose-Einstein condensate, I'll tell you that."

Copyright 1999 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.




Spooky teleportation study brings future closer
October 22, 1998
Top scientific advance of year: Universe will keep expanding
December 18, 1998





The Rowland Institute for Science
Harvard University
Stanford University
INSTITUTE FOR THEORETICAL PHYSICS: Program on Bose Einstein Condensation
Bose Einstein Condensate Links -- Erich Mueller


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"Beam Me Up"

An experiment confirms that teleportation
is possible--at least for photons.


The Innsbruck Experiment


Captain Kirk and his crew do it all the time with the greatest of ease: they discorporate at one point and reappear at another. But this form of travel long has seemed remote to the realm of possibility. Now, however, it turns out that in the strange world of quantum physics, teleportation is not only theoretically possible, it can actually happen.

Image: University of Innsbruck

ANTON ZEILINGER and his coworkers designed the first experiment to verify quantum teleportation.

One group of researchers at the University of Innsbruck in Austria published an account of the first experiment to verify quantum teleportation in the December 11 issue of Nature. And another team headed by Francesco De Martini in Rome has submitted similar evidence to Physical Review Letters for publication. Neither group sent a colleague to Katmandu or a car to the moon. Yet what they did prove is still pretty startling. Anton Zeilinger, De Martini and their colleagues demonstrated independently that it is possible to transfer the properties of one quantum particle (such as a photon) to another--even if the two are at opposite ends of the galaxy.

Until recently, physicists had all but ruled out teleportation, in essence because all particles behave simultaneously like particles and like waves. The trick was this: they presumed that to produce an exact duplicate of any one particle, you would first have to determine both its particlelike properties, such as its position, and its wavelike properties, such as its momentum. And yet doing so would violate the Heisenberg uncertainty principle of quantum mechanics. Under that principle, it is impossible to ever measure wave and particle properties at the same time. The more you learn about one set of characteristics, the less you can say about the other with any real certainty.

Image: Andre Berthiaume

IBM CREW Six researchers--Richard Jozsa, William K. Woolters, Charles H. Bennett(back row, left to right) Gilles Brassard, Claude Crepeau and Asher Peres (front row)--proposed quantum teleportation in 1993.

In 1993, though, an international team of six scientists proposed a way to make an end-run around the uncertainty principle. Their solution was based on a theorem of quantum mechanics dating to the 1930s called the Einstein-Podolsky-Rosen effect. It states that when two particles come into contact with one another, they can become "entangled." In an entangled state, both particles remain part of the same quantum system so that whatever you do to one of them affects the other one in a predictable, domino-like fashion. Thus, the group showed how, in principle, entangled particles might serve as "transporters" of sorts. By introducing a third "message" particle to one of the entangled particles, one could transfer its properties to the other one, without ever measuring those properties.
Bennett's ideas were not verified experimentally until the Innsbruck investigators performed their recent experiment. The researchers produced pairs of entangled photons and showed they could transfer the polarization state from one photon to another.

Teleportation still has one glitch: In the fuzzy realm of quantum mechanics, the result of the transfer is influenced by the receiver's observation of it. (As soon as you look at, say, Bones, he will look like something else.) So someone still has to tell the receiver that the transformation has been made so that they can correctly interpret what they see. And this sort of communication cannot occur at faster-than-light speeds. Even so, the scheme has definite applications in ultrafast quantum computers and in utilizing quantum phenomena to ensure secure data transmission [see QUANTUM CRYPTOGRAPHY, Charles H. Bennett, Scientific American, October 1992].

For now, though, it will be a long time before a real Scotty beams up a living Captain Kirk.

--By Alan Hall, contributing writer


Quantum teleportation at the University of Innsbruck

Download copies of Innsbruck journal articles

Quantum research at IBM

Quantum information from Los Alamos National Laboratory





Quantum Teleportation


Teleportation is the name given by science fiction writers to the feat of making an object or person disintegrate in one place while a perfect replica appears somewhere else. How this is accomplished is usually not explained in detail, but the general idea seems to be that the original object is scanned in such a way as to extract all the information from it, then this information is transmitted to the receiving location and used to construct the replica, not necessarily from the actual material of the original, but perhaps from atoms of the same kinds, arranged in exactly the same pattern as the original. A teleportation machine would be like a fax machine, except that it would work on 3-dimensional objects as well as documents, it would produce an exact copy rather than an approximate facsimile, and it would destroy the original in the process of scanning it. A few science fiction writers consider teleporters that preserve the original, and the plot gets complicated when the original and teleported versions of the same person meet; but the more common kind of teleporter destroys the original, functioning as a super transportation device, not as a perfect replicator of souls and bodies.


Two years ago an international group of six scientists, including IBM Fellow Charles H. Bennett, confirmed the intuitions of the majority of science fiction writers by showing that perfect teleportation is indeed possible in principle, but only if the original is destroyed. Meanwhile, other scientists are planning experiments to demonstrate teleportation in microscopic objects, such as single atoms or photons, in the next few years. But science fiction fans will be disappointed to learn that no one expects to be able to teleport people or other macroscopic objects in the foreseeable future, for a variety of engineering reasons, even though it would not violate any fundamental law to do so.

Until recently, teleportation was not taken seriously by scientists, because it was thought to violate the uncertainty principle of quantum mechanics, which forbids any measuring or scanning process from extracting all the information in an atom or other object. According to the uncertainty principle, the more accurately an object is scanned, the more it is disturbed by the scanning process, until one reaches a point where the object's original state has been completely disrupted, still without having extracted enough information to make a perfect replica. This sounds like a solid argument against teleportation: if one cannot extract enough information from an object to make a perfect copy, it would seem that a perfect copy cannot be made. But the six scientists found a way to make an end-run around this logic, using a celebrated and paradoxical feature of quantum mechanics known as the Einstein-Podolsky-Rosen effect. In brief, they found a way to scan out part of the information from an object A, which one wishes to teleport, while causing the remaining, unscanned, part of the information to pass, via the Einstein-Podolsky-Rosen effect, into another object C which has

never been in contact with A. Later, by applying to C a treatment depending on the scanned-out information, it is possible to maneuver C into exactly the same state as A was in before it was scanned. A itself is no longer in that state, having been thoroughly disrupted by the scanning, so what has been achieved is teleportation, not replication.

As the figure to the left suggests, the unscanned part of the information is conveyed from A to C by an intermediary object B, which interacts first with C and then with A. What? Can it really be correct to say "first with C and then with A"? Surely, in order to convey something from A to C, the delivery vehicle must visit A before C, not the other way around. But there is a subtle, unscannable kind of information that, unlike any material cargo, and even unlike ordinary information, can indeed be delivered in such a backward fashion. This subtle kind of information, also called "Einstein-Podolsky-Rosen (EPR) correlation" or "entanglement", has been at least partly understood since the 1930s when it was discussed in a famous paper by Albert Einstein, Boris Podolsky, and Nathan Rosen. In the 1960s John Bell showed that a pair of entangled particles, which were once in contact but later move too far apart to interact directly, can exhibit individually random behavior that is too strongly correlated to be explained by classical statistics. Experiments on photons and other particles have repeatedly confirmed these correlations, thereby providing strong evidence for the validity of quantum mechanics, which neatly explains them. Another well-known fact about EPR correlations is that they cannot by themselves deliver a meaningful and controllable message. It was thought that their only usefulness was in proving the validity of quantum mechanics. But now it is known that, through the phenomenon of quantum teleportation, they can deliver exactly that part of the information in an object which is too delicate to be scanned out and delivered by conventional methods.


This figure compares conventional facsimile transmission with quantum teleportation (see above). In conventional facsimile transmission the original is scanned, extracting partial information about it, but remains more or less intact after the scanning process. The scanned information is sent to the receiving station, where it is imprinted on some raw material (eg paper) to produce an approximate copy of the original. In quantum teleportation two objects B and C are first brought into contact and then separated. Object B is taken to the sending station, while object C is taken to the receiving station. At the sending station object B is scanned together with the original object A which one wishes to teleport, yielding some information and totally disrupting the state of A and B. The scanned information is sent to the receiving station, where it is used to select one of several treatments to be applied to object C, thereby putting C into an exact replica of the former state of A.

To learn more about quantum teleportation, see the following articles:

C.H. Bennett, G. Brassard, C. Crepeau, R. Jozsa, A. Peres, and W. Wootters, "Teleporting an Unknown Quantum State via Dual Classical and EPR Channels", Phys. Rev. Lett. vol. 70, pp 1895-1899 (1993)
(the original 6-author research article).
Tony Sudbury, "Instant Teleportation", Nature vol.362, pp 586-587 (1993) (a semipopular account).
Ivars Peterson, Science News, April 10, 1993, p. 229. (another semipopular account).
Samuel Braunstein, A fun talk on teleportation

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