Re: Quantum Cryptography can not work

Peter Pearson <ppearson@xxxxxxxxxxxxxxx> writes:

On 25 Feb 2007 03:08:46 -0800, azeltsman2@xxxxxxxxxxx wrote:
[snip]
What if he uses a laser amplifier, making a number of identical
photones from initial one? And then sending one of them to the
receiver?

As I understand it, you cannot clone photons, even with a
laser amplifier. I am not a physicist, but when quantum
cryptography was invented I worked among laser physicists,
and I asked around (and got contradictory answers). Among
much unfamiliar stuff was the notion that the medium of your
laser amplifier inevitably has a nonzero probability of
spontaneous emission, which would preserve the necessary
uncertainty over the state of the original photon.

If I recall correctly, this same anti-cloning principle
must be invoked in analysis of the "EPR Paradox" to prevent
faster-than-light transmission of information. Specifically,
one recipient's uncertainty about the state of the other
recipient's discriminator depends on his getting only one
chance to make a measurement on the photon he receives.

To me, having quantum mechanics and special relativity
rescued from contradiction by a seemingly arbitrary detail
of quantum electrodynamics always smacked of a
deus-ex-machina ending. I hope somebody who really knows
this stuff will post a followup that explains everything.

??? It is NOT and arbitrary detail. It is crucial.
Were it arbitrary you should be able to come up with a situation in which
one could use the entanglement to signal faster than light.

.

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