Re: Quantum Cryptography can not work

Mike Amling wrote:

Peter Pearson wrote:
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.

The probability of emission into a given state is proportional to 1
plus the number of photons already in that state, if I recall Einstein's
formula correctly. So, the amplifier is more likely to emit a clone of
the input photon than a photon in any other state.


Quantum crypto interception relies on the cloning of single photons, not
where there are many photons in the same state.

The maximum probability that a single photon will be cloned is 5/6 (or more
accurately that the output photons will have the same polarisation as the
original - up to 2/3 of the photons can be cloned, and of the remaining 1/3
half will have the same polarisation at random).

This is a firm physical limit due to the nature of the universe. Figures
approaching this limit can be achieved for single photons in practice (with
expensive equipment).

Unfortunately you can't tell whether the polarisation of the original photon
has been changed by the cloning process in the remaining 1/3 of cases, so
you can't just try to repeatedly clone it several times. It may also be hard
to tell which photon is which, depending on setup.

At least that's what I've heard. I don't really understand it all that well,
bra-ket notation leaves me by :(

Quantum crypto (or at least good quantum crypto) takes this 5/6 into
account.



BTW most varieties of operational QC do not involve entanglement, although
some may do.




--
Peter Fairbrother

.

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