Re: Optical repeaters

[tcmay@got.net (Timothy C. May)]

At 12:45 AM 2/20/96, Alexandra Griffin wrote:

(about inline optical amplifiers/regenerators)

>You're right, I do remember reading about these somewhere... didn't
>realize they were already in use.
>
>Even so, I still don't think such a repeater would pass quantum-crypto
>signals, excepting any photons that happened to just "leak" directly
>through.  Your useful quantum state information resides in the
>individual photons originally sent, and any even the optical repeaters
>you describe achieve gain by by gating in *more* photons under the
>incoming signal's control.  In so doing it will collapse the
>wavefunctions of these incoming photons.

Given that a quantum cryptography system depends on *single photons* to
work, I'm not sure that talk of amplifification makes sense. Between source
and receiver, a photon either makes it or doesn't. If it makes it, it makes
it will its full "quanta" of energy, of course. If it doesn't make it, due
to tunnel-penetrating the walls of the fiber or scattering off an impurity
in the fiber, then it just doesn't make it, so no
amplification/regeneration is possible. (Regenerate _what_?)

But the quantum measurement issue, aside from the above, is an interesting one.

We have to be careful here (and I'm including myself, not just using the
royal "we"). It isn't clear to me that the amplification/regeneration
process counts as making a measurement, from some recent work I've read
(sorry, don't recall the references, but could be a recent issue of
"Scientific American"). In interference measurements, the wave function
collapses if individual photons are counted and recorded (whatever
"recorded" really means...) and the interference pattern vanishes. If the
photons are not counted and/or recorded, the pattern reappears.

By analogy, it is not clear to me that a simple regeneration mechanism,
with no local observer or recording apparatus, will collapse the wave
function. Seems to me an experiment may have already been done along these
lines: separate fibers producing an interference pattern and then these
inline amps added...if the interference pattern remains, as I would expect,
then the amps/regenerators did not constitute a "measurement" in QM terms.


--Tim May

Boycott espionage-enabled software!
We got computers, we're tapping phone lines, we know that that ain't allowed.
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