on 10 Oct 2006 02:35:28 -0700, sez:

` Wayne Throop wrote:
` :
` : Am I correct in thinking that this technology is FTL, and not subject
` : to any kind of range restriction or interference/ signal blockage?
` :
` : Can we therefore imagine that some future interplanetary mission might
` : take with it a lump of entangled particles (having left the entangled
` : 'mates' of those particles back on Earth), allowing the spacecraft and
` : mission control to exchange data instantaneously, no matter how far
` : away the craft travels?
`
` No. Or rather, you can imagine it (and, eg, Stross did in Singluarity
` Sky and Iron Sunrise), but there's not much of a justification for it
` in the theory and practice of quantum so-called-teleportation.
`
` Because, in order to accomplish the so-called-teleportation, you
` must 1) make a measurement at the souce, 2) send a message about what
` you found to to the destination, and finally you can 3) cause the
` "quantum state" to "teleport". Note specifically step 2.

` Oh, OK thanks. Does anyone know of a decent, layman-friendly
` explanation of how this works? Even the wikipedia article on the
` subject is full of incomprehensible heiroglyphics...

There are masses of books and articles on Bell's Inequality, and
a lesser number on entanglement and Sneaky Entanglement Tricks.
For lucidity, I would recommend first Barnard D'Espagnat's article
in SciAm on Bell's Inequality, I believe it was something like
Nov. 1979. Then there is David Mermin's article in Physics Today,
which I think was reprinted in a book somewhere. My favourite
book on the subject is David Albert's "Quantum Mechanics and
Experience". The ideas are mathematically easy in the sense
that you require no knowledge of QM to understand the main
concepts, but the logical implications are quite profound, and
you really have to be awake to keep up.

"Quantum Teleportation" is basically an extension of the apparent
remote influence demonstrated by Bell's Inequality, but it is
a more recent activity, and there is much less written so far.
A friend of mine is currently reading a book called "Entanglement",
which he is enjoying (don't know the author), but he has no
physics background (yet), and I haven't seen it so I can't say
more about it.

In sloppy pop-QM terminology, the idea behind the teleportation
is to "collapse the wave function" of one half of the entangled
set at the "sending" location, which thereby "instantaneously"*
sets the state of the other half of the entangled set. However,
the state at collapse can't be controlled, it can only be set
by the act of observation, and the outcome is QM-random, so
the state of the system at the remote location also takes on
its new state randomly, and thus no information about the "send"
location can be extracted from the collapse event.

*I think it is more accurate there to say "cause to have always
been", essentially choosing one of the Everett-Wheeler "many
worlds". As you can see, thinking of it that way gets away
from the notion that you are doing something superluminally.
You can hum along to Steely Dan's "Pretzel Logic" while you
study this stuff...


--
================================================== ========================
Pete Vincent
Disclaimer: all I know I learned from reading Usenet.