[honeyed_coffee]

The spooky part occurs when two parties who share this entangled state know what measurement to perform. If the measurement choice aligns for both parties, their outcomes can be correlated precisely. If the measurement choices are not aligned, the outcomes are also random

[magicalhippo]

> If the measurement choices are not aligned, the outcomes are also random

Just to be a bit pedantic, as it can otherwise lead to some confusion: the measurement outcomes are always random.

If the particles are entangled, then it is the correlations that are not random.

[taeric]

This is still a part that annoys me. I have asked why you can't use the correlations to facilitate communication, and people always seem to think I'm asking why you can't do this per particle. I get that the the individual measures are basically useless on their own. Question is if the correlations can be confirmed so well, why can't that be used?

[magicalhippo]

> 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]

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]

> 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.

[drwiggly]

From what I'm gathering.

  Alice measures at angle X, gets value V1

  Calls Bob on the phone, okay I measured angle X.

  Bob measures at angle X, also gets value V1

  Bob measures at angle Y, gets value V2.

  Bob calls Alice back says, okay I measured angle Y.

  Alice measures angle Y, also gets V2.

The correlation here is nobody can do other measurements while the other party is in the process of measuring. Each party can't know the other party is done until traditional communication has happened.

If each party acted independently they would randomly change the state on the other side and each party would get what appears to be random values.

[taeric]

I think my mind bend is more over 3 actors. Note that my understanding, also, is that it has been shown that changing a detector changes what is detected at the other detector.

    A is sending entangled stuff to B and C.
    B measures and gets a set of angles that tells them what C would be measuring
    C changes what they are measuring.  

The question is, how rapidly does the "spooky" distance change happen? I get that it would not be communication between A and B or C. I similarly get that you could not coordinate between B and C. But, from all of the framings I've seen so far, I don't understand why the change between B and C is not faster than speed of light.

(And just to rapidly get it out there, I fully expect that I'm merely misunderstanding something here.)

Edit: Also, to add, my understanding is that they are not "getting angles" per se, but would be seeing distributions. Which is why you would need more than 1 particle, as it were. So, you would say of the X I have recorded, 30% have been blue, 70% have been green. I suppose the concern is that you have no way of knowing when the "100%" mark is done until after classical communication, such that it is impossible to know what the final distribution you are measuring is? Effectively?

[honeyed_coffee]

>The question is, how rapidly does the "spooky" distance change happen? I get that it would not be communication between A and B or C. I similarly get that you could not coordinate between B and C. But, from all of the framings I've seen so far, I don't understand why the change between B and C is not faster than speed of light.

It's because measurements at B do not convey any information to C while the measurements are performed and vice versa. Unless B calls C to inform them of the choice of measurement setting, C will not know the measurement outcome at B's side. This is true even if they know that they share entangled states prior to prior to performing measurements.

> Edit: Also, to add, my understanding is that they are not "getting angles" per se, but would be seeing distributions. Which is why you would need more than 1 particle, as it were. So, you would say of the X I have recorded, 30% have been blue, 70% have been green. I suppose the concern is that you have no way of knowing when the "100%" mark is done until after classical communication, such that it is impossible to know what the final distribution you are measuring is? Effectively?

There has to be post processing of data where they drop the results of rounds where their measurement choices don't match. This is important because of what's called non commuting measurements. Measurements in one setting don't give us any information about measurement outcome in another setting. So effectively at one end, they have to record their measurement choice and corresponding outcomes of said measurement. And when comparing the data, the participants only have to keep the outcomes of rounds where the measurement choice is the same at both end

[taeric]

I'm reading that as the distributions that are seen are not stable. It may be that you got 30% this time, but 40% next time. On the exact same setup. (Obviously making up numbers.) Yes, you know that the other side saw something, but that is obviously useless.

Framing I saw was more of a truth table like where B/C have known states they can be in that each lead to a known distribution of outcomes. It was not clear that the known distribution was only an observed distribution.