Science News - article on Quantum Crypto

[Sherry Mayo]

> 
> 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?