[magicalhippo]

> 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]

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]

> 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]

Cool, thanks for sticking with me in this! I definitely took it to be that the individual detectors were replicable at the individual level. Guessing that is not claimed and was an assumption in my reading. Certainly fits intuition better.

I suppose all that is left in the intuition busting, is how the probabilities don't add up as expected?

[magicalhippo]

> I suppose all that is left in the intuition busting, is how the probabilities don't add up as expected?

Lets imagine electrons are objects in a program, then the "electron class" has a private field containing a seed value to a pseudo-random number generator (ie deterministic), and the two electrons are initialized with the same seed value.

Further imagine that performing a measurement of an electron amounts to taking the seed, generating a random number between 0 and 360 degrees (sample value), and then comparing that random number to the measurement angle. If the sample value and the measurement angle is closer than +/- 90 degrees we say the measurement result is up, otherwise down.

Alright, so, if we imagine that when Charlie prepared the electrons, he creates two "electron objects", and passes one to Alice and the other to Bob.

If Charlie prepares entangled electrons, he'll ensure both instances have the same seed value. If he wants to create regular non-entangled electrons, he'll make each have a random seed value.

For non-entangled electrons, Alice and Bob will not see any correlation if they later compare notes.

For entangled electrons, if Alice and Bob uses the same angle they must get the same result per definition[1]. And indeed one can find the correlation as a function of the difference in angle, and it's a linear function from perfect correlation if the angles are the same (zero difference) to zero correlation (perfect anti-correlation) when the angles are 180 degrees apart.

However on real, entangled electrons in the lab things are different. There you'll find that the correlation is higher than the linear function when the difference is smaller than 90 degrees, and less than the linear function when the difference is greater than 90 degrees[2].

Thus if we measure the entanglement at not just 0 and 90 degrees difference but also 45 degrees difference, we'll find that our lab measurements do not agree with our simulated measurements.

Hence we conclude that entangled electrons do not behave like small objects that were created with the same "hidden value", ie the seed value in my example.

That's the essence of Bell's theorem and the tests of it (at least according to my memory).

[1]: Note that measuring real entangled electrons Alice and Bob will get exactly the opposite result, they're perfectly anti-correlated, but this matters not for this explanation and not worrying about it will make the explanation easier.

[2]: IIRC it goes like cos(a/2)^2 or something along those lines, ie https://www.wolframalpha.com/input?i=plot+%7B1-abs%28x%2Fpi%...

[taeric]

I thought the hidden variable idea was proven not to hold, though? Like, I thought that was the point? That the behavior observed can only be explained using the state of the remote measure as part of the explanation?

I have not looked back at the book I read. I definitely remember it had examples that were not paired off. I'm assumingy memory is simply off.

[magicalhippo]

Yes, that's what I was trying to say.

Hidden variables, like my "electron objects", would give a linear relationship.

However what quantum mechanics predicts and what we measure in the lab is a non-linear relationship that for some angles yield stronger correlations. Hence the phrase "violating Bell's inequality".

After Bell people devised other inequalities for other experimental setups which also can be used to rule out local hidden variables. A popular example are the CHSH inequalities[1], which is easier to realize experimentally, and give a stronger disagreement with hidden variables.

I've never seen the particles not paired up, and I don't see how that would work.

The whole point is that according to quantum mechanics, the pair of entangled particles aren't two separate systems, but that they must be treated as having a single state.

[1]: https://en.wikipedia.org/wiki/CHSH_inequality