In 1993 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. In subsequent years, other scientists have demonstrated
teleportation experimentally in a variety of systems, including single photons,
coherent light fields, nuclear spins, and trapped ions. Teleportation
promises to be quite useful as an information processing primitive, facilitating
long range quantum communication (perhaps unltimately leading to a "quantum
internet"), and making it much easier to build a working quantum
computer. 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.
In the past, the idea of teleportation was not taken very 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. By contrast, 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 and links: