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Quantum teleportation and communication



Tim May writes:

> Imagine a pair of photons sent in opposite directions. With different
> polarizations, but "tangled." Observer A measures a polarization of "1."
> 
> He then knows that Observer B will measure "0."
> 
> All that is revealed is a _correlation_, a kind of structure built into the
> Universe. Interesting, but not so weird as it seems.  (And this is not any
> kind of "action at one site instantaneously changing the state far away."
> No more so than sending two envelopes out, one with a "1" inside and the
> other with a "0" inside changes things instantaneously.....)
> 
> No signal sending is possible because neither observer can "change" the
> polarization of a photon.

Tim May knows no more about quantum mechanics than he does about
cryptography.  He is wrong about the nature of the correlation between
the two photons.

Bell's theorem shows that if local realism (a technical concept which is
hard to deny) holds, then when Observer A changes the way he performs
his measurements, what Observer B sees does change.  It is completely
mistaken to think of the "two envelope" analogy as applying to quantum
correlations.  There is something much stranger going on here.

Don't trust what Tim May writes.  He is not only a hypocrite (complaining
about chemistry discussions while adding to an equally off-topic physics
thread), but a fool as well.