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:
