Alan Anderson wrote:
wrote:

Also, how does the receive location know that the sender has collapsed
the waveforms? Surely the only way to check is to observe the receive
particle, which would collapse it anyway... or is that the purpose of
the classical communications channel?

That's exactly the purpose of the channel, yes. Information about the
result of the sender's measurement is transmitted to the receiver, who
then arranges an interaction which yields a perfect copy of the sender's
particle.

Arranges an interaction... Isn't that kind of like saying "WOW! I've
discovered a way of turning my friend's house blue from the other side
of the world! AlI have to do is paint my own house house blue, then
email my friend and tell him to paint *his* house blue as well! It
works, look at that! It's blue! Isn't that incredible?"

What would happen if the receiver were to arrange an interaction that
should yield a *different* result?

If so, then looking at the receive particle you cannot infer *anything
whatsoever* about what may or may not have happened at the send
location?

You got it. In the typical experiment dealing with Bell's Inequality,
it's only after the two parties compare notes that anything other than
randomness can be seen.

So you're saying that until they compare notes they are each a
Schroedinger's cat to the other?

How is that useful for anything?

With suitable care, it can be used to make remote measurements of
quantum states. That could be part of the "readout" from a quantum
computer.

How can you do a remote measurement of a quantum state if, as
previously agreed, 'you cannot infer anything whatsoever about what may
or may not have happened at the send location?' My brain hurts.

The basic phenomenon is *already* being used to implement absolutely
secure communication channels -- look up quantum cryptography.

I've had a look at the wikipedia article (yeah, I know, wiki isn't
everything) and quickly gotten lost, but I'll keep trying. Also, from
what I can infer, it isn't so much "absolutely secure" as "someone
could be eavesdropping, but if they were you'd know about it."