7 ms·

Wait, why is Quantum Entanglement a non-starter? How does that require a conventional channel? Once you have the two entangled, assuming you have some way to

Wait, why is Quantum Entanglement a non-starter? How does that require a conventional channel? Once you have the two entangled, assuming you have some way to change the state of one with out breaking the entanglement, then how is that not enough for communication?

metalliqaz 8y ago
Because once the particles are entangled, one has to be moved by conventional means to the destination while the other stays behind.
- dbingham 8y ago
  Sure, but you could still create a two way walkie talkie that couldn't be eavesdropped on and would allow faster than light communication this way, theoretically.
  - simcop2387 8y ago
    You can make it secure from mitm with this but it doesn't allow ftl. You can't use the entangled particles themselves to do the communication since you cant control how the superposition collapses when you measure it. This allows you to get two streams that nobody but you and the party knows but they're fundamentally random and unpredictable, you can't encoded information into it, just use it as a one time pad essentially.
    - bduerst 8y ago
      But that in itself is a signal, right? Say you have two sets of two entangled particles, all four particles in similar states. You have one particle from each set, your partner has the corresponding particles in the sets. You check both particles you have, and: - if both particles have similar state, then your partner hasn't checked any of their particle states, because neither have collapsed. - if one particle has collapsed and is in a different state than the other particle, then you know your partner has at least checked the state of one of particle sets, possibly both. If you partner is 50 light years away and checks it only seconds prior to your check, then they are signalling you at faster-than-light travel, right?
      - danielsju6 8y ago
        But you have to communicate the states of each other's particles (as far as you can tell random noise) to compare/contrast and glean influence, so it's only suitable for encryption of classically transmitted messages. Think faster-than-light XOR ;)
        bduerst 8y ago
        Got it, so the state is random for all particles until it is measured by your or your partner, afterwhich the other particle state is known. But the act of measuring or the decaying doesn't tell you when (or if) they measured it because the state never changes, it just becomes known.
        int_19h 8y ago
        It's not random; it's a superposition. It is perhaps less nonsensical (in a "common sense" way) to think of it in terms of the many-worlds interpretation. The moment you have created the photons, you have forked the universe - but observers on both sides are still identical in both worlds, because they haven't done anything that'd allow them to figure out which universe they're in. So for each observer, we treat both "versions" of that observer as one and the same. Now, when one of the observers measures their photon, they figure out which fork they're in. And then in that universe, the other observer measuring it will also get the same result, of course, proving that they're also in that same universe. And in the other universe, the second observer will get the opposite result, again, proving that they're in that other universe. But neither measurement actually caused anything to happen, nor did they get any information about the opposite observer in their universe as a result of it.
  - jerf 8y ago
    abdullahkhalids has the correct answer. Even if you have the entangled particles already in place, you can't communicate with them. The real proof is that if you could, we'd already have this technology and high-speed traders would already be using it. If it could exist, we have all the tools to build it; it's far simpler than even a 4-qubit quantum computer, which we've also built. We don't have it because entanglement can't be used that way.
  - semi-extrinsic 8y ago
    Nope, you couldn't. From all the experiments and theory we have on entanglement, you can't use it to communicate faster than light.
- SigmundA 8y ago
  Which is still extremely useful for things like probes. Instant comm's to Mars rover or even voyager. Of course no ones figured out how to make use of entanglement for information transfer. Pops up a lot in Sci-Fi, reading Three-Bodied Problem now and the aliens sent "sophons" basically probes the size of protons at near the speed of light to earth from Alpha Centauri, they arrived in a few years while their invasion fleet will take 400 years to get here. The sophons spy and interfere with earth communicating with aliens via quantum entanglement.
  - teraflop 8y ago
    > Of course no ones figured out how to make use of entanglement for information transfer. Saying "no one's figured it out" is misleading because it creates the impression that it's an engineering problem to be solved. According to our best understanding of quantum mechanics, the laws of physics fundamentally do not permit faster-than-light communication, whether through entanglement or any other mechanism.
    - Filligree 8y ago
      Well, of course they don't. How would you even begin to build a simulator for something like that? Communications overhead between the nodes is bad enough as it is.
    - SigmundA 8y ago
      I was trying to imply that its probably the best avenue for research, and also we may never figure it out, might be a dead end. However I would say the reason it is used so much in sci-fi is it currently the one way we can cause action at a distance faster than light, which is so tantalizingly close to communication.
      - codethief 8y ago
        I wouldn't say we're close to FTL transfer of information in any meaningful way. In particular, > we can cause action at a distance is not true, either.
abdullahkhalids 8y ago
You still need to transfer a message using a classical channel to actually communicate anything. Remember, there is no known way to communicate faster-than-light. https://en.wikipedia.org/wiki/No-communication_theorem https://en.wikipedia.org/wiki/No-communication_theorem
- dbingham 8y ago
  Following this rabbithole lead me here, which explained it in a way that made sense to me: https://www.lesswrong.com/posts/DY9h6zxq6EMHrkkxE/spooky-action-at-a-distance-the-no-communication-theorem https://www.lesswrong.com/posts/DY9h6zxq6EMHrkkxE/spooky-act... Just in case anyone else is struggling with this, as I was :)
  - snakeboy 8y ago
    Good article! This part makes me laugh though >Here's something that, as a kid, I couldn't get anyone to explain to me: "Why can't you signal using an entangled pair of photons that both start out polarized up-down? By measuring A in a diagonal basis, you destroy the up-down polarization of both photons. Then by measuring B in the up-down/left-right basis, you can with 50% probability detect the fact that a measurement has taken place, if B turns out to be left-right polarized." He must have been a pretty smart kid...
  - zamalek 8y ago
    The way I understand it is that you still need a CLK signal, entanglement is the DATA pin. The primary purpose of entanglement is privacy. A sufficiently advanced species may choose to use entanglement to hide from the rest of the galaxy, and the CLK radio burst could be made to look suspiciously like natural phenomena (FRBs). However, a major point of contention is that humanity occurs far too soon in the life of the universe. It has taken life on Earth 3.5 billion years to evolve to us, that means that there have been just under 4 epochs of human evolutions in the age of the universe (and just over 1 epochs of human evolutions in the age of Earth). Chances are, using sci-fi nomenclature, that we are the "ancients." We have quite a lot to get done for those that follow.
  - 8y ago
jasonwatkinspdx 8y ago
First, you have to transport one of the entangled pair to your remote location. > assuming you have some way to change the state of one with out breaking the entanglement Second, this isn't possible. When you measure the entangled pair, you know they'll be correlated: if one is A, the other is B, and vice versa. But when you measure one of the particles, you have no idea if you're going to get A or B. You don't have a way to force the entanglement to go a direction you want. So it doesn't serve as a communication channel.
nwallin 8y ago
> assuming you have some way to change the state of one with out breaking the entanglement, This isn't possible. Not only is it quantum mechanically impossible to change the state of one without breaking the entanglement, it is quantum mechanically impossible to observe the state of one of them without breaking the entanglement. Remember that in quantum mechanics "interacting," "changing the state," and "observing" are all the same thing.
- Mertax 8y ago
  Can you know whether or not a particle is currently entangled? With observation of only 1 of the entangled particles? Say you had millions of entangled particles, you ship their companions to the other end of the galaxy. You now have a million bit array to encode data into for 1 time use? I’m guessing you can’t know whether a partical is currently in a state of entanglement... EDIT: my question has already be asked & answered several times...one such example: https://physics.stackexchange.com/questions/207169/can-i-tell-from-one-particle-of-an-entangled-pair-whether-the-other-particle-has https://physics.stackexchange.com/questions/207169/can-i-tel...

metalliqaz 8y ago

Because once the particles are entangled, one has to be moved by conventional means to the destination while the other stays behind.

dbingham 8y ago
Sure, but you could still create a two way walkie talkie that couldn't be eavesdropped on and would allow faster than light communication this way, theoretically.
- simcop2387 8y ago
  You can make it secure from mitm with this but it doesn't allow ftl. You can't use the entangled particles themselves to do the communication since you cant control how the superposition collapses when you measure it. This allows you to get two streams that nobody but you and the party knows but they're fundamentally random and unpredictable, you can't encoded information into it, just use it as a one time pad essentially.
  - bduerst 8y ago
    But that in itself is a signal, right? Say you have two sets of two entangled particles, all four particles in similar states. You have one particle from each set, your partner has the corresponding particles in the sets. You check both particles you have, and: - if both particles have similar state, then your partner hasn't checked any of their particle states, because neither have collapsed. - if one particle has collapsed and is in a different state than the other particle, then you know your partner has at least checked the state of one of particle sets, possibly both. If you partner is 50 light years away and checks it only seconds prior to your check, then they are signalling you at faster-than-light travel, right?
    - danielsju6 8y ago
      But you have to communicate the states of each other's particles (as far as you can tell random noise) to compare/contrast and glean influence, so it's only suitable for encryption of classically transmitted messages. Think faster-than-light XOR ;)
      - bduerst 8y ago
        Got it, so the state is random for all particles until it is measured by your or your partner, afterwhich the other particle state is known. But the act of measuring or the decaying doesn't tell you when (or if) they measured it because the state never changes, it just becomes known.
        int_19h 8y ago
        It's not random; it's a superposition. It is perhaps less nonsensical (in a "common sense" way) to think of it in terms of the many-worlds interpretation. The moment you have created the photons, you have forked the universe - but observers on both sides are still identical in both worlds, because they haven't done anything that'd allow them to figure out which universe they're in. So for each observer, we treat both "versions" of that observer as one and the same. Now, when one of the observers measures their photon, they figure out which fork they're in. And then in that universe, the other observer measuring it will also get the same result, of course, proving that they're also in that same universe. And in the other universe, the second observer will get the opposite result, again, proving that they're in that other universe. But neither measurement actually caused anything to happen, nor did they get any information about the opposite observer in their universe as a result of it.
- jerf 8y ago
  abdullahkhalids has the correct answer. Even if you have the entangled particles already in place, you can't communicate with them. The real proof is that if you could, we'd already have this technology and high-speed traders would already be using it. If it could exist, we have all the tools to build it; it's far simpler than even a 4-qubit quantum computer, which we've also built. We don't have it because entanglement can't be used that way.
- semi-extrinsic 8y ago
  Nope, you couldn't. From all the experiments and theory we have on entanglement, you can't use it to communicate faster than light.
SigmundA 8y ago
Which is still extremely useful for things like probes. Instant comm's to Mars rover or even voyager. Of course no ones figured out how to make use of entanglement for information transfer. Pops up a lot in Sci-Fi, reading Three-Bodied Problem now and the aliens sent "sophons" basically probes the size of protons at near the speed of light to earth from Alpha Centauri, they arrived in a few years while their invasion fleet will take 400 years to get here. The sophons spy and interfere with earth communicating with aliens via quantum entanglement.
- teraflop 8y ago
  > Of course no ones figured out how to make use of entanglement for information transfer. Saying "no one's figured it out" is misleading because it creates the impression that it's an engineering problem to be solved. According to our best understanding of quantum mechanics, the laws of physics fundamentally do not permit faster-than-light communication, whether through entanglement or any other mechanism.
  - Filligree 8y ago
    Well, of course they don't. How would you even begin to build a simulator for something like that? Communications overhead between the nodes is bad enough as it is.
  - SigmundA 8y ago
    I was trying to imply that its probably the best avenue for research, and also we may never figure it out, might be a dead end. However I would say the reason it is used so much in sci-fi is it currently the one way we can cause action at a distance faster than light, which is so tantalizingly close to communication.
    - codethief 8y ago
      I wouldn't say we're close to FTL transfer of information in any meaningful way. In particular, > we can cause action at a distance is not true, either.

abdullahkhalids 8y ago

You still need to transfer a message using a classical channel to actually communicate anything. Remember, there is no known way to communicate faster-than-light. https://en.wikipedia.org/wiki/No-communication_theorem https://en.wikipedia.org/wiki/No-communication_theorem

dbingham 8y ago
Following this rabbithole lead me here, which explained it in a way that made sense to me: https://www.lesswrong.com/posts/DY9h6zxq6EMHrkkxE/spooky-action-at-a-distance-the-no-communication-theorem https://www.lesswrong.com/posts/DY9h6zxq6EMHrkkxE/spooky-act... Just in case anyone else is struggling with this, as I was :)
- snakeboy 8y ago
  Good article! This part makes me laugh though >Here's something that, as a kid, I couldn't get anyone to explain to me: "Why can't you signal using an entangled pair of photons that both start out polarized up-down? By measuring A in a diagonal basis, you destroy the up-down polarization of both photons. Then by measuring B in the up-down/left-right basis, you can with 50% probability detect the fact that a measurement has taken place, if B turns out to be left-right polarized." He must have been a pretty smart kid...
- zamalek 8y ago
  The way I understand it is that you still need a CLK signal, entanglement is the DATA pin. The primary purpose of entanglement is privacy. A sufficiently advanced species may choose to use entanglement to hide from the rest of the galaxy, and the CLK radio burst could be made to look suspiciously like natural phenomena (FRBs). However, a major point of contention is that humanity occurs far too soon in the life of the universe. It has taken life on Earth 3.5 billion years to evolve to us, that means that there have been just under 4 epochs of human evolutions in the age of the universe (and just over 1 epochs of human evolutions in the age of Earth). Chances are, using sci-fi nomenclature, that we are the "ancients." We have quite a lot to get done for those that follow.
- 8y ago

jasonwatkinspdx 8y ago

First, you have to transport one of the entangled pair to your remote location. > assuming you have some way to change the state of one with out breaking the entanglement Second, this isn't possible. When you measure the entangled pair, you know they'll be correlated: if one is A, the other is B, and vice versa. But when you measure one of the particles, you have no idea if you're going to get A or B. You don't have a way to force the entanglement to go a direction you want. So it doesn't serve as a communication channel.

nwallin 8y ago

> assuming you have some way to change the state of one with out breaking the entanglement, This isn't possible. Not only is it quantum mechanically impossible to change the state of one without breaking the entanglement, it is quantum mechanically impossible to observe the state of one of them without breaking the entanglement. Remember that in quantum mechanics "interacting," "changing the state," and "observing" are all the same thing.

Mertax 8y ago
Can you know whether or not a particle is currently entangled? With observation of only 1 of the entangled particles? Say you had millions of entangled particles, you ship their companions to the other end of the galaxy. You now have a million bit array to encode data into for 1 time use? I’m guessing you can’t know whether a partical is currently in a state of entanglement... EDIT: my question has already be asked & answered several times...one such example: https://physics.stackexchange.com/questions/207169/can-i-tell-from-one-particle-of-an-entangled-pair-whether-the-other-particle-has https://physics.stackexchange.com/questions/207169/can-i-tel...