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Scientists teleport two different objects



 
 
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  #23  
Old October 12th 06, 05:28 AM posted to sci.space.policy
pete[_1_]
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Posts: 104
Default Scientists teleport two different objects

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.
  #24  
Old October 12th 06, 05:35 AM posted to sci.space.policy
[email protected]
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Posts: 12
Default Scientists teleport two different objects


Alan Anderson wrote:
wrote:

Am I correct in thinking that this technology is FTL, and not subject
to any kind of range restriction or interference/ signal blockage?


The technology is definitely limited by the speed of light. There has
to be a regular communication channel in order to let the receiving end
know what the result of the "measurement" was at the sending end.

[There does seem to be something FTL going on with the entangled states
of the "probe" particles (see Bell's Inequality), but if it's real, it's
apparently something we mere mortals cannot access. Instantaneous
communication of hidden variables is strictly reserved for the universe
itself.]



Ummm, sending the "entangled" photon is definitely "speed of
light"...however...I got a question!!!!

Imagine two pairs (or sets) of black holes...

Could they be positioned in such way that....
Photons can be trapped in perpetual orbit that passes these photons
back and forth between the black holes...

I picture one photon of the entangled pair is put into perpetual orbit
around one pair or set of black holes, the other photon is sent merrily
on its way to its destination 100K LYs "thataway" to be captured by the
second set of black holes. You create a sufficient number of entangled
photon pairs, sending one photon to the one B/H set, the other to the
other B/H set. I ignore the problem of identifying the individual
photon pairs.

But after all photons are orbiting their respective B/Hs, do you have a
FTL signaling system? Can one change the appropriate quantum state of
one entangled photon and have its entangled partner reflect that
change? IOW, are entangled pairs...reuseable?

If so, then 100,000 years later, we could teleport from "here" to
"there" as often as we wished.

............well, it's a thought.......

  #26  
Old October 12th 06, 12:01 PM posted to sci.space.policy
[email protected]
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Posts: 24
Default Scientists teleport two different objects


pete wrote:
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...


{Snip lots of useful book references, thanks.]

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.


Thanks. So...
Particle A collapses, and in doing so forces particle B to collapse. It
might be that they collapse into the same state, or into different
states. Is that right?

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?

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? How is that useful for anything?

My head hurts, I need a beer:
http://www.angryflower.com/schrod.gif

  #27  
Old October 12th 06, 12:32 PM posted to sci.space.policy
Alan Anderson
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Posts: 335
Default Scientists teleport two different objects

wrote:

Particle A collapses, and in doing so forces particle B to collapse. It
might be that they collapse into the same state, or into different
states. Is that right?


Nope. The whole thing depends on the phenomenon of entangled quantum
states, whereby whatever state A ends up in is also what B ends up in.

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.

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.

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.

The basic phenomenon is *already* being used to implement absolutely
secure communication channels -- look up quantum cryptography.
  #28  
Old October 12th 06, 02:33 PM posted to sci.space.policy
[email protected]
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Posts: 24
Default Scientists teleport two different objects


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."

  #29  
Old October 12th 06, 03:46 PM posted to sci.space.policy
[email protected]
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Posts: 24
Default Scientists teleport two different objects


wrote:
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?"


OK, I've just read
http://pass.maths.org.uk/issue35/fea...ert/index.html and
(assuming it's accurate) that has helped a lot. I think I have a handle
on it now. you can pretty much ignore my previous post.

I can also see why entanglement would be useless for FTL comms - The
actual data communicated is random. You could collapse your waveforms
at points A and B simulateously and read the results, and even know (or
at least assume) that you have the exact same results as the person at
the other end, but all you're going to get is a string of utterly
random numbers - you can't stamp your own message on them, which makes
them pretty much useless for communication. The only reason this is
useful for cryptogrophers is that strings of utterly random numbers can
be used as crypto keys.

WRT crypto I also now appreciate that it's not the message that's
transmitted securely, just the key- and once the key is known to be
secure, you can make the (encrypted) message as public as you like.

Still not sure how it helps quantum computers, but maybe I'll be able
to figure it out for myself now that I've got this far.

Thanks for the answers everyone.

BTW, I have to say that now I have a rudimentary grasp of how it works,
I find the term "teleport" somewhat misleading. All that's happened is
that two particles have assigned the same property.

 




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