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Great question! This gets to the heart of why quantum teleportation has any value at all. So, before QM there was already a sense in which you could teleport a
by pontus 6y ago
Great question! This gets to the heart of why quantum teleportation has any value at all.
So, before QM there was already a sense in which you could teleport an object: simply measure its state perfectly and send that information to another location and have them reconstruct that state particle by particle. In principle the new system would be indistinguishable from the original system and you could claim that you've teleported it. Now, with the discovery of quantum mechanics, this process no longer works because there is no way to measure the complete state of a quantum system. For example, you could measure the position of each particle to arbitrary accuracy or you can measure the speed of every particle to arbitrary position, but you can't do both (Heisenberg's uncertainty principle). So, it would seem like one could not construct a perfect replica of a quantum system in a new location by measuring its state in the original position.
The cleverness of quantum teleportation is that you use entanglement to sort of short circuit this limitation. You let entanglement do the heavy lifting to sort of "copy" the state from one location to another and then perform a measurement in the original location to uncover just enough information so that the person in the second location can manipulate its system to reconstruct the original state.
It's sort of like reconstructing the state without actually knowing what that state is.
Now, an interesting side effect of the quantum version is that the first measurement of the original system is necessarily destructive. As such, it's not like you'll end up with two copies of the same thing (which is what would happen in the classical version) so there's no discussion necessary around the distinction between teleportation and cloning. Classically you'd be cloning the system but quantum mechanically you'd really truly be teleporting it (in fact, there's a result in quantum mechanics called "the no cloning theorem" that proves that cloning in QM is impossible).
- sebmellen 6y agoVery interesting. If I understand correctly, does this mean cloning is a functionally impossible task? I've always been entertained by the paradoxes where someone is teleported ala Michael Crichton's Timeline, and is then (due to some glitch) "duplicated", leading to interesting quandaries about "who is who". If Penrose is right about consciousness [0], this means all these fantastical paradoxes are just fantasy, right? [0]: https://bigthink.com/paul-ratner/why-a-genius-scientist-thinks-our-consciousness-originates-at-the-quantum-level https://bigthink.com/paul-ratner/why-a-genius-scientist-thin...
- pontus 6y agoCorrect, cloning in even the simplest sense of a single particle state (let alone a whole human) is impossible in quantum mechanics.
- tomrod 6y agoIt sounds like state send is partial using the "phone" analogy. Can there be triplicates of entanglement?
- vez- 6y agoYup https://en.wikipedia.org/wiki/W_state https://en.wikipedia.org/wiki/W_state
- pontus 6y agoThere are limits to how much three systems can be entangled with each other. It turns out that one system cannot be maximally entangled with two different systems. In fact, these types of arguments are often used in studying information paradoxes in black holes.
- FeepingCreature 6y agoThough cloning a human may be possible if the brain does not rely on quantum state. Obligatory "warm and wet" objection.
- comfyinnernet 6y agoI don't think you would need exact duplication for these situations to arise.
- chrisweekly 6y agoHey yeah I remember reading Penrose's "The Emperor's New Mind" in maybe 1998?, an interesting take on the nature of consciousness... fascinating stuff.
- drdeca 6y agoIndeed there is a “no cloning theorem”. Iirc it essentially comes from the fact that time evolution is linear, and a function that sends stateOfIntetest \otimes constantState to stateOfInterest \otimes stateOfInterest for all values of stateOfInterest would be quadratic?
- mrtesthah 6y agoIt sounds like start trek’s transporter beams may be more accurately described than we thought.
- maxerickson 6y agoThey repeatedly treat the pattern like data. Riker splits, Tuvix!, the one where the other doctor ages and gets de-aged in the transporter, the one where Scotty is stored in it for decades, etc.
- Snitch-Thursday 6y agoThat's exactly what I was thinking! Running far too much into fandon, this holds also for the fictional 'pattern buffer'. It has to examine the original AND communicate that state back to the duplicate quick enough or the 'clone' will be inaccurate, maybe even enough to kill someone a la Star Trek: The Motion Picture.
- freeflight 6y agoIt's also where the meme comes from how Star Trek transporers are actually suicide booths [0]. [0] https://arstechnica.com/gaming/2017/09/is-beaming-down-in-star-trek-a-death-sentence/ https://arstechnica.com/gaming/2017/09/is-beaming-down-in-st...
- edge17 6y agoMaybe I'm dense, but I still don't understand. The cloning explanation made sense to me, but to the original question - how does the recipient know the message is done being sent without the sender picking up the phone and calling the recipient...? i.e. Is there some equivalent of a termination code/header sort of thing that the recipient is looking for in the 'bit stream' or whatever? Or am I not even thinking about this in terms of the right analogy? edit: Thank btw, this comment was fascinating to me.
- pontus 6y agoThe receiver does not know that a message has been sent until the first person contacts them classically. It's a common mistake to think that quantum teleportation is a new way of sending information. It's really a way to use classical communication in order to leverage entanglement to bypass various limitations of quantum mechanics. So, the two people communicating would e.g. start out together and create a pair of entangled systems A and B. The person in possession of system B would then travel far away. The person in possession of system A then decides that they want to teleport a new system C to the person far away. They do this by placing system C next to system A and then performing a measurement on the combined system A+C causing these two states to become entangled. We now have an implicit entanglement between system C and system B that is far away. The person in possession of system A+C now picks up the phone and calls the other person to tell them what the outcome of their measurement on A+C was. The person far away then uses this information to determine a way to manipulate their state B in a certain way (the particular way in which they need to do this depends on the outcome of the measurement of A+C). Once that manipulation is complete, the system they have in their possession (B) is now in the quantum state that C was originally in. The system C, unfortunately has been destroyed in the process.
- throwaway888abc 6y agoThanks for all your explanations here
- stakkur 6y agoIt's a common mistake to think that quantum teleportation is a new way of sending information. It's really a way to use classical communication in order to leverage entanglement to bypass various limitations of quantum mechanics. Yes. This is the key point, I think, and it didn't seem well-communicated in the article.
- darau1 6y agoSo we'd still be limited by the speed of classical communication in sending the object's state to the new location? I hope I'm understanding this correctly.
- pontus 6y agoYes, that's correct
- colechristensen 6y agoYes, to achieve entanglement in the first place you have to either do something to two objects at a distance and that action can’t exceed the speed of light (and often is light) or you have to entangle two objects and then move them to their destination. Teleportation is achieved by doing a specific action on one and then communicating to the other to do a specific action to the other. In no case can information be transmitted faster than light.
- wruza 6y agoit would seem like one could not construct a perfect replica of a quantum system It was always confusing why this replica has to be perfect. Even two “you” separated by a nanosecond in time are not perfect replicas at all. Why not to allow some impreciseness and teleport “quantumly uncertain” bodies instead of perfect copies? You can’t even know what’s perfect or not by the same principle. Moreover, maybe our structure allows very imperfect copying while retaining the same functionality after few minutes of teleache.
- rocqua 6y agoSo, once the recipient gets the classic message, they can instantly set their local system to the same state as was measured by the sender's system? Can this be used to 'pause' a system, by delaying the transmission of the classic message? Or do you get the state the original system would have had, had you not measured it?
- pluto9 6y ago> Now, an interesting side effect of the quantum version is that the first measurement of the original system is necessarily destructive. As such, it's not like you'll end up with two copies of the same thing (which is what would happen in the classical version) so there's no discussion necessary around the distinction between teleportation and cloning. Is it possible to have three entangled systems? It seems that would allow sending the state information from one to the other two, thus destroying the original but leaving two "copies". Apologies if the answer to this is obvious, I'm a total layman when it comes to this stuff.