Talk:Quantum entanglement

Standard error of sign regarding information and entropy.

Short before the sentences:

″The reversibility of a process is associated with the resulting entropy change, i.e., a process is reversible if, and only if, it leaves the entropy of the system invariant. Therefore, the march of the arrow of time towards thermodynamic equilibrium is simply the growing spread of quantum entanglement.[83] This provides a connection between quantum information theory and thermodynamics.″

... all entropy formulas, whether Shannon's or 'von Neumann' tell about possibilities and/or bandwidth. Real data transferred via classic or quantum methods show always the reverse sign, because a single of the many possibilities has been chosen for transfer. In the same way growing quantum entanglement does not increase but reduces entropy. For sure the internal order by entanglement is even the reverse of disorder maximization by thermodynamic equilibrium. If [83] is indirectly cited, it tells simply non-sense. Please drop the sentences above and the reference from the article. Many thanks!

At least 10,000 times faster than light

Wikipedia now says:

"communications at the speed of light would have taken longer—in one case, 10,000 times longer—than the interval between the measurements".

Is it the same thing as the this? :

https://www.extremetech.com/extreme/150207-chinese-physicists-measure-speed-of-einsteins-spooky-action-at-a-distance-at-least-10000-times-faster-than-light

"Chinese physicists measure speed of Einstein's 'spooky action at a distance': At least 10,000 times faster than light. By Sebastian Anthony March 7, 2013"

If yes, could we make the wikipedia more understandable for an average human or even an average educated person reading wikipedia? Just say the speed is at lest 10^4 speed of light?

Has the speed not be estimated after year 2013, to verify results?

217.140.214.197 (talk) 11:05, 23 May 2024 (UTC)

Quantum entanglement lead

Hi, pretty surprised to see your revert (https://en.m.wikipedia.org/w/index.php?title=Quantum_entanglement&diff=prev&oldid=1253878041) of my edit (https://en.m.wikipedia.org/w/index.php?title=Quantum_entanglement&diff=prev&oldid=1253686245).

Not going to waste time disputing it, just want to let you know that one of the hallmarks of a high-quality reference text, textbook, encyclopedic article etc.– even one concentrating on a highly special subject – is providing a broad enough outlook in introduction.

As an occasional reader of Wikipedia articles, I would definitely prefer the version I have suggested; furthermore, I would appreciate reading a lead which doesn't mislead a reader by dropping links to closely related subjects as in this case, in *any* article.

And as a peer reviewer and an editor of manuscripts and proceedings since 1990s, I can tell you that an author who'd send me a paper containing the statement which I have corrected, would see his paper returned with the recommendation to avoid such blunders in future for his own sake. 188.66.35.87 (talk) 18:58, 28 October 2024 (UTC)

I find rather misleading your suggestion that there is a well-defined trio that form the core of the quantum-classical disparity: quantum entanglement, uncertainty principle and wave–particle duality. Such a list is merely a matter of taste. Tercer (talk) 21:07, 28 October 2024 (UTC)

Didn't plan to get back to this, but the *text* of my edit made no claims or assertions of any well-defined trios, I just linked to WPD, UP and DSE, so please refrain from putting in my mouth what I didn't say in the article text. But when it comes to current wording, which calls QE "a primary feature of quantum mechanics" and puts it "at the heart of disparity..." – now that is not only not the best wording for a WP article, but misleading indeed, as generally accepted views of QM do not assign to QE some special or unique role within QM, hence my edit.

Do you have access to classical texts on QM (Feynman's lectures, writings of Bohr), or at least to recognised and widely used graduate-level textbooks (Sakurai, Shankar etc.)? The reason why I'm asking is because it appears you are not very familiar with the subject if you *really* believe that current wording praising the role of QE is good and find my correction misleading. But the fact is, you won't find a QM textbook introducing QE in the same fashion as in the lead, viz. that it's the "primary feature..." simply because it's not – you'd see a chapter dedicated to it at best, along with chapters on other phenomena (and to the best of my memory, not even a dedicated chapter in the textbooks I mentioned above).

And as far as some of the brightest minds are concerned, you won't see them assigning special or central role to QE within QM either: it's well known that Bohr assigned high importance to complementarity principle, and Feynman to WPD; he is quoted as saying ‘a phenomenon which is impossible... to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery [of quantum mechanics].’ (quoting from https://www.nature.com/articles/ncomms6814).

Is that wording, hyping the role of QE, by chance, a quotation from the NYT article? It appears to be behind a paywall, so I can't even check it myself without subscribing or buying it, which I don't plan to do. In general, I would advise against using sources like New York Times as supporting citations in articles like this one. Popular articles tend to be a lightweight reading, are rarely rigorous in academic sense, often written in sensationalistic style, and WP guidelines might in fact not recommend it too (is that correct? I'm not intimately familiar with all of them).

So I would suggest not only fixing current wording, rewriting it in less emotional style and bringing it in line with common knowledge, which doesn't place QE in some special position as far as the disparity of classical and quantum mechanics is concerned, but also replacing the paywalled NYT article with a proper source in line with WP guidelines, or just removing it (statements which are common knowledge don't require a citation).

Do you agree? 217.118.83.166 (talk) 18:36, 29 October 2024 (UTC)

The article claims it's a primary feature, not the primary feature. Tercer (talk) 18:44, 29 October 2024 (UTC)

Nitpicking like that is not an example of constructive communication, especially considering that you see precise quotation from the article at the start of my message and that I spent time explaining to you things which should be well-known to an experienced editor, as well as to someone who is competent to contribute to articles on QM. But all right, I'll take that as a "yes" to my last question and will fix the sentence.

More importantly, do you realise that such conduct as yours (unfounded reverting, manipulating quotations, strange nitpicking) is very effective at only one thing: completely killing motivation in any further contribution to this article, as well as any other, where you are active in the same fashion? A few days ago I considered in earnest allocating some time to taking a closer look at several articles on QM to fix inaccuracies or add some missing content, but seeing how it goes, I reviewed my plans. I have better things to do with my time, than seeing a strange revert and then wasting time explaining the obvious.

And although I'm not highly familiar at all with WP's numerous guidelines, there is an essay and a policy which I consider rather important, in particular for the growth of the number of contributors to WP, and would highly recommend you to adhere to, as long as you don't aim to discourage and demotivate people willing to contribute something: WP:ROWN and WP:OWN. 188.66.32.76 (talk) 20:07, 30 October 2024 (UTC)

It is not nitpicking, it is the main point. You are complaining that the article claims entanglement is the only core distinguishing feature of quantum mechanics, but it does no such thing. Tercer (talk) 22:38, 30 October 2024 (UTC)

No need to misrepresent again what I say, the reasons why I suggest the rewrite are described in detail in my 2nd message. Besides, I asked you clearly if you agree with the suggestion and expected as clear an answer from you, as well as any valid arguments, references to WP guidelines and RS in support of your opinion, but you presented none of that.

So no clear answer, no arguments, but instead nit-picking and another revert from you. Shall we ask for third opinion then?

Request for third opinion

Please choose the version which you consider more appropriate, or feel free to suggest your own. Argumentation of the disputing parties is above. Also please comment whether the paywalled NYT article used for unclear purpose should be kept or removed. Any other comments regarding the dispute are also welcome.

Current version:

The topic of quantum entanglement is at the heart of the disparity between classical and quantum physics: entanglement is a primary feature of quantum mechanics not present in classical mechanics.

Suggested rewrite, which reflects that generally accepted views of quantum mechanics do not assign to quantum entanglement central or unique role, as far as disparity of quantum mechanics and classical physics is concerned, uses more encyclopedic language, and removes paywalled NYT article (purpose of the source completely unclear and no explanation provided by the opponent; NYT is a very strange choice of source to cite in the article on quantum entanglement (WP:V, WP:RS)):

Along with the uncertainty principle and wave–particle duality demonstrable in the double-slit experiment, quantum entanglement is one of phenomena of quantum mechanics, not present in classical physics. 217.118.83.168 (talk) 19:22, 31 October 2024 (UTC)

I think the current version is adequate. Listing other phenomena that are also non-classical just makes the sentence longer without bringing any real clarity. The current version says a primary feature, not the primary feature (or the quintessential feature, etc.). The current version simply does not say that entanglement is the only non-classical feature of quantum mechanics.

I would agree with removing the New York Times article, on the general principle that pop science is not suitable reference material when it comes to quantum physics. Replace it with a textbook, or just cut it entirely, since that paragraph is there to summarize what comes later. The fact that the intro is intended to be a summary of the main article also argues against mentioning wave-particle duality or the uncertainty principle there, because the latter is barely mentioned and the former doesn't appear at all. XOR'easter (talk) 22:11, 31 October 2024 (UTC)

I broadly agree with XOR'easter. I do find that Overbye is one of the best pop-sci writers but there are many alternative sources. Johnjbarton (talk) 17:10, 1 November 2024 (UTC)

XOR'easter, thanks for responding to the invitation and a thoughtful reply. Some very good points there, and I fully see your reasoning, but I also want to make two more comments.

Firstly, after spending some time with WP guidelines, "a primary feature" looks to me as a borderline case, and "at the heart of disparity" a case falling directly under MOS:PUFFERY. Secondly, as I mentioned before, quantum entanglement is never introduced, or referred to, as "a primary feature of quantum mechanics" in professional literature on QM (classical texts, widely used grad-level textbooks, etc.); just one of phenomena, not more primary than others. I think it's obvious that in general WP articles should stick firmly to established or universally adopted characterisation and terminology and avoid inventing something different (there could well be relevant guideline, pointer to which is highly appreciated).

So with all due respect, I really fail to see why this puffery, explicitly not recommended in the guidelines, should be kept. As for mentioning UP and WPD – as I said before, as a *reader*, I would definitely prefer a lead of any article to give me a more or less broad perspective, preferably with links to "same-rank" phenomena as long as they are few, as in this case, and my suggestion is naturally aligned with that preference. But I fully see your reasoning too, and if you are convinced that omitting them is a better choice than including from the viewpoint of offering readers a better source of knowledge (the main goal to keep in mind when evaluating alternatives), then so be it.

So, how about "Quantum entanglement is one of phenomena of quantum mechanics, not present in classical physics." then?

I suggest that we converge on a variant which fully conforms to WP guidelines (feel free to offer your own formulation), thus laying good groundwork for cleaning up the rest of the article to very high standards and extending it with essential content, and do it. That's a serious undertaking, and I would definitely appreciate some company and division of work and responsibilities, rather than do everything single-handedly. Thanks again for the highly constructive input here. 188.66.35.184 (talk) 20:56, 1 November 2024 (UTC)

Contrary to your claim: eg Horodecki et al say "In this way entanglement is that feature of quantum formalism which makes it impossible to simulate quantum correlations within any classical formalism." I think your view unnecessarily narrow. Imagine the "heart of the disparity" being large and accommodating oh so many disparities that entanglement makes the cut. Imagine 10 primary differences and entanglement makes the cut. There is no QM without entanglement. We're not deceiving readers.

By the I think you are focusing on the wrong end of the problem. Per WP:LEAD the intro is a summary so improve the article before getting too wrapped up in the intro. Johnjbarton (talk) 21:20, 1 November 2024 (UTC)

The article already gives a rather famous quote from Schrödinger: "I would not call [entanglement] one but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought." XOR'easter (talk) 17:32, 2 November 2024 (UTC)

Johnjbarton, MOS:PUFFERY has nothing to do with my views, it's one of the guidelines to follow when contributing content, and any content, which doesn't conform to this, or any other, guideline is not a subject of discussion, it's a subject to fix on sight – simple as that. Regarding your example, keep in mind there are rules and there are exceptions.

XOR'easter: Is it natural to expect some other characterisation from the one who essentially introduced the term and wrote a paper on it (not to mention that certain WP guidelines can cover cases like this)? And since there are no more comments to my last message, do I get it right that there is no more disagreement with the suggested rewrite, and that version is fine with you? 188.66.32.88 (talk) 20:26, 2 November 2024 (UTC)

Paradox edit

There seems to be some misunderstanding concerning my recent edits to the "Paradox" section. I replaced the largely unsourced section with content based on a review in one of the top physics journals, Reviews of Modern Physics, by a well-known physicists Asher Peres and Daniel R. Terno. The review has over 800 citations. Part of their article talks directly about quantum paradoxes and uses entanglement in one of the examples. Of course we can debate how well I paraphrased the article and there might be conflicting sources (but other sources like Zellinger are along the same lines). The source is

• Peres, A., & Terno, D. R. (2004). Quantum information and relativity theory. Reviews of Modern Physics, 76(1), 93.

Here I quote the two key paragraphs I used:

• Many physicists, perhaps a majority, have an intuitive, realistic worldview and consider a quantum state as a physical entity. Its value may not be known, but in principle the quantum state of a physical system would be well defined. However, there is no experimental evidence whatsoever to support this naive belief. On the contrary, if this view is taken seriously, it may lead to bizarre consequences, called ‘‘quantum paradoxes.’’ These so-called paradoxes originate solely from an incorrect interpretation of quantum theory, which is thoroughly pragmatic and, when correctly used, never yields two contradictory answers to a well-posed question. It is only the misuse of quantum concepts, guided by a pseudorealistic philosophy, that leads to paradoxical results.
• As a simple example, consider our two observers, conventionally called Alice and Bob,4 holding a pair of spin 1/2 particles in a singlet state. Alice measures sz and finds +1, say. This tells her what the state of Bob’s particle is, namely, the probabilities that Bob would obtain +/-1 if he measures (or has measured, or will measure) s along any direction he chooses. This is purely counterfactual information: nothing changes at Bob’s location until he performs the experiment himself, or receives a message from Alice telling him the result that she found. In particular, no experiment performed by Bob can tell him whether Alice has measured (or will measure) her half of the singlet. A seemingly paradoxical way of presenting these results is to ask the following naive question. Suppose that Alice finds that s_z=1 while Bob does nothing. When does the state of Bob’s particle, far away, become the one for which s_z=-1 with certainty? Although this question is meaningless, it may be given a definite answer: Bob’s particle state changes instantaneously.

I tried to make my summary follow the source closely. The previous content had one source "The Cosmic Compendium: Interstellar Travel" according to the publisher "an in-depth look at interstellar travel". Johnjbarton (talk) 16:58, 1 November 2024 (UTC)

Of course there are conflicting sources. You are giving only the neo-Copenhagen side of the story defended by Asher Peres. It would be equally inappropriate to use a Many-Worlds RS to present the Many-Worlds point of view as the truth, or a Bohmian RS to present the Bohmian point of view as truth, or a collapse model RS to present the collapse models point of view as truth. As poorly-sourced the current text is, it is both correct and neutral. Tercer (talk) 21:31, 1 November 2024 (UTC)

You are making claims but not providing sources. You are claiming the content is correct but it cannot be verified. If you have a source for this content please add it. You are claiming the content is neutral I can point you to Wikipedia policy:

• All encyclopedic content on Wikipedia must be written from a neutral point of view (NPOV), which means representing fairly, proportionately, and, as far as possible, without editorial bias, all the significant views that have been published by reliable sources on a topic.

The content I added is clearly supported by a reliable source. The existing content is not. All of the significant view that have been published by reliable sources are in the content I added. Johnjbarton (talk) 01:39, 2 November 2024 (UTC)

What I'm claiming is simply that whether the quantum state is real is a matter of interpretation. Claiming in Wikivoice that it is not real is a clear violation of NPOV. The neo-Copenhagen interpretation defended by Peres and others says that it's not real. Other interpretations disagree. Many-Worlds, Bohmian mechanics, and collapse models, all say that it is real. We can't take any of their of views as the truth without violating NPOV.

I don't think I need a source for this, what I'm stating is the blinding obvious. Your own quote by Peres shows that he's aware that he is defending a controversial position, though: Many physicists, perhaps a majority, have an intuitive, realistic worldview and consider a quantum state as a physical entity. Tercer (talk) 15:53, 2 November 2024 (UTC)

Please re-read my edit. It made no claim regarding the reality of the quantum state. My edit concerned the origin of "paradox" as given in an excellent reference. The reference points to attempts to use "pseudorealistic philosophy" and "incorrect interpretation" as the origin of paradox. The reference goes on to describe the imaginary existence of particles before measurement as the origin of the entanglement paradox. None of the other QM interpretations you list are relevant here because they all agree with the results of QM.

My edit did not say anything about "truth".

Quoting Peres out of context does not change the reliability of this source. Peres' point is clear from the context and it is not controversial but widely discussed from Einstein/Bohr on. As we have learned over many decades physicists yearn for an intuitive model for QM. I think anyone who ever reads about QM believes in an intuitive model, albeit a different one every week. The failure of these informal models gives rise to paradox. That's his point. Johnjbarton (talk) 19:04, 2 November 2024 (UTC)

When you make a statement in Wikivoice it is implicit that it is the truth, or at least that it reflects scientific consensus. But there is no consensus behind Peres' position, and he is well aware of that.

And yes, you are making a statement about the reality of the quantum state: Peres' position is that the quantum state is not real, and that this sort of nonlocality is an illusion arising from the mistaken assumption that the state is real. Quoting from your edit: Attempts to imagine the quantum states of the particles as a physical entity gives rise to paradoxical effects and The paradox is only a result of imagining the second particle's spin as a physical reality rather than as a result of the overall measurement.. Tercer (talk) 13:33, 3 November 2024 (UTC)

I would agree it would be better to change

• Attempts to imagine the quantum states of the particles as a physical entity gives rise to paradoxical effects

to

• Attempts to imagine the quantum states of the individual particles as a physical entity gives rise to paradoxical effects

Johnjbarton (talk) 17:10, 3 November 2024 (UTC)

That's still pure Copenhagen. We can't state such a thing in Wikivoice. Tercer (talk) 17:16, 3 November 2024 (UTC)

Your stand is very curious. On the one hand you label the content I want as "pure Copenhagen" despite the lack of any connection. On the other hand you are defending the unsourced content which is expressed in the "collapse" jargon associated primarily with Heisenberg and Von Neumann. It seems to me the point of view issue you are so concerned about applies to the current content instead. Johnjbarton (talk) 02:02, 7 November 2024 (UTC)

Setting aside the exact phrasing of the edit, I think that "there are no real paradoxes, only failures of classical intuition" is a sentiment that holds true across interpretations. XOR'easter (talk) 19:15, 2 November 2024 (UTC)

I think the definition of "paradox" that you're using is not helpful. Quoting from paradox: A paradox is a logically self-contradictory statement or a statement that runs contrary to one's expectation. If one insists on having an actual logical contradiction very few paradoxes exist. On the other hand, the use of "paradox" to mean counter-intuitive result allows us to talk about the paradoxical aspects of quantum entanglement, the EPR paradox, etc. Tercer (talk) 13:39, 3 November 2024 (UTC)

I will agree to use the second definition,

• A paradox is a ... statement that runs contrary to one's expectation.

which seems the same as your

• "paradox" to mean counter-intuitive result

or XOR'easters

• "only failures of classical intuition"

This matches the way Peres uses the term. Johnjbarton (talk) 17:14, 3 November 2024 (UTC)

The source

• Nielsen, Michael A.; Chuang, Isaac L. (2000). Quantum Computation and Quantum Information. Cambridge University Press. pp. 112–113. ISBN 978-0-521-63503-5.

discusses the EPR paradox using the singlet state example and never mentions "collapse". This confusing terminology is not needed. Johnjbarton (talk) 17:44, 7 November 2024 (UTC)

Major clean-up (+ GA) effort

How many would be willing to undertake a proper clean-up of this article (rewriting poorly written portions, adding essential content, reviewing bibliography, replacing wikipuffery and other bad style with proper encyclopedic language etc.), and if all goes well, aim for acquiring GA status? That's a huge undertaking for a single person, but a manageable task for a competent team, working towards that goal. 188.66.32.88 (talk) 20:30, 2 November 2024 (UTC)

While I appreciate your enthusiasm, in my experience team efforts on Wikipedia take the form of combat not cooperation. For this article in particular I think you will need exceptionally strong sources and a pragmatic mindset to avoid getting caught up in long discussions over one sentence at a time. I suggest you start with one section rather than an overhaul to begin. Johnjbarton (talk) 21:29, 2 November 2024 (UTC)

I would also encourage you to register for a named user account. Every edit by an IP addr user is suspect. I suppose most editors hesitate to revert edits by named editors especially if they have seen the name before, but don't hesitate for IP users. Johnjbarton (talk) 01:25, 3 November 2024 (UTC)

Sources for "Meaning" section.

The paper

• Schroeder, Daniel V. (2017-11-01). "Entanglement isn't just for spin". American Journal of Physics. 85 (11): 812–820. arXiv:1703.10620. doi:10.1119/1.5003808. ISSN 0002-9505.

has numerous simple examples of entanglement in the context of using the concept in undergraduate courses on quantum mechanics. Near the beginning the paper gives a simple definition:

• ...the underlying concept: that for any quantum system with more than one degree of freedom, the vast majority of allowed states exhibit “correlations” or “non-separability.”

Schrodinger's original paper:

• Schrödinger, Erwin (1935). "Discussion of probability relations between separated systems". Mathematical Proceedings of the Cambridge Philosophical Society. 31 (4): 555–563. Bibcode:1935PCPS...31..555S. doi:10.1017/S0305004100013554. S2CID 121278681.

has this definition:

• Another way of expressing the peculiar situation is: the best possible knowledge of a whole does not necessarily include the best possible knowledge of all its parts,...

I added these sources to the article and my edits were reverted. I ask that my changes be restored. Johnjbarton (talk) 01:58, 7 November 2024 (UTC)

You replaced In entanglement, one constituent cannot be fully described without considering the other(s), which is a straightforward mathematical property of an entangled state, with In entanglement, measurements of properties become correlated, which is completely useless. Merely being correlated is not specific to entanglement; moreover, your sentence changes the subject from the state itself to measurements, which are not talked about in this paragraph at all.

I'm really exasperated at your insistence in editing articles you don't understand. I have better things to do than to undo your damage, and it's really not my job to teach you quantum mechanics. Tercer (talk) 12:43, 7 November 2024 (UTC)

The claim in the article:

• In entanglement, one constituent cannot be fully described without considering the other(s),

is not sourced. There is no way to verify the wording or you claim about it. It is neither straightforward nor mathematical. It uses words like "fully described" and "considering" that are either vague or technical, we can't tell without a reference.

The claim in my edit

• In entanglement, measurements of properties become correlated,

is sourced and a quote is available as an aid to reader. This is a simple sentence: entanglement implies correlation. The sentence does not say correlation is specific to entanglement as you assert.

I also removed the word "local" which is not mentioned in the sources I have read so far. It may be ok, but this extra word should not be included without sourcing because non-locality is discussed elsewhere in the article.

Please restore my changes. If you find sources we can discuss how to best present the content to represent all of the sources. Johnjbarton (talk) 16:24, 7 November 2024 (UTC)

It is neither straightforward nor mathematical. Yet another demonstration that you don't understand the subject. I'm not going to waste my time with this. Tercer (talk) 10:34, 8 November 2024 (UTC)

The essential character of this Meaning section is that a simple definition of entanglement exists. In contrast the opposite character appears in authoritative reviews. For example the Horodecki et al Rev. Mod. Phys. article says:

• "The fundamental question in quantum entanglement theory is which states are entangled and which are not. Only in a few cases does this question have simple answer."

So our content should focus on those few cases but outline what allows the simple definition. The simple definition applies for distinguishable particles in pure states, not generally. Johnjbarton (talk) 02:42, 10 November 2024 (UTC)

Naturally entangled systems

The two sentence section "Naturally entangled systems" claims:

• The correct ionization energy can be calculated only by consideration of electron entanglement.

and cites

• Frank Jensen. Introduction to Computational Chemistry. Wiley, 2007, ISBN 978-0-470-01187-4

However that source never uses the word "entanglement" based on Google Books search. The claim makes no sense. On the one hand, the non-separability of the single electron states for a multi-electron atom is trivially true and the indistinguishability of the electrons means they are entangled. On the other hand "consideration of electron entanglement" is not how quantum chemists think about multielectron systems, as evidenced by the absence of the term in the source. I propose to delete this section. Johnjbarton (talk) 02:38, 7 November 2024 (UTC)

Unverified and incorrect content in section Pure states.

The section on Pure states a paragraph sourced to

• Nielsen, Michael A.; Chuang, Isaac L. (2000). Quantum Computation and Quantum Information. Cambridge University Press. pp. 112–113. ISBN 978-0-521-63503-5.

The paragraph uses the technical term "collapse" two times incorrectly, for example:

• Alice measures 0, and the state of the system collapses to $|0\rangle _{A}|1\rangle _{B}$.

Collapse or state reduction converts a superposition into a single eigenstate. In this example, we have a singlet state superposing two eigenstates of the system two measurements, one by Alice and one by Bob. Both measurements must be completed before we have an eigenstate of the composite system. Alice's device is only one part of the apparatus. I checked the source cited in this paragraph. It never uses the word "collapse" nor does it discuss state reduction in connection with the content on page 112-113.

The paragraph also concludes incorrectly that

• Thus, system B has been altered by Alice performing a local measurement on system A.

Alice's actions to do not cause any change, that is why the correlations effects are so puzzling. The source says it this way:

• Its as if the second qubit knows the result of the measurement on the first no matter how the first is measured.

This is fundamentally different than the article content in both the agent of change and the tone of causality ("has been altered" vs "as if"). I worded the paragraph to reflected the very straight-forward content in the source. My change was reverted. Please restore my edit. Johnjbarton (talk) 17:11, 8 November 2024 (UTC)

The conclusion of renowned physicist N. David Mermin's Physics Today article (>700 citations on Google Scholar) says:

• "Orthodox quantum metaphysicians would, I believe, say no, nothing has changed at A as the result of the measurement at B; what has changed is our knowledge of the particle at A. (Somewhat more spookily, they might object to the naive classical assumption of localizability or separability implicit in the phrases "at A" and "at B.""

This directly contradicts our content:

• "Thus, system B has been altered by Alice performing a local measurement on system A."

Ref is

• Mermin, N. D. (1985). Is the moon there when nobody looks? Reality and the quantum theory. Physics today, 38(4), 38-47.

My correction should be restored. Johnjbarton (talk) 18:51, 11 November 2024 (UTC)

Another reference that contradicts our content is

• Susskind, Leonard; Friedman, Art; Susskind, Leonard (2014). Quantum mechanics: the theoretical minimum; [what you need to know to start doing physics]. The theoretical minimum / Leonard Susskind and George Hrabovsky. New York, NY: Basic Books. ISBN 978-0-465-06290-4.

On page 266, the section "Entanglement and Locality" ends with

• "Nothing that happens at Bob's end had any immediate effect on Alice's density matrix...this means that Alice's view of her subsystem (her statistical model) remains exactly as it was. This remarkable result may seem surprising for a maximally entangled system, but it also guarantees that no faster-than-light signal has been sent."

Johnjbarton (talk) 00:21, 15 November 2024 (UTC)

Quantum decoherence

The article needs a summary section on Quantum decoherence and its relationship to entanglement. Johnjbarton (talk) 17:20, 12 November 2024 (UTC)

And it probably says too much about the emergence of time/emergence of spacetime business, which is a much more niche concern (and currently rather badly sourced). XOR'easter (talk) 21:25, 12 November 2024 (UTC)

Yes, the sad state of that article is actually what set me on editing Wikipedia and it still seems like a big challenge. There are lots of long review articles by a few people but the concrete results to hand-waving ratio is distressingly low. Zurek in particular has his jargon which is defined in terms of other jargon so it's hard to summarize for an article.

Nevertheless a few bits, from say Schlosshauer's review, are relevant to entanglement, but they are pretty much the opposite of what the articles says now. In QM there is no such thing as "breaking" entanglement, just more entanglement or less. Getting less is extremely hard. That is why we have ultra-high vacuum chambers, monochromators, and all the other elaborate physics equipment. The Peres section around pg 117 is another demonstration. Getting more entanglement is easy and decoherence is our name for entanglement with the environment. That much can be sourced and should be in the article.

One particular, special entanglement with the environment can, under circumstances not clearly understood (by anyone other that Zurek at least ;-), give rise to classical information, a measurement. This low level process involving massive amounts of entanglement gets summarized in a quantum model as state reduction. I think this work is still speculative and too complex to summarize in an article on entanglement but I keep a look out for a source.

In my opinion, the current wording in the article, that "Entanglement is broken when ... a measurement is made." implies a physical process by using physical language. I don't believe that can be sourced. The correct description is not low-level physics: the measurement gives us knowledge of a particular superposition term and we can use that knowledge to predict correlated results. That is easy to source also. Johnjbarton (talk) 23:53, 12 November 2024 (UTC)

Unverified claim concerning decoherence.

The article currently says:

• Entanglement is broken when the entangled particles decohere through interaction with the environment; for example, when a measurement is made.

with a reference:

• Asher Peres, Quantum Theory: Concepts and Methods, Kluwer, 1993; ISBN 0-7923-2549-4, p. 115.

However, when one reads the reference, nothing like this is said. The section in the reference is entitled "Decorrelation of an entangled state". The discussion concerns converting a pure state in superposition into a pure direct product state. There is no discussion of quantum decoherence or of measurement. Environmental interaction is not discussed, it is not a discussion of general principles, and it does use language like entanglement is "broken". This reference does not verify to the content.

Quantum decoherence is considered to be a consequence of entanglement and there is quite a lot of work connecting entanglement, decoherence and measurement, but this reference is used incorrectly in the article. My correction of this sentence was reverted. Please restore my correction. Johnjbarton (talk) 19:03, 12 November 2024 (UTC)