John Denker wrote:
> At 02:04 PM 1/18/01 -0800, [EMAIL PROTECTED] wrote:
> >  I don't think she could learn much with a single photon,
> I'm not so sure about that.  Remember, photon counters (which measure 
> A_dagger A) are not the only measuring devices in the world.  There are 
> also voltmeters (which measure A_dagger plus A).  For low-amplitude analog 
> signals, the voltmeter is vastly more informative.  I have not yet cobbled 
> up a believable apparatus for measuring the polarization angle of a single 
> photon, but I don't think it would be terribly hard to do so.

I'd like to hear more about how this is possible in theory, even without
regard to issues of practicality.  I know you mentioned EPR entangled
photons but I don't see how you could use them.  (Actually I did come
up with way, but it would also have allowed FTL communications so I'm
pretty sure it wouldn't have worked...)

With the voltage meter, I think what you're measuring is essentially
energy, which is a continuous Hilbert space, so one measurement can
provide a lot of information.  But photon polarization is a two basis
state system so I don't see how one measurement can give you much.

If Eve really can accurately determine the rotation angle by passing
just a single photon through the system that really shoots down the SC
Net idea.


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