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I'm confused, as I'm an enthusiast but definitely no physicist. The article states that quantum information is being teleported via entanglement. However, it w

I'm confused, as I'm an enthusiast but definitely no physicist. The article states that quantum information is being teleported via entanglement. However, it was my understanding that one cannot transfer information in such a way as the "information" is only revealed when interacting with the particle. Could someone perhaps clarify what's going on?

pontus 6y ago
You entangle two systems but in order to actually complete the teleportation you need to measure one system and then convey the outcome of that measurement to the other party. This information is needed by the second party in order for them to be able to correctly collapse the state of their system into one that is identical to the original system being teleported. The information that the first party must convey to the receiving party must be sent in a classical way (e.g. a phone call).
- junon 6y ago
  This makes zero sense. If the information must be conveyed classically anyway, what's the point?
  - ed25519FUUU 6y ago
    It clearly makes no practical sense at the moment outside of research.
    - lscharen 6y ago
      There are quantum communication links for satellites that guarantee that the data streams are not tampered with. https://directory.eoportal.org/web/eoportal/satellite-missions/q/quess https://directory.eoportal.org/web/eoportal/satellite-missio... https://spectrum.ieee.org/tech-talk/computing/networks/quantum-drone https://spectrum.ieee.org/tech-talk/computing/networks/quant...
      - foobiekr 6y ago
        Less "tampered with" (because they can be trivially blocked or corrupted) and more "uninspected."
  - marcosdumay 6y ago
    The point is to test physics models.
  - tgb 6y ago
    If you want to transmit 1 quantum bit (qubit), then you need to transmit 2 classical bits. Why is this useful? Because otherwise you have to carry the qubit over by hand. It's really just "quantum ethernet" not "quantum teleportation". There are actually other uses, too, about error tolerance, allowing you to quality-control some steps of the computation and repeat them if necessary without risking damaging the results of other steps.
    - 8note 6y ago
      You still have to carry the qubit over by hand, but you can do it asynchronously to the change
      - tgb 6y ago
        I think that undersells the advantage. You have to carry a qubit over, but it could be done even before know what you will need to send.
  - abdullahkhalids 6y ago
    Physical qubits are much more sensitive to transmission noise than physical bits. If you want to transmit qubits, teleportation allows you to do it with higher fidelity than by physical transmission. Why do you want to transmit qubits? For various emerging quantum information technologies, each with different potential economic impact.
    - pueblito 6y ago
      Does this mean quantum gizmos will ‘teleport’ the data over space instead of using wires? Like, will a quantum processor pull the data in the quantum ram using teleportation?
      - abdullahkhalids 6y ago
        Well, not that short range! We are talking about where you have to transmit the qubits at least a few dozen meters. So perhaps distributed quantum computers spread across a university, city or country. Also, remember, you still need wired or wireless transmission of bits to do the teleportation protocol.
      - colechristensen 6y ago
        It means quantum “networking” will be transmitted over classical communications channels.
    - spurgu 6y ago
      This simple explanation clicked for me, thank you!
  - 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 ago
      Very 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...
  - ChrisLomont 6y ago
    Superdense coding allows sending two bits of classical information by only sending one qubit physically. Ideally this will double transfer rates. And in any case it provides another "modality" for data communication, which may provide tradeoffs that have benefits in other directions. I'm pretty sure (haven't been in the field a while) that superdense coding the densest coding that one can gain using quantum entanglement. https://en.wikipedia.org/wiki/Superdense_coding https://en.wikipedia.org/wiki/Superdense_coding
  - 8note 6y ago
    Supposing we get enough qubits to support teleporting you, we can regularly entangle qubits and ship them around the world, then when you want to travel, we can teleport you, and the trip time will be how long it takes to send the message. This cuts your trip time down from hours long flights to seconds long trips
    - roywiggins 6y ago
      It might be easier just to destructively scan and reconstruct you classically and ignore the quantum stuff. If the person who steps out of the booth still thinks they're "you", problem solved, and without dealing with quantum states. How much do you care that your cells are all quantum-mechanically identical post-teleport? Not much!
  - dheera 6y ago
    The point is actually in even transferring an arbitrary quantum state from one quantum particle to another, and given an entangled pair existing in advance. Transferring a quantum state from one particle to another isn't an easy or obvious task, because you can't measure it or you would collapse it. Note that "classical" is just stating that a classical channel is good enough to serve that purpose. A channel that preserves quantum state can of course be used, it's just that that isn't required for this to work. All classical phenomena are quantum, we just use the word "classical" to describe subsets of quantum phenomena acting qualitatively in ways that are consistent with macroscopic phenomena.
  - im3w1l 6y ago
    Suppose Alice and Bob have a pair of entangled particles. But Bob wants to give his part of the pair to Charlie. Quantum teleportation lets him do that over the phone.
- raspasov 6y ago
  This is a very high quality discussion on QM and explanations from pontus, keep it up!
- deleted 6y ago
  [deleted]
- Buttons840 6y ago
  ELI5: I have a red ball and a blue ball. I randomly place each ball into one of two boxes. I give you one box, and keep the other box for myself, then wave my magic wand and transport you 5,000 light years away. You look inside your box and see a red ball. In that moment you learn that I have a blue ball in my box. You gained that information in an instant, about a box 5,000 light years away. (This is an explanation I've heard. But I don't personally know if it's accurate.)
  - chiefofgxbxl 6y ago
    (My understanding; I only took one course in quantum mechanics) Arguably the information that you "gained" was predetermined when you decided which colored ball to place in which box. You didn't gain any knowledge that you couldn't have known before you were teleported 5,000 ly away. What's also more difficult is that the balls wouldn't have definitive colors, but exist in a probabilistic state / superposition. But you couldn't send the message "I have a red ball" to the individual 5,000 ly away because collapsing your ball's wave will be random and you can't determine which color you want. You could "mistakenly" collapse your ball to blue, not red.
    - russdill 6y ago
      This big missing piece here is that it's not a simple red/blue property. Spin is the easiest to understand. When you measure, the spin will either measure up, or down. The key though is you can measure spin against any angle. If you measure the spin of the two particles at the same angle, you will get opposite answers. And you can choose what angle to measure at* long after the particles have separated. * There is one theory of QM that removes the possibility of choice from the universe.
  - pontus 6y ago
    I talked about this elsewhere in this thread (just with gloves instead of colored balls).
  - Kranar 6y ago
    While what you say is true, it doesn't have anything to do with quantum mechanics. I'm sure the ancient Greeks would have had no problem understanding that if you put a red ball in one box and a blue ball in another box then if you open a box and see a blue ball you'd know that the red ball is in the other box. In quantum mechanics, entanglement is more about the fact that until a measurement is performed on one of the balls, both balls are simultaneously blue and red and will behave as if it was in a superposition of both colors up until a measurement is performed. By extension, any other property that depends on the color of those balls will also be entangled with the balls and behave in a superposition of whatever properties are entangled. For example if a washing machine washes clothes with hot water if the blue ball is in box A, and washed with cold water if the blue ball is in box B, then that washing machine will be washing clothes with hot and cold water until a measurement is performed on any of the balls or the washing machine itself. The clothes being washed will simultaneously be expanding (from hot water) and contracting (from cold water). Only once a measurement is performed on any part of the entangled system will every property of the system collapse into a definite state of red or blue, hot or cold, expanded or contracted.
deleted 6y ago
[deleted]
fastball 6y ago
We had digital data storage before we had the internet. Back then, if you wanted to share digital information with someone else, you had to save that information to a hard disk (or similar) and physically transport it to their location. Then we invented the internet and now you can transmit digital information between digital computers over wires / EM signals. Quantum teleportation / entanglement does the same but for quantum state rather than just digital state, which allows quantum computers to communicate with each other. Without this technology, quantum computers would not be able to communicate with each other over a network. So you still need to use wires / EM waves in order to transmit the data, but the data you are transmitting is quantum. Technically you're not actually sending qubits over the wire though, you're sending 2 classical bits per quibit which are able to tell the other system what each qubit in the target system is supposed to look like, but effectively you can pretend you're sending qubits over the wire.
- jiofih 6y ago
  If sending a pair of bits over the wire accurately describes this, then what is the achievement of transmission over 44km? Wouldn’t it simply be limited to our existing fiber coverage? Feels like something is missing here
  - fastball 6y ago
    Sorry, I was mixing terms. You need to send classical information over the wire, but you're also sending photonic qubits thru fiber, just not as a communication method. The fiber is how you get the entangled qubit to the other side of the network (entangled qubits are generated together). But again, this is not how you are communicating information. Technically this doesn't need to happen over fiber, you just need to somehow have entangled qubits in two separate locations, and obviously if we're talking about entangled photons, fiber is probably the best way to separate them. Information is communicated once the entangled photon has reached the other side of the network, at which point a measurement at one end of the network is made and communicated (classically). Once again though, the purpose of the classical bits is not actually to send information, but rather because any measurement of one part of the entangled system results in its collapse (Copenhagen interpretation), and thus a collapse of the entangled quantum system as a whole. So if you have a quantum computer doing some calculation with qubits and those qubits have been entangled with others at the other side of the network, you can perform measurements on the qubits at one end and this will collapse the qubits at the other end. This would not be possible to do as you'd expect from classic digital stuff, where you can just send the info over the wire. The distance is limited because of fidelity loss in fiber - you actually need your single photonic qubit (entangled with another qubit at the source) to make it from one end of the network to the other. In order to extend the range you can use quantum repeaters, which are effectively just a sequence of entanglements. As an aside, higher quality fiber made in microgravity (ZBLAN) could also help bring these transmission distances up.

pontus 6y ago

You entangle two systems but in order to actually complete the teleportation you need to measure one system and then convey the outcome of that measurement to the other party. This information is needed by the second party in order for them to be able to correctly collapse the state of their system into one that is identical to the original system being teleported. The information that the first party must convey to the receiving party must be sent in a classical way (e.g. a phone call).

junon 6y ago
This makes zero sense. If the information must be conveyed classically anyway, what's the point?
- ed25519FUUU 6y ago
  It clearly makes no practical sense at the moment outside of research.
  - lscharen 6y ago
    There are quantum communication links for satellites that guarantee that the data streams are not tampered with. https://directory.eoportal.org/web/eoportal/satellite-missions/q/quess https://directory.eoportal.org/web/eoportal/satellite-missio... https://spectrum.ieee.org/tech-talk/computing/networks/quantum-drone https://spectrum.ieee.org/tech-talk/computing/networks/quant...
    - foobiekr 6y ago
      Less "tampered with" (because they can be trivially blocked or corrupted) and more "uninspected."
- marcosdumay 6y ago
  The point is to test physics models.
- tgb 6y ago
  If you want to transmit 1 quantum bit (qubit), then you need to transmit 2 classical bits. Why is this useful? Because otherwise you have to carry the qubit over by hand. It's really just "quantum ethernet" not "quantum teleportation". There are actually other uses, too, about error tolerance, allowing you to quality-control some steps of the computation and repeat them if necessary without risking damaging the results of other steps.
  - 8note 6y ago
    You still have to carry the qubit over by hand, but you can do it asynchronously to the change
    - tgb 6y ago
      I think that undersells the advantage. You have to carry a qubit over, but it could be done even before know what you will need to send.
- abdullahkhalids 6y ago
  Physical qubits are much more sensitive to transmission noise than physical bits. If you want to transmit qubits, teleportation allows you to do it with higher fidelity than by physical transmission. Why do you want to transmit qubits? For various emerging quantum information technologies, each with different potential economic impact.
  - pueblito 6y ago
    Does this mean quantum gizmos will ‘teleport’ the data over space instead of using wires? Like, will a quantum processor pull the data in the quantum ram using teleportation?
    - abdullahkhalids 6y ago
      Well, not that short range! We are talking about where you have to transmit the qubits at least a few dozen meters. So perhaps distributed quantum computers spread across a university, city or country. Also, remember, you still need wired or wireless transmission of bits to do the teleportation protocol.
    - colechristensen 6y ago
      It means quantum “networking” will be transmitted over classical communications channels.
  - spurgu 6y ago
    This simple explanation clicked for me, thank you!
- 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 ago
    Very 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...
- ChrisLomont 6y ago
  Superdense coding allows sending two bits of classical information by only sending one qubit physically. Ideally this will double transfer rates. And in any case it provides another "modality" for data communication, which may provide tradeoffs that have benefits in other directions. I'm pretty sure (haven't been in the field a while) that superdense coding the densest coding that one can gain using quantum entanglement. https://en.wikipedia.org/wiki/Superdense_coding https://en.wikipedia.org/wiki/Superdense_coding
- 8note 6y ago
  Supposing we get enough qubits to support teleporting you, we can regularly entangle qubits and ship them around the world, then when you want to travel, we can teleport you, and the trip time will be how long it takes to send the message. This cuts your trip time down from hours long flights to seconds long trips
  - roywiggins 6y ago
    It might be easier just to destructively scan and reconstruct you classically and ignore the quantum stuff. If the person who steps out of the booth still thinks they're "you", problem solved, and without dealing with quantum states. How much do you care that your cells are all quantum-mechanically identical post-teleport? Not much!
- dheera 6y ago
  The point is actually in even transferring an arbitrary quantum state from one quantum particle to another, and given an entangled pair existing in advance. Transferring a quantum state from one particle to another isn't an easy or obvious task, because you can't measure it or you would collapse it. Note that "classical" is just stating that a classical channel is good enough to serve that purpose. A channel that preserves quantum state can of course be used, it's just that that isn't required for this to work. All classical phenomena are quantum, we just use the word "classical" to describe subsets of quantum phenomena acting qualitatively in ways that are consistent with macroscopic phenomena.
- im3w1l 6y ago
  Suppose Alice and Bob have a pair of entangled particles. But Bob wants to give his part of the pair to Charlie. Quantum teleportation lets him do that over the phone.
raspasov 6y ago
This is a very high quality discussion on QM and explanations from pontus, keep it up!
deleted 6y ago
[deleted]
Buttons840 6y ago
ELI5: I have a red ball and a blue ball. I randomly place each ball into one of two boxes. I give you one box, and keep the other box for myself, then wave my magic wand and transport you 5,000 light years away. You look inside your box and see a red ball. In that moment you learn that I have a blue ball in my box. You gained that information in an instant, about a box 5,000 light years away. (This is an explanation I've heard. But I don't personally know if it's accurate.)
- chiefofgxbxl 6y ago
  (My understanding; I only took one course in quantum mechanics) Arguably the information that you "gained" was predetermined when you decided which colored ball to place in which box. You didn't gain any knowledge that you couldn't have known before you were teleported 5,000 ly away. What's also more difficult is that the balls wouldn't have definitive colors, but exist in a probabilistic state / superposition. But you couldn't send the message "I have a red ball" to the individual 5,000 ly away because collapsing your ball's wave will be random and you can't determine which color you want. You could "mistakenly" collapse your ball to blue, not red.
  - russdill 6y ago
    This big missing piece here is that it's not a simple red/blue property. Spin is the easiest to understand. When you measure, the spin will either measure up, or down. The key though is you can measure spin against any angle. If you measure the spin of the two particles at the same angle, you will get opposite answers. And you can choose what angle to measure at* long after the particles have separated. * There is one theory of QM that removes the possibility of choice from the universe.
- pontus 6y ago
  I talked about this elsewhere in this thread (just with gloves instead of colored balls).
- Kranar 6y ago
  While what you say is true, it doesn't have anything to do with quantum mechanics. I'm sure the ancient Greeks would have had no problem understanding that if you put a red ball in one box and a blue ball in another box then if you open a box and see a blue ball you'd know that the red ball is in the other box. In quantum mechanics, entanglement is more about the fact that until a measurement is performed on one of the balls, both balls are simultaneously blue and red and will behave as if it was in a superposition of both colors up until a measurement is performed. By extension, any other property that depends on the color of those balls will also be entangled with the balls and behave in a superposition of whatever properties are entangled. For example if a washing machine washes clothes with hot water if the blue ball is in box A, and washed with cold water if the blue ball is in box B, then that washing machine will be washing clothes with hot and cold water until a measurement is performed on any of the balls or the washing machine itself. The clothes being washed will simultaneously be expanding (from hot water) and contracting (from cold water). Only once a measurement is performed on any part of the entangled system will every property of the system collapse into a definite state of red or blue, hot or cold, expanded or contracted.

deleted 6y ago

[deleted]

fastball 6y ago

We had digital data storage before we had the internet. Back then, if you wanted to share digital information with someone else, you had to save that information to a hard disk (or similar) and physically transport it to their location. Then we invented the internet and now you can transmit digital information between digital computers over wires / EM signals. Quantum teleportation / entanglement does the same but for quantum state rather than just digital state, which allows quantum computers to communicate with each other. Without this technology, quantum computers would not be able to communicate with each other over a network. So you still need to use wires / EM waves in order to transmit the data, but the data you are transmitting is quantum. Technically you're not actually sending qubits over the wire though, you're sending 2 classical bits per quibit which are able to tell the other system what each qubit in the target system is supposed to look like, but effectively you can pretend you're sending qubits over the wire.

jiofih 6y ago
If sending a pair of bits over the wire accurately describes this, then what is the achievement of transmission over 44km? Wouldn’t it simply be limited to our existing fiber coverage? Feels like something is missing here
- fastball 6y ago
  Sorry, I was mixing terms. You need to send classical information over the wire, but you're also sending photonic qubits thru fiber, just not as a communication method. The fiber is how you get the entangled qubit to the other side of the network (entangled qubits are generated together). But again, this is not how you are communicating information. Technically this doesn't need to happen over fiber, you just need to somehow have entangled qubits in two separate locations, and obviously if we're talking about entangled photons, fiber is probably the best way to separate them. Information is communicated once the entangled photon has reached the other side of the network, at which point a measurement at one end of the network is made and communicated (classically). Once again though, the purpose of the classical bits is not actually to send information, but rather because any measurement of one part of the entangled system results in its collapse (Copenhagen interpretation), and thus a collapse of the entangled quantum system as a whole. So if you have a quantum computer doing some calculation with qubits and those qubits have been entangled with others at the other side of the network, you can perform measurements on the qubits at one end and this will collapse the qubits at the other end. This would not be possible to do as you'd expect from classic digital stuff, where you can just send the info over the wire. The distance is limited because of fidelity loss in fiber - you actually need your single photonic qubit (entangled with another qubit at the source) to make it from one end of the network to the other. In order to extend the range you can use quantum repeaters, which are effectively just a sequence of entanglements. As an aside, higher quality fiber made in microgravity (ZBLAN) could also help bring these transmission distances up.