Quantum
Teleportation
From IBM
1995
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
Related Links:
Quantum
Information Page
 TOP
|
