Quantum teleportation and communication

[Tim May <tcmay@got.net>]

Steve asked in another message about where one learns about some of this
stuff. As always, "Scientific American" is a good place to look for
articles. They've had many articles on Bell's Theorem, nonlocality, the EPR
paradox/nonparadox, the Aharanov-Bohm results, and quantum teleportation. A
SciAm article will explain, with pictures, in ways we can't here. (Plus the
articles are carefully written, by experts--we waste too much time here
getting lost in misunderstandings about the meanings of terms of art.)
"Science News" is sometimes good, too. And "New Scientist" and "Nature."
Any library should have plenty of articles.

There are also dozens of popular books on these subjects, ranging from "In
Search of Schrodinger's Cat" to "Where Does the Weirdness Go?"

And the Web of course has vast resources. Just taking a glance, I see 2000
hits on "quantum teleportation" with HotBot. And 200 hits on "faster than
light communication," for example. Other, more refined searches are trivial
to do. Some of these articles are reviews, some are primers, some are
pointers to more info.


Anyway, back to "quantum weirdness," which, so far, has not looked very
weird. (To me, at least. But, like most physicists, it seemed weird to me
at first but then became "just the way things are" after a few months.)


At 9:20 PM -0800 1/27/98, Steve Schear wrote:

>I don't understand why a command/synch channel is required.  Why aren't the
>coding techniques commonly used in telecom and disk data encoding adequate
>to both synchonize and convey data?

Imagine a pair of photons sent in opposite directions. With different
polarizations, but "tangled." Observer A measures a polarization of "1."

He then knows that Observer B will measure "0."

All that is revealed is a _correlation_, a kind of structure built into the
Universe. Interesting, but not so weird as it seems.  (And this is not any
kind of "action at one site instantaneously changing the state far away."
No more so than sending two envelopes out, one with a "1" inside and the
other with a "0" inside changes things instantaneously.....)

No signal sending is possible because neither observer can "change" the
polarization of a photon. They can certainly pick a sequence of photons to
measure, and thus get various polarization values. But it can't send a
"message" to the other site because the series of photons picked forms the
key, and that key must itself be sent. Similarities to one time pads,
obviously.

More info in the sources named. Avoid the pop science treatments by Nick
Herbert, unless other, more mainstream sources are also consulted at the
same time. And definitely avoid the "psi" nonsense of Jack Sarfatti.

Gribbin's books are pretty good.

--Tim May

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