Re: Quantum communication might be possible?

In article <20060122024635.A521812F383@xxxxxxxxxxxxxxxxxxxx>,
Greg Egan <gregegan@xxxxxxxxxxxxxxx> wrote:
>
>What I am describing is simply the production and transmission of as
>many individual pairs of entangled photons as required to fulfill the
>purpose at hand. The number of bits available is unlimited because the
>number of photon pairs you can create and distribute is unlimited. This
>is not the most efficient scheme, but it's conceptually the simplest, so
>it's worth understanding what's happening here before considering all
>the various permutations and refinements.

Well, er, yes, but ....

The trouble is that doesn't give any advantage over physically
distributing the only two copies of a one-time pad. I am happy
to agree that it is possible - just not that it is useful.

>The crucial point is that an "entangled channel" shouldn't be mistaken
>for some kind of quantum walkie-talkie that you can pump information
>through without having other means of communication. If you give Alice
>and Bob an entangled channel but otherwise isolate them from each other
>completely, they won't be able to communicate anything.

Yes. What I understood the various papers and seminars to mean was
that the "entangled channel" communicated only noise - but that can
then be used to secure a classical channel.

If the best that it is possible to do is to set up a small number
(say < 10^9) of entangled objects, and read them once, then I am
99% certain that the technology is of essentially no practical use.
I.e. 99% of its potential uses are more simply and equally securely
done by using one-time pads stored on memory sticks.


Regards,
Nick Maclaren.

.

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