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Newsgroups: sci.crypt
Subject: Re: Quantum cryptography: a flawed premise?
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References: <141@lorien.OCF.LLNL.GOV>
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In article <141@lorien.OCF.LLNL.GOV> pearson6@llnl.gov writes:
>Recent exciting and ingenious articles on "quantum cryptography" [1,2]
>seem to bring this interesting field close to practical fruition.
>One thing bothers me, though, and I wonder if it bothers anybody else.
>Explanations of the technique's strength rely heavily on the
>fact that measurements made on a single photon cannot possibly give
>a complete knowledge of its original state of polarization.
>While this basic fact is true, the articles seem not to consider the
>possibility of "amplifying" the photon into a large number of identically
>polarized photons, and then making many independent measurements on those
>duplicate photons, from which the original state of polarization can
>indeed be deduced with the necessary precision.

You quite correcly state that photons can be "amplified" or "cloned" 
in the process of stimulated emission.  Stimulated photons posess
all the properties of the original photon, including phase and
polarization information.  One would think, classically, that one
could perform different measurements on each of the cloned photons,
put all thte results together, and say with precision all the 
properties of the original photon.

Quantum mechanics doesn't let you do this, however.  In stimulated
emission, the incident and stimulated photons have the same wave 
function, so a measurement on one photon can destroy information 
and randomize behavior of all the "clones" as well, no matter where 
they are.

The fact that this happens simultaneously over distances appears to
contradict either relativity or causality or both, and was noted by
Einstein, Podolsky and Rosen as a \"paradoxical" prediction and
thus a flaw in quantum theory.  Experiments have proved the effect real,
however, and it has become known as the EPR effect.

(To further digress, the philosophical implications of this "spooky
action at a distance continue to be debated.  See Richard A. Healey`s
article in the June `92 Philosophy of Science.)

In article <67497@cup.portal.com> Hal_-_Finney@cup.portal.com writes:
>You can't clone photons.  If you could, faster-than-light communication
>would be possible through the Bell's theorem experiment.

Strangely, this doesn't seem to be true.  I never quite understood the 
proof, but apparently, no "information" is passed between the
photons, even though their future behavior is altered by tests on
other photons.  The effect cannot be exploited for faster-than-light
communication.  I'm sure someone on sci.physics could give a decent
explanation, but I don't have any citations handy.

Joe Thomas
jthomas@access.digex.com