![[ IMAGE: Quantum Teleportation ]](teleport1.gif)
Millions of us have seen the famous Star Trek series, which enjoyed a cult fan following because of its uniqueness in more ways than one. We watched in fascination as Captain James T. Kirk transported himself from a nearby planet to his spaceship by saying all but 4 words: "Beam me up Scotty!!" This phenomenon of "disassembling" Captain Kirk and "reassembling" him at another place is referred to in scientific terms as Teleportation. 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. However, it turns out that in the strange world of quantum physics, teleportation is not only theoretically possible, it can actually happen.
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.
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.
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.
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 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.
![[ IMAGE: Classical Facsimile Transmission ]](teleport2.gif)
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. 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.
This page last modified on
16/03/2002
Contact: karthik_abhiram@yahoo.com
[Home]