3 ms·

> I have asked why you can't use the correlations to facilitate communication But how could they? Charlie prepares a pair of entangled electrons and sends on

> I have asked why you can't use the correlations to facilitate communication But how could they? Charlie prepares a pair of entangled electrons and sends one each to Alice and Bob. Alice performs a spin measurement along some angle and, entirely randomly, gets either up or down as a result. Alice and Bob decide on their measurement settings and measure a bunch of electrons using the same angle for each measurement. They can even agree in advance so they both know which angle the other will use. After the run, Alice will have a bunch of measurement results which are roughly 50% up and 50% down. Bob too will have a bunch of measurement results which are roughly 50% up and 50% down. Assuming ideal detectors and such, there will be no discernible pattern to the ups and downs for either. Only if the afterwards come together and compare their results pair for pair will they see the quantum correlations between the value in each pair. For some angles, they're more likely to be anti-correlated, and for some angles there doesn't seem to be any correlation. That is, if they both used the same angle, the they'll find that each time Alice measured up then Bob measured down, and every time Alice measured down then Bob measured up. And if they used a similar but not equal angle, then it's more likely that when Alice measured up then Bob measured down, and vice versa. And since they by now know that this experiment has been done before and the predictions of quantum mechanics hold, they can even predict this result. However what good does it do for Alice? After all, regardless of measurement settings Bob will measure a uniform 50/50 distribution of ups and downs.

taeric 3y ago
Agreed that it doesn't do Alice any good, necessarily; but it seems that it does get information between Bob and Charlie? If the detector at Bob's site influences what Charlie would see at an aggregate level, do they have to wait for the end of the experiment to know? Couldn't they make an inference at every hour on what the detector was doing at the other end? Even if they were a lightyear away from each other. If the answer really is that they have to wait for the end of the entire experiment, I think that settles it for me. Will think some more on it. (And again, as noted, I have not thought that hard on this. Even with my odd "would this work" you need some way to get entangled particles sent across stupid large distances. Which... already seems silly?)
- magicalhippo 3y ago
  > If the detector at Bob's site influences what Charlie would see at an aggregate level Charlie doesn't see anything. He sends the electrons here and there. He's just produced the entangled electrons, he hasn't measured them. If he did he would destroy the entanglement and ruin the experiment (which is what secure quantum communication is about). Unless he gets some reply (say a photon or electron sent by Bob), he doesn't know what either measure. But if he does get a return particle then they're just communicating classically, so why not just pick up a phone?
  - taeric 3y ago
    Apologies, I screwed up the names there. I was thinking down thread where I had A sending. So, A sends, B has a detector, C has a detector. Framing I've seen had it such that depending on the setting of B's detector, C would get a different result. (And vice versa.) Now, I am assuming I saw an incomplete framing where this is only true if they communicate back to A? Stated differently, the framing I saw was that the "spooky" action was somehow setting detector C to a specific setting would cause a different reading in detector B. And this was done in such a way that B could not know that C had changed. But, simply getting a new reading at B means that either A or C has changed, necessarily? And again, going off old memory. Never my area of study, such that I assume I am misunderstanding. It is frustrating because most "pointing out my mistake" assumes I care about individual protons. I'm saying if we can agree to have A set to send with constant rate, then barring that getting broken, it seems you have a scheme whereby B and C can know what they are doing in aggregate.
    - magicalhippo 3y ago
      > Framing I've seen had it such that depending on the setting of B's detector, C would get a different result. That was the entire point of my initial post: there's no discernible difference in the actual individual measurement results regardless of detector settings. The quantum correlations only show up if someone compares both measurements pair by pair. And to do so, regular communication must happen. Many sources are very sloppy when it comes to phrasing this, so you're not alone in being confused. I too thought like you way back, thinking it could be used for communication.

taeric 3y ago

Agreed that it doesn't do Alice any good, necessarily; but it seems that it does get information between Bob and Charlie? If the detector at Bob's site influences what Charlie would see at an aggregate level, do they have to wait for the end of the experiment to know? Couldn't they make an inference at every hour on what the detector was doing at the other end? Even if they were a lightyear away from each other. If the answer really is that they have to wait for the end of the entire experiment, I think that settles it for me. Will think some more on it. (And again, as noted, I have not thought that hard on this. Even with my odd "would this work" you need some way to get entangled particles sent across stupid large distances. Which... already seems silly?)

magicalhippo 3y ago
> If the detector at Bob's site influences what Charlie would see at an aggregate level Charlie doesn't see anything. He sends the electrons here and there. He's just produced the entangled electrons, he hasn't measured them. If he did he would destroy the entanglement and ruin the experiment (which is what secure quantum communication is about). Unless he gets some reply (say a photon or electron sent by Bob), he doesn't know what either measure. But if he does get a return particle then they're just communicating classically, so why not just pick up a phone?
- taeric 3y ago
  Apologies, I screwed up the names there. I was thinking down thread where I had A sending. So, A sends, B has a detector, C has a detector. Framing I've seen had it such that depending on the setting of B's detector, C would get a different result. (And vice versa.) Now, I am assuming I saw an incomplete framing where this is only true if they communicate back to A? Stated differently, the framing I saw was that the "spooky" action was somehow setting detector C to a specific setting would cause a different reading in detector B. And this was done in such a way that B could not know that C had changed. But, simply getting a new reading at B means that either A or C has changed, necessarily? And again, going off old memory. Never my area of study, such that I assume I am misunderstanding. It is frustrating because most "pointing out my mistake" assumes I care about individual protons. I'm saying if we can agree to have A set to send with constant rate, then barring that getting broken, it seems you have a scheme whereby B and C can know what they are doing in aggregate.
  - magicalhippo 3y ago
    > Framing I've seen had it such that depending on the setting of B's detector, C would get a different result. That was the entire point of my initial post: there's no discernible difference in the actual individual measurement results regardless of detector settings. The quantum correlations only show up if someone compares both measurements pair by pair. And to do so, regular communication must happen. Many sources are very sloppy when it comes to phrasing this, so you're not alone in being confused. I too thought like you way back, thinking it could be used for communication.