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. ---------:---------:---------:---------:---------:---------:---------:---- Timothy C. May | Crypto Anarchy: encryption, digital money, tcmay@got.net 408-728-0152 | anonymous networks, digital pseudonyms, zero W.A.S.T.E.: Corralitos, CA | knowledge, reputations, information markets, Higher Power: 2^756839 - 1 | black markets, collapse of governments. "National borders aren't even speed bumps on the information superhighway."