Is the reality of Quantum Entanglement still an open question?

If we have two entangled particles A and B as an example, has there been any experiment measuring particle A's and freezing that state for a time and measuring B's value anytime later? Most of the experiments I see are continuous streams of photons or particles, split-ted and fed to detectors. But not two isolated systems clearly showing that "spooky action at a distance".

## Comments

You have multiple issues/questions here.

- You question whether Bell's Theorem actually precludes all possible hidden variable theories.
- You question whether experimental tests of entanglement are "unambiguous."
- In addition, you have some questions about at least some of the details of the experimental setup.

First off, Bell's Theorem is a (mathematical) theorem in the true sense of the term: That means that it has been proven (mathematically) that if the conditions of the theorem are met, then the consequences will follow, without fail. However, it may be important for you to know that the conditions, of the theorem, are, essentially, the definitions of (Bell's) hidden variable-type theories. So the results of the theorem are how the measurement statistics will work out for such hidden variable-type systems (Bell's Inequality).

Now, while Bell's definition of a hidden variable-type theory is eminently reasonable, and appears to be rather broad, there is a *possibility* that it may not cover all possible hidden variable-type theories. Therefore, if one were to be able to formulate a hidden variable-type theory that does not fall under the definition given by Bell, then Bell's Theorem cannot be used to constrain the measurement statistics.

Now, as far as the EPR-type (entanglement) experiments are concerned, I don't remember whether the EPR paper made such things clear, or whether they simply assumed their audience already knew, but there are a number of important experimental considerations:

- You must have at least two systems (particles will do) that are in a state of "entanglement" (at least two entangled particles). (Entanglement is a particular type of Quantum Mechanical (QM) state that has no true Classical analog.)
- You must be able to make at least two different measurements on each of the systems, and the nature of each measurement must be such that one cannot make more than one such measurement on a QM system without interference (the QM operators must not commute), like measurements of position and momentum in the same direction.
- You must make the measurements on all of the systems such that there is no possible causal connection between the various measurements.

Without these conditions, there are Classical, speed of light or slower, means whereby the results predicted by QM could be simulated (no "spooky action at a distance" required). All the special care that goes into creating good EPR-type (entanglement) experiments is really all about closing all the possible "loopholes" whereby the QM results could, possibly, be simulated by Classical means.

So, your question about whether there are any "unambiguous" entanglement experiments really boils down to whether there have been any such experiments performed that closed all the possible "loopholes".

Unfortunately, as far as I know, right now, there are none, though the possible "loopholes" keep getting smaller and smaller, with more of them being closed almost with each new experiment.

David

What I'm thinking is that Quantum Entanglement can be clearly shown to be true by the following experiment, if it is possible to perform:

Two entangled particles A and B are separated at a distance. We hold/measure the spin of particle A and then at some time later we measure the spin of particle B if we get the same anti-correlation at any time in the future then we may prove entanglement.

In other words nearly eighty years after the EPR publishing Einstein's (et. al.) critiques of quantum theories still have not yet been disproven. The fact that a self-proclaimed non-physicist is having "nightmares" and is still dreaming ways of disproving it speaks VOLUMES.

Good Luck!

P.S. Schrodinger STOLE his "cat" from Einstein.

Please go back and reread the three (3) "important experimental considerations", above.

What you seem to be proposing cannot "prove" (Quantum) entanglement, using your proposed setup, because one can obtain the same anti-correlation (or correlation, as the case may be) with a purely Classical, hidden variable theory with causal (speed of light or less) interactions (absolutely no need for any "spooky action at a distance").

Do you see that?

David

*it also says that we cannot be definite about it*. For one thing, entanglement disappears once you made the measurement. It is like saying "it is true (entanglement) but you cannot be certain of it".

I'm not quite sure what you mean by "definiteness", here. Are you wanting some set of measurements that don't have to be analyzed statistically?

If we were only looking for "spin up" vs. "spin down", or "left-hand polarized" vs "right-hand polarized", etc. Then, yes, we could be "definite", but we would also, again, be able to get the same result using a local, Classical, causal, deterministic (hidden variable) theory. So we would not be able to distinguish between such vs. Quantum Mechanics (QM).

If one is attempting to determine whether QM is a better fit to experiment/observation, one needs to find experiments/observations that (preferably, maximally) distinguish between the alternatives.

Such is the case for distinguishing between any theoretical alternative explanations.

You say: "QM says that two pairs [actually, two particles of an entangled pair] separated at a distance would be entangled and *it also says that we cannot be definite about it*." Now, if by "*we cannot be definite about it*" you mean that the statistical nature of the measurements on entangled pairs of particles are such that one cannot know, just from measuring one particle, what measurement must have been performed on the other particle (so as to be able to use such entangled pairs of particles for "faster than light" communication), yes, you are quite correct.

However, if the two researchers have already agreed upon a particular measurement, with only two possible outcomes (up-down, left-right, etc.) then one can be certain (given 100% detection efficiency, of course) what the results will be or were of the measurement of the other, just from a measurement of the one on hand. Very definite, but indistinguishable from having the particles already predetermined.

If you go a step further, and have the two researchers set up a protocol of such two state experiments (say start with left-right, then up-down, then two left-right, then ...), then, once again, such can be quite definite. On the other hand, a sufficiently "knowledgeable" predetermined sequence of particle pairs can accomplish the same thing.

So, to "definitely" show that one has Quantum Entanglement, rather than some (sophisticated) local, Classical, causal, deterministic (hidden variable) system, instead, one must be more clever. Unfortunately, this means that neither researcher can know what the other is doing, until they come together and compare notes.

The Bell inequality is all about the limits on the statistics that a (Bell-type) hidden variable theory could achieve under such a situation. Then compare that to the Quantum Entanglement case, and chose measurements so as to maximize the statistical differences, and voila! You have something that is as "definite" as we seem to be able to get, in trying to distinguish such possibilities.

Does this help? Or did I completely miss the "definiteness" concept you were driving at?

David

However, if the two researchers have already agreed upon a particular measurement, with only two possible outcomes (up-down, left-right, etc.) then one can be certainDavid, I may misunderstand you here, but what he talks about is "counter/contra factual definiteness", and that can only be empirically seen in the statistics at arbitrary relative angles (between Alice and Bob's measurement devices). So what you write here is at least misleading.

Johann: Deleting my comment while keeping the pseudoscientists that still advertise Joy Christian may not be so wise.

It is certainly quite possible that what Johann is trying to refer to is, indeed, "counter/contra factual definiteness". However, I would prefer he be the one to express that that is what he was trying to get at, since there are many other possible things he could be alluding to or seeking.

David

P.S. I have to agree about the "pseudoscientists that still advertise Joy Christian" thing, but this is Johann's 'blog, after all.

" You say: "QM says that two pairs [actually, two particles of an entangled pair] separated at a distance would be entangled and

*it also says that we cannot be definite about it*." Now, if by "

*we cannot be definite about it*" you mean that the statistical nature of the measurements on entangled pairs of particles are such that one cannot know, just from measuring one particle, what measurement must have been performed on the other particle (so as to be able to use such entangled pairs of particles for "faster than light" communication), yes, you are quite correct."

I appreciate your help in correcting my misconceptions Dave, you're a very good teacher.

P.S.

Nightmares are to continue, since now its clearer that QM is extremely weird.

Careful Mr Cruz, poking the bear (elucidating flaws in modern scientific theories) may result in a violent mauling by said bear… (if you are lucky they will simply ignore you)