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I have read this question:

Correlation vs. entanglement for composite quantum system

Entangled states can produce nonclassical correlations, but this is not necessarily the case.

So far so good.

For instance, for a two-particle system of spinful particles with the constraint that the total spin is zero, you always have the correlation that if you measure the spin of one particle to be up (in one direction), the spin of the other particle will be down (in that direction). This is true regardless of whether the system is in an entangled state."

This is exactly what I am asking. What more does in this case entanglement add? Will entanglement change the measurement other then what correlation of the total spin=0 gives?

These do not answer my question.

I have read this question:

Why is quantum entanglement considered to be an active link between particles?

Quantum Entanglement - What's the big deal?

where Luboš Motl says in a comment:

Entanglement is nothing else than correlation between two objects ("subsystems") and this correlation is always a consequence of their mutual contact or common origin in the past.

These answers state that QM entangelement is always based on a spatial mutual contact or common origin in the past.

Understanding quantum entanglement.. help me validate this analogy!

If we both choose to open our envelopes from the bottom, we always (or nearly always) find papers of identical colors. Now try telling a story like yours --- where the envelopes carry true information about what's "really" in the other envelope --- that fits these facts. Good luck.

Now this summarizes my question basically. What is the reasoning behind this difference between the two statements? One says QM entanglement is just correlation. The other one says it is not, because QM entanglement gives you examples in the envelopes that classical correlation cannot explain. Which one is right?

Question:

  1. Will an entangled system give different measurement then just correlation?
Árpád Szendrei
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  • @probably_someone I have read that one and I just edited to make it clear. I am asking if entanglement adds something more nonclassical then just the classical correlation that is explained in the answers there. That answer is for a mathematical correlation vs entanglement which is understandable. What I am asking is why does entanglement add more then the classical sock in the box eigenvalue subsets? – Árpád Szendrei Jan 22 '20 at 23:38
  • See my answer here: https://physics.stackexchange.com/a/330571/4993 – WillO Jan 22 '20 at 23:57
  • Does this answer your question? Understanding quantum entanglement.. help me validate this analogy! – WillO Jan 22 '20 at 23:58
  • @WillO your answer seems to be somewhat OK for me, but I am asking for a reasoning whether the classical is the same or QM entanglements adds more, and you say that basically "If we both choose to open our envelopes from the bottom, we always (or nearly always) find papers of identical colors. Now try telling a story like yours --- where the envelopes carry true information about what's "really" in the other envelope --- that fits these facts. Good luck." – Árpád Szendrei Jan 23 '20 at 00:55
  • @WillO I do understand if you choose from the top then it will be opposite color, and if one from the top and one from the bottom then opposite. But what do you mean by if they both choose from the bottom, then it is identical color? That is exactly my question here, that reasoning explained, because that is the difference between classical and QM. I will edit my question. – Árpád Szendrei Jan 23 '20 at 00:56
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    Arpad: The point precisely is that the facts described about the envelopes are classically impossible, but that entangled particles do behave that way. Therefore yes, entanglement is different from classical correlation. – WillO Jan 23 '20 at 01:12
  • @WillO that is exactly my point. I agree with you. Maybe I did not express myself clearly. Then what is the reasoning behind the statement "Entanglement is nothing else than correlation between two objects ("subsystems") and this correlation is always a consequence of their mutual contact or common origin in the past." – Árpád Szendrei Jan 23 '20 at 01:15
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    Does this answer your question? Correlation vs. entanglement for composite quantum system –  Jan 23 '20 at 01:36
  • @BenCrowell "For instance, for a two-particle system of spinful particles with the constraint that the total spin is zero, you always have the correlation that if you measure the spin of one particle to be up (in one direction), the spin of the other particle will be down (in that direction). This is true regardless of whether the system is in an entangled state." This is exactly what I am asking. What more does in this case entanglement add? Will entanglement change the measurement other then what correlation of the total spin=0 gives? I will edit. – Árpád Szendrei Jan 23 '20 at 01:41
  • https://en.wikipedia.org/wiki/Bell_test_experiments – d_b Jan 23 '20 at 01:49
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    This is another example of seeing "nothing is better than ice cream" and "broccoli is better than nothing", and concluding "broccoli is better than ice cream". – knzhou Jan 23 '20 at 01:58
  • @knzhou ""For instance, for a two-particle system of spinful particles with the constraint that the total spin is zero, you always have the correlation that if you measure the spin of one particle to be up (in one direction), the spin of the other particle will be down (in that direction). This is true regardless of whether the system is in an entangled state." This is exactly what I am asking. What more does in this case entanglement add? Will entanglement change the measurement other then what correlation of the total spin=0 gives? Is this not a specific question? – Árpád Szendrei Jan 23 '20 at 01:59
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    That's specific, but there's a bigger problem here: you've written 24 questions and answers on quantum entanglement, but the basic question of whether entanglement is different from classical correlations is the most basic possible question. Your particular question here is literally the first question every asked in this field, and it was answered by Bell over 50 years ago. My point, as I've told you many times, is that trying to learn physics from internet posts by word association is extremely inefficient and will leave your understanding with massive gaps like this one. Just get a book! – knzhou Jan 23 '20 at 02:07
  • This is the same thing as writing 24 questions and answers about systems programming, but then turning around and asking what an integer is. Or writing 24 questions and answers about Shakespeare, and then asking what a noun is. – knzhou Jan 23 '20 at 02:09
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    @knzhou I agree with you. Maybe the problem is I am not expressing myself clearly enough. I just wanted to know if a correlated system of two particles can have sum of spin=0 meaning the measurement will always give you for one particle the inverse the other particle's spin. But entanglement will even give you some type of correlation for spin on another axis (which cannot classically be explained), and I just tried to ask for a explanation on the difference, so entanglement should not be just correlation. – Árpád Szendrei Jan 23 '20 at 03:23
  • I think John Bell's experiment simply shows that there is no hidden variables meaning there is no classical correlation see Bell's inequality. Hope this help. :) – user6760 Jan 23 '20 at 07:23
  • related: https://physics.stackexchange.com/q/429136/955 – lurscher Jan 23 '20 at 16:57

1 Answers1

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(need 50+ rep to comment here)

I see some of what you mean. Here is a focal point in your question body:

This is exactly what I am asking. What more does in this case entanglement add? Will entanglement change the measurement other then what correlation of the total spin=0 gives?

You haven't read any clear description yet (that you understand) about the results of measuring entangled particles. You ask if every pair adds up to the same total. No.

For instance, for a two-particle system of spinful particles with the constraint that the total spin is zero, you always have the correlation that if you measure the spin of one particle to be up (in one direction), the spin of the other particle will be down (in that direction). This is true regardless of whether the system is in an entangled state."

This part of your question body explains that a system can provide these results if the particles are not entangled. So certain types of systems behave in that manner. The behavior has nothing to do with entanglement.

If we measure a property of one particle we know something about it that is no longer a probability. We will then find the other particle is also no longer just in a probability state.

That is entanglement. It means the two particles gain a fixed state when we measure one of them.

The state they gain, whether it is spin-up, spin-down or any other position has nothing to do with the entanglement. The results are random, unpredictable, no pattern.

FacingDownZ
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