Entanglement swapping and Bohmian mechanics

[Demystifier]

Are there any experiments that demonstrate and entanglement between a measurement apparatus and a photon? That would certainly settle a lot of questions.

Actually there are such experiments. (With the aid of AI today it is not difficult to find literature that discusses such experiments.) One example is https://arxiv.org/abs/1806.10615 , where the apparatus is a mechanical oscillator containing about $10^{10}$ atoms.

 

[DrChinese]

Actually there is. (With the aid of AI today it is not difficult to find literature that discusses such experiments.) One example is https://arxiv.org/abs/1806.10615 , where the apparatus is a mechanical oscillator containing about $10^{10}$ atoms.

That’s not what I asked. At all. Plenty of experiments have been done showing entanglement between large quantum objects/systems.

But you just said (other post) that computer bits in RAM somewhere are entangled with a photon mid flight. That’s a pretty big claim, for a lot of reasons. I’m not sure you really sense how crazy that sounds. And how obviously speculative that is, without the slightest evidence whatsoever that it actually occurs. Even you acknowledge that there can be only one bit of information that is entangled with that photon at any one time.

Seriously, anybody can make up stuff, knowing it can never be proven or disproven. Tiny invisible turtles, all the way down – and all that.

Now, please understand this carefully: if that’s what Bohmian mechanics says, that’s what it says. That’s precisely what I’m asking about and what I want to learn. I’ve asked some questions, and you’ve given me some answers. And I sincerely appreciate that, as well as your time.

 

[Demystifier]

That’s not what I asked. At all. Plenty of experiments have been done showing entanglement between large quantum objects/systems.

But you just said (other post) that computer bits in RAM somewhere are entangled with a photon mid flight. That’s a pretty big claim, for a lot of reasons. I’m not sure you really sense how crazy that sounds. And how obviously speculative that is, without the slightest evidence whatsoever that it actually occurs. Even you acknowledge that there can be only one bit of information that is entangled with that photon at any one time.

Seriously, anybody can make up stuff, knowing it can never be proven or disproven. Tiny invisible turtles, all the way down – and all that.

Now, please understand this carefully: if that’s what Bohmian mechanics says, that’s what it says. That’s precisely what I’m asking about and what I want to learn. I’ve asked some questions, and you’ve given me some answers. And I sincerely appreciate that, as well as your time.

Well, to be honest, the claim that it is just the bit in the computer chip, and nothing else, that is entangled with the photon is an oversimplification. It may be approximately true during a relatively short time, but not for very long. In reality, it is much more complicated. In a sense, it is the whole laboratory containing the apparatus, the human observer, the air in the laboratory, the photons emitted to the Universe from the laboratory, etc., that is entangled with the photon. Needless to say, this entanglement is practically impossible to prove experimentally. But that's what our theories say, based on the theory of decoherence, that the photon is ultimately entangled with the whole laboratory and its environment. Are you familiar with the decoherence theory?

Anyway, if you don't believe such explanations based on decoherence theory, what is your explanation of entanglement swapping? Do you think that standard QM can explain it? Or do you think that it's a big mystery?

 

[DrChinese]

1. Well, to be honest, the claim that it is just the bit in the computer chip, and nothing else, that is entangled with the photon is an oversimplification. … In a sense, it is the whole laboratory containing the apparatus, the human observer, the air in the laboratory, the photons emitted to the Universe from the laboratory, etc., that is entangled with the photon. Needless to say, this entanglement is practically impossible to prove experimentally. But that's what our theories say, based on the theory of decoherence, that the photon is ultimately…

2. Anyway, if you don't believe such explanations based on decoherence theory, what is your explanation of entanglement swapping? Do you think that standard QM can explain it? Or do you think that it's a big mystery?

1. That’s essentially what I’m hearing from you. I accept that as your answer.

2. Swapping with delayed choice: Pretty big mystery would be fair.

Of course, Asher Peres, Anton Zeilinger and a large cast of other equally great scientists put this together without the slightest assistance from Bohmian theory. That’s not a criticism, it’s just fact. You can interpret that anyway you like.

Every interpretation seems to make some kind of assumption not present standard QM. BM does, MWI does, etc. They say an atheist believes in exactly one fewer God than a theist. I often question assumptions. I strongly suspect you do too. Ditto for any reader following along this thread.

So when you make statements, the likes of which I have never seen or heard before, what is strange about me asking? There are literally dozens of interpretations of quantum mechanics. Presumably all but one, at best, is wrong. Hopefully you follow what I’m saying and why.

My personal goal is to reconcile advanced modern experiment with some of these interpretations. I strongly believe that many cannot survive deep questioning. Why do I believe that? BM, MWI and many other interpretations were developed 50-60+ years ago. (Of course, orthodox theory is even older Lol.) So there’s a pretty good chance that some of them may not be able to perform effectively against these newer experiments. Bohmian mechanics, as I mentioned earlier, has the advantage that it is non-local by design. That should be an advantage.

 

[Demystifier]

Of course, Asher Peres, Anton Zeilinger and a large cast of other equally great scientists put this together without the slightest assistance from Bohmian theory.

I don't have problems with that. Most interpretations agree that the photon B is entangled with the whole laboratory and its environment, there is nothing particularly Bohmian about that assertion.

In fact, I can even prove a version of this claim theoretically without hand waving, by an reductio ad absurdum. Assume the opposite, that after the measurement of A, the B is not entangled with anything. I think that's what you intuitively feel to be the case, that B is no longer entangled with anything. But it was entangled with something before, so if later it is no longer entangled with anything, this breaks one of the main laws of QM. Namely, unitarity implies that von Neumann entropy of a closed system cannot change, that's a theorem. But if B ceases to be entangled with anything, this means that the whole Universe comes into a state of the form $\rho_B \rho_{\rm rest}$, where
$$\rho_B = \frac{1}{2} \left[ |+\rangle\langle +| + |-\rangle\langle -|\right]$$
is a mixed state with non-zero entropy, which increases von Neumann entropy of the Universe. And since the Universe is a closed system, this increases von Neumann entropy of the closed system, thus violating the theorem. In other words, to satisfy the theorem, the photon must be entangled with something. And that "something" must be a part of the Universe, so the photon must be entangled with some part of the Universe. The theorem does not say which part is that, but it says that it must be entangled with some part. Q.E.D.

A side remark. The above analysis is also closely related to the black hole information paradox. So if you want an example of a problem with quantum entanglement that is really hard to resolve, don't study entanglement swapping (there are no many people in the world who think it's a big problem), study black hole information paradox instead. See e.g. my https://arxiv.org/abs/2405.05617 and references therein.

 

[gentzen]

What’s your handle there? I was looking, maybe post #401 was a bad place to start?

The exchange starts with his statement in #398 and my reaction in #405. It ends with his reaction in #408:

Verschränkungen gehören nicht zum sprachlichen Repertoire der TI. Man sieht sie nämlich nur bei Wellenfunktionen, sie beschreiben also nur winzigste Systeme (ohne Temperatur), insbesondere keine Detektorten.

Die Beschränkung der "Sichtbarkeit" von Verschränkung auf "winzigste Systeme" wird wohl schon richtig sein. Aber die "nur bei Wellenfunktionen" und "(ohne Temperatur)" Teile stimmen vermutlich nicht ganz. Damit man von (bipartiter) Verschränkung sprechen kann, [...] Weil einige Systeme aber "ohne Vermittler" kaum miteinander interagieren (z.B. Photonen, solange [...]), ist manchmal sogar Zimmertemperatur kein Problem.

Die Verschränkung sieht man nur, wenn der Zustand rein ist; in einer Dichtematrix sieht man statt dessen Korrelationen.

Naja, laut deutscher und englischer Wikipedia ist es im Allgemeinen halt NP-schwer zu entscheiden, ob bei einem gemischen Zustand Verschränkung vorliegt:
[...]
[...] Aber welches Bild auch immer sich dabei ergeben würde, es ändert nichts daran, dass man auch bei gemischen Zuständen von Verschränkung spricht, und diese geeignet quantifiziert.

Es ändert auch nichts daran, das dies für die TI vollkommen irrelevant ist.

 

[gentzen]

Umm, never thought that nor said that. Particle A can be maximally entangled with system BC.

That is exactly the point, that neither you nor Demystifier ever said (or thought) anything about "bipartite," "tripartite," or "multipartite".

But C cannot be maximally entangled with D at the same time on the same basis.

C can be entangled with D. And D can even be maximally entangled with a subsystem of C. And therefore, perhaps a bit surprisingly, D can be maximally entangled with C.

The question of whether C can be maximally entangled with D is "just" a question of language. I would argue against such language, because it creates too much confusion. So if the Hilbert space $H_D$ has a smaller dimension than the Hilbert space $H_C$, the approprite languge for me is that C is never maximally entangled with D, but D can be maximally entangled with C.

Let us assume that "the larger system" C is maximally entangled with D. What do we have to compute if we want to know whether A is maximally entangled with BC? We have to trace out D (and any other relevant environment) and check the entanglement of the resulting density matrix on $H_A \otimes H_{BC}$. That was the point I brought up in my correction of the expert we both know:

Damit man von (bipartiter) Verschränkung sprechen kann, muss man den "globalen" Hilbertraum in ein Tensorprodukt der beiden verschränkten Systeme A und B, sowie der Umgebung C faktorisieren, also H=A⊗B⊗C. Dann muss man in der "globalen" Dichtematrix auf H die partielle Spur über die Umgebung C nehmen. Man erhält eine Dichtematrix auf A⊗B, die prinzipiell eine Verschränkung zwischen A und B enthalten könnte.

The point with the density matrix is not just ignored by that expert, but also by Demystifier (and you). Let me try to check how Demystifier reacts when I try to correct him.

 

[Demystifier]

Let me try to check how Demystifier reacts when I try to correct him.

No problem at all.

 

[gentzen]

Note that the entanglement monogamy is never broken. Initially, B is maximally is entangled with A. Then, it is maximally entangled with the electron, no longer with A. Then, it is maximally entangled with the amplified signal, no longer with the electron. Then, it is maximally entangled with the bits in the computer, no longer with the amplified signal.

Why should it no longer be entangled with the electron, just because the signal got amplified? In a quantum computer, you would have to perform an uncomputation on the electron, if you want the entanglement to go away. If you don't do this, and "trace out the electron", the entanglement between amplified signal and B will be nearly gone.

And the same for the bits. You can store them in a huge error correcting code in a quantum computer if you want, and perform an uncomputation on everything else. But if you don't do this, then nearly no entanglement will be left, after tracing out the relevant environment.

Well, to be honest, the claim that it is just the bit in the computer chip, and nothing else, that is entangled with the photon is an oversimplification. It may be approximately true during a relatively short time, but not for very long.

I think it is not an oversimplification, but simply wrong. You have to include everything where the relevant information can still be found, even if the information already got deluded over many degrees of freedom.

Missing a few of such degrees of freedom is not yet fatal, because if only a small part of the relevant information can be found there, then the entanglement will still be nearly intact, even after tracing out those few degrees of freedom.

In reality, it is much more complicated. In a sense, it is the whole laboratory containing the apparatus, the human observer, the air in the laboratory, the photons emitted to the Universe from the laboratory, etc., that is entangled with the photon.

That is probably overkill, as explained above. Only those part which include a substantial part of the information need to be included. So the rest of the Universe can often be ignored.

 

[DrChinese]

1. Most interpretations agree that the photon B is entangled with the whole laboratory and its environment, there is nothing particularly Bohmian about that assertion.

2. Assume the opposite, that after the measurement of A, the B is not entangled with anything. I think that's what you intuitively feel to be the case, that B is no longer entangled with anything.

1. I would absolutely dispute your statement in every respect. But that is just a side comment.

2. In orthodox QM, photon B is still entangled to photon A, even though photon A no longer exists. Ma et al: entanglement can be "produced a posteriori, after the entangled particles have been measured and may no longer exist".

It takes an interpretation to describe it otherwise. Which in and of itself is not an issue.

You describe the ongoing entanglement of photon B as being with the photon A apparatus et al. That does work! As I say, if that’s your description, that’s your description. Yes, it raises a lot of flags to me. But I’ve been interested in your take. So all good.

 

[Morbert]

2. In orthodox QM, photon B is still entangled to photon A, even though photon A no longer exists.

That's not orthodox QM. That's a particular account, layered on top of orthodox QM. Alternative accounts do not have B still entangled with A.

 

[DrChinese]

That's not orthodox QM. That's a particular account, layered on top of orthodox QM. Alternative accounts do not have B still entangled with A.

Yes, it is oQM - as I quoted from the authors. A couple of points.

1. I bolded the word “still”. B is still entangled with A, even though A does not still exist. Although it seems nonsensical according to classical norms, that’s the orthodox rule. Experimentalists do not see anything as “missing” in this description. This IS the minimum.

2. It is a requirement for B to be in an entangled state in order to participate in a swap. Being formerly entangled with A is sufficient; that’s referred to as “quantum nonlocality” in thousands of papers (just do a title search in Arxiv).

If in BM: you say B is presently entangled with A’s measurement apparatus, that added assumption works too. Which I assume is why @Demystifier has taken the time to clarify this assumption/hypothesis/foundational point and present it to me here. I wanted to understand how Bohmians addressed this, and he has explained.

 

[Matterwave]

1. I bolded the word “still”. B is still entangled with A, even though A does not still exist. Although it seems nonsensical according to classical norms, that's the orthodox rule. Experimentalists do not see anything as “missing” in this description. This IS the minimum.

I am interested in how you read the original papers. The quote from Ma et. al.

There [Peres Gedankenexperiment], entanglement can be “produced a posteriori, after the entangled particles have been measured and may no longer exist.” In this work we report the first realization of Peres' thought experiment.

Is quoting directly from Peres:

In the present article, I propose an even more
paradoxical experiment, where entanglement is produced a posteriori, after the entangled
particles have been measured and may no longer exist.

It seems you take this sentence to be Peres talking at face value, is that correct?

When I read this, however, I see Peres as setting up a rhetorical device which he will tear down later. Why do I read it this way? Because of the conclusion of the Peres paper, in the section "The Paradox":

There can be no doubt that the particles that were independently produced and tested
by Alice and Bob were uncorrelated and therefore unentangled. Each one of these particles may well have disappeared (eg, been absorbed) before the next particle was produced, and before Eve performed her tests. Only the records kept by the three observers remain, to be examined objectively.

I read this as saying "just the experimental facts are objective".

How can the appearance of entanglement arise in these circumstances? The point is
that it is meaningless to assert that two particles are entangled without specifying in
which state they are entangled, just as it is meaningless to assert that a quantum system
is in a pure state without specifying that state [9].

I read this as him saying sentences like "A and B are entangled" have no meaning unless you specify the exact state. There's no objective fact about whether A and B are entangled or not, there is just the description given by the state vector.

Finally, he states:

In summary, there is nothing paradoxical in the experiments outlined above. However,
one has to clearly understand quantum mechanics and to firmly believe in its correctness to see that there is no paradox.

I read this as him saying he doesn't actually see a paradox. And hence why I think the quote "I propose an even more paradoxical experiment" from his abstract is a rhetorical device.

In other words, I see Peres as holding an almost absolutely instrumentalist view where we should just set up experiments, "shut up and calculate", and match the calculations to the results of the experiments. In such a worldview, which he holds to be *the* world view of quantum mechanics, no paradoxes may arise. The world simply follows the mathematics of the formalism and that's that.

Hence, I am interested to know how you read the paper as backing up your claim "B is still entangled with A, even though A does not still exist." as being "standard quantum mechanics".

I am sure you read the paper very differently than me! And I certainly can be very deeply mistaken. I don't pretend that I can see into the mind of Peres. I only give my accounting of how I read it. :)

 

[Demystifier]

In orthodox QM, photon B is still entangled to photon A, even though photon A no longer exists.

This view is not foreign to me. I have studied similar ideas in the context of black hole information paradox, e.g. here https://arxiv.org/abs/0905.0538 . In essence, in such situations we have an entanglement between the future and the past. Some 15 years ago I thought of such a view as promising, but I never thought of it as standard or orthodox. I still don't.

 

[Demystifier]

It is a requirement for B to be in an entangled state in order to participate in a swap. Being formerly entangled with A is sufficient; that’s referred to as “quantum nonlocality” in thousands of papers (just do a title search in Arxiv).

But this nonlocality is a nonlocality over time, while the usual nonlocality in QM refers to nonlocality over space. Note that nonlocality over time can, in principle, even be explained in classical terms. Namely, to explain the correlation between the future and the past in classical terms, it is not necessary that information travels faster than light. In this sense, the nonlocality over time is not a true nonlocality.

 

[DrChinese]

I am interested in how you read the original papers. … It seems you take this sentence to be Peres talking at face value, is that correct? … I read this as saying "just the experimental facts are objective".

I read this as him saying sentences like "A and B are entangled" have no meaning unless you specify the exact state. …
In other words, I see Peres as holding an almost absolutely instrumentalist view where we should just set up experiments, "shut up and calculate", and match the calculations to the results of the experiments. In such a worldview, which he holds to be *the* world view of quantum mechanics, no paradoxes may arise. The world simply follows the mathematics of the formalism and that's that.

Hence, I am interested to know how you read the paper as backing up your claim "B is still entangled with A, even though A does not still exist." as being "standard quantum mechanics".

Of course I’m well familiar with the Peres paper. And he’s famous for saying things like “just the experimental facts are objective”.

Well, it’s an experimental fact that our photon B must be entangled to participate in a swap. And you aren’t gonna find Peres discussing entanglement between that photon and bits in a computer anywhere. None of the experimentalists use lingo anything remotely more or less than what I’m saying. Obviously, they use generally accepted, terminology, wording, and scientific technique. In other words, orthodox quantum mechanics. You are free to dissect or interpret their quoted words anyway you like.

Ma et al:"produced a posteriori, after the entangled particles have been measured and may no longer exist"

As to what state A and B are initially entangled in: that is plainly stated in each paper. |Ψ−〉usually.

 

[DrChinese]

This view is not foreign to me. I have studied similar ideas in the context of black hole information paradox, e.g. here https://arxiv.org/abs/0905.0538 . In essence, in such situations we have an entanglement between the future and the past. Some 15 years ago I thought of such a view as promising, but I never thought of it as standard or orthodox. I still don't.

Umm, exactly what else do you think Megidish is if not oQM? It’s an experimental demonstration of exactly that, 2012. Entanglement across time… and I posted that reference on PF the same week it came out.

Now one of my points is that Bohmians seem to be playing catch-up with the state of the art for orthodox quantum mechanics. (And you sort of just made my point.) Which is why I ask probing questions trying to learn more. For those that are not well acquainted with the Ma and Megidish papers, a better understanding of those may be enlightening.

Ma https://arxiv.org/pdf/1203.4834
Megidish https://arxiv.org/pdf/1209.4191

For those that are confident that there is nothing new for the Bohmian to learn, these papers won’t mean much. For anyone else, they should be raising questions - regardless of your preferred interpretation.

 

[javisot]

Umm, never thought that nor said that. Particle A can be maximally entangled with system BC. But C cannot be maximally entangled with D at the same time on the same basis.

There’s a huge difference between a quantum system and a macroscopic object. And when it comes to entanglement, trying to talk about them on equal terms is virtually nonsensical.

I) For example, let’s talk about entangled photons and measurement apparati. I send entangled photon A to a polarization measurement apparatus. Some (Bohmians) seem to claim that apparatus is now entangled with photon B. Now, exactly what part of the apparatus is entangled to B? It first goes through a polarizing beam splitter. Is that what becomes entangled? Then later photon A continues on to a detector and is absorbed. Is that when it becomes entangled? Is the polarizer entangled with the detector and photon B?

II) And whatever apparatus is entangled with photon B: does this new entanglement have any impact on entangled electrons in atoms, composing the apparatus? Exactly what is entangled with what?

III) And suppose I send another entangled photon E (its partner is F) into the measurement apparatus while it is still entangled with B. Are B and F now entangled as well? Is the measurement apparatus entangled with both B and F simultaneously? By the way, this experiment is easily performed.

Hey, if that’s your hypothesis, just let me know! This is basically what I’m asking in the first place !! We take an initial assumption and direct one self towards reasonabledeductions.

Your doubts are reasonable. When we say "two particles are entangled", regarding spin, for example, we mean that if one has spin-up, the other has spin-down (to put it in simple terms).

So, when we say a particle is entangled with a measuring device, does that mean the particle has spin-up "and the measuring device has spin-down"?

No, that is not actually what we mean. What we mean is that there is a quantum correlation between the particle's properties and the values the measuring device will yield.

 

[Matterwave]

Your doubts are reasonable. When we say "two particles are entangled", regarding spin, for example, we mean that if one has spin-up, the other has spin-down (to put it in simple terms).

A state like $|01\rangle$ where "one has spin up, the other has spin down" is a product state, not an entangled one.

Whereas a state like
$$\frac{1}{\sqrt{2}}(|00\rangle+|11\rangle)$$
Where both have spin down or both have spin up is entangled.

The closer definition of entanglement (at least for pure states) is "not a product state", not whether "one is spin up, the other is spin down".

 

[martinbn]

A state like $|01\rangle$ where "one has spin up, the other has spin down" is a product state, not an entangled one.

Whereas a state like
$$\frac{1}{\sqrt{2}}(|00\rangle+|11\rangle)$$
Where both have spin down or both have spin up is entangled.

The closer definition of entanglement (at least for pure states) is "not a product state", not whether "one is spin up, the other is spin down".

I will point out that sometimes entanglement is used to mean no -classical coreletions.

 

[Demystifier]

Ma https://arxiv.org/pdf/1203.4834
Megidish https://arxiv.org/pdf/1209.4191

For those that are confident that there is nothing new for the Bohmian to learn, these papers won’t mean much. For anyone else, they should be raising questions - regardless of your preferred interpretation.

This can be puzzling only to those who have not yet decided which interpretation of QM they will use, be it Bohmian, many-world, Copenhagen, consistent histories, QBism, whatever. Once you fix your interpretation, there is nothing puzzling about those experiments. But if you have not fixed your interpretation, because neither looks convincing to you, and you want experiments themselves to lead you towards the right interpretation, then you are likely to make up a new interpretation for every significantly new experiment, so at the end you stay confused, because you still don't have one interpretation applicable to all the experiments. So my advice for anyone is: First fix your interpretation, and then interpret particular experiments with your chosen interpretation, not the other way around.

 

[Matterwave]

Of course I’m well familiar with the Peres paper.

I wasn't asking if you are familiar with Peres or not, I am sure you are, I was asking about how you interpret the statements he made in that paper. If you would rather not or you are too busy, I understand.

And you aren’t gonna find Peres discussing entanglement between that photon and bits in a computer anywhere.

Maybe you are confusing me for @Demystifier? I have never mentioned bits in a computer in any post in this thread that I can remember.

You are free to dissect or interpret their quoted words anyway you like.

Sure, and I shared my reading. That's all I did, and I was curious how you read those statements. :)

Ma et al:"produced a posteriori, after the entangled particles have been measured and may no longer exist"

So I take it, you read this completely at face value and give it as the evidence for your statement:

Well, it’s an experimental fact that our photon B must be entangled to participate in a swap.

?

 

[Motore]

It is a requirement for B to be in an entangled state in order to participate in a swap. Being formerly entangled with A is sufficient; that’s referred to as “quantum nonlocality” in thousands of papers (just do a title search in Arxiv).

You’ve got the quotes right, but I think you’re reading too much into what they mean by “entangled.” It sounds like you’re imagining some kind of physical link stretching back through time, connecting a photon that’s already gone with one that’s still around. I don’t think that’s what Peres (or Zeilinger) mean here.

If we stick with the usual Copenhagen-style view of QM, the quantum state isn’t really a physical object. It’s a way of describing the probabilities for different outcomes. So when physicists say that photon A is entangled with photon B, even if A has already been destroyed, they’re talking about the correlations between the measurements associated with A and B.

And Victor’s delayed choice doesn’t change the history of the particles. As I understand it, it’s basically a matter of deciding afterward how to sort and pair the data you already collected. So whether one of the photons still exists at that point isn’t really relevant to the correlation you find in the data.
https://arxiv.org/html/2508.13431v1

 

[Morbert]

Yes, it is oQM - as I quoted from the authors. A couple of points.

1. I bolded the word “still”. B is still entangled with A, even though A does not still exist. Although it seems nonsensical according to classical norms, that’s the orthodox rule. Experimentalists do not see anything as “missing” in this description. This IS the minimum.

The authors do not present this account as orthodox QM. They present it as an account. There are alternative accounts.

What you would need to provide is authors refuting those accounts.

Umm, exactly what else do you think Megidish is if not oQM? It’s an experimental demonstration of exactly that, 2012. Entanglement across time… and I posted that reference on PF the same week it came out.

The significance of the Megidish paper is in the performance of the interesting experiment. Alternative, forward-in-time accounts of the experiment produce the same observed correlations.

 

[javisot]

A state like $|01\rangle$ where "one has spin up, the other has spin down" is a product state, not an entangled one.

Whereas a state like
$$\frac{1}{\sqrt{2}}(|00\rangle+|11\rangle)$$
Where both have spin down or both have spin up is entangled.

The closer definition of entanglement (at least for pure states) is "not a product state", not whether "one is spin up, the other is spin down".

I know; I take it for granted that anyone who has read what I said understands that the "spin-up" and "spin-down" terminology is a linguistic simplification adopted for the sake of the conversation.

The essence of what I said is this: talking about two entangled particles is not the same as talking about the entanglement of one particle and a measuring device.

 

[Demystifier]

Megidish https://arxiv.org/pdf/1209.4191

One more comment. Already in the abstract it says "Any part of a quantum system that has finished evolving ..." I think it's one of the main sources of confusion. In non-relativistic QM, there is no such thing as "part of a system that has finished evolving". In experiments with photons one might think that such thing exists, because the photon can be destroyed so it can "finish evolving". However, in my opinion, such a way of thinking is wrong, or at least deeply misleading. Particle destruction is described by relativistic QFT. In QFT (relativistic or not), a particle is not "a part of a quantum system". Instead, a particle is an excitation of the whole system. A part of the system, instead, is a spatial region with finite volume $V$. It never finishes evolving. In particular, if a particle localized in $V$ is destroyed, the evolution in $V$ does not stop. The destroyed particle does not turn into nothing. It turns into another state, conserving energy and everything else that needs to be conserved, so the state in $V$ keeps evolving.

 

[Sambuco]

Yes, it is oQM - as I quoted from the authors. A couple of points.

Not really. It is quite clear in those works that the authors take an information-based, $\psi$-epistemic interpretation. The very idea that two system become entangled after they have been measured is interpretation-dependent. Furthermore, if by orthodox-QM we mean the textbook formulation (a wavefunction evolving forward-in-time), such a correlation does not necessarily represent genuine entanglement, as discussed in the Cohen paper, where it is called "counterfactual entanglement".

Lucas.

 

[gentzen]

I would say that it is oQM, in the sense that oQM gives you the freedom to talk like that, as long as you don‘t overstretch and say things which contradict the mathematical formalism of QM.

Of course, the trouble with oQM is that you already need both a good intuition for QM and a good technical fluency with the mathematical formalism of QM to operate within those constraints. Many of us are not convinced that DrChinese has those required good intuitions and technical fluency.

 

[Sambuco]

I would say that it is oQM, in the sense that oQM gives you the freedom to talk like that, as long as you don‘t overstretch and say things which contradict the mathematical formalism of QM.

That seems like a reasonable position to me.

Lucas.

 

[Morbert]

I would say that it is oQM, in the sense that oQM gives you the freedom to talk like that

The sense in which @DrChinese means it is he believes oQM forecloses alternative ways to talk about the experiment, such as a forward-in-time account. It of course doesn't.