Bob writes:
Does anyone know if the new in-line optical amplifiers (not switches!) have any effect on quantum crypto messages?
Yes, any active devices in your communications path would be unable to function without making some kind of classical measurement on the photons involved (e.g. measuring phase relative to a definite test angle, if phase is what's being modulated), thereby collapsing the wavefunction and spoiling any special properties afforded by being able to send photons down the line without "looking at them." Optical repeaters have to pass your signal through an intermediate electronic stage anyway, since we have no purely optical valve/transistor equivalents (bosons don't interact with each other at all).
I am not sure this is correct. In-line optical amplifiers work by stimulated emission like a laser rod. There is no intermediate electronic stage. The amplifier is a section of fibre that is doped with a rare earth element. The rare earth atoms are boosted into a metastable high energy state using a power source around the fibre. Passing signals (photons) stimulate the decays of the metastable states releasing more photons and boosting the signal. I am not sure what the effect of this on the polarisation characteristics of the signal is but my *hunch* is that the polarisation characteristics would be preserved in the amplified signal. Sherry ps Any laser physicists in the house?