Quantum Teleportation
IBM Research:
Quantum Teleportation
NOTES AND COMMENTS: One physicist who re-opened the
discussion on local hidden variables in QM is physicist
David Bohm.
His theory on the implicate order may have been a theory ahead of its
time. Below is an article that can be found on IBM's research WEB
page. Here are some scientists who take the results of the EPR
experiments seriously and feel these experiments
validate QM to the degree that the fantastic sci-fi idea of
teleportation can be accomplished. Read on....
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.
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.
http://www.research.ibm.com/