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Scientists teleport two different objects
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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
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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....... |
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Scientists teleport two different objects
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#26
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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 |
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Scientists teleport two different objects
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#28
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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
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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. |
#30
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Scientists teleport two different objects
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