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)

I completely rewrote the "Paradox" subsection in User:Johnjbarton/sandbox/quantum_entanglement_concept#Paradox Johnjbarton (talk) 17:40, 25 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)

Another reference that directly contradicts our content:

• Peres, Asher (1993). Quantum Theory: Concepts and Methods. Kluwer. ISBN 0-7923-2549-4. page 154 says:

" If the first observer got a bad education in quantum theory and believes that the pair of particles has, at each instant, a definite wave function, he will say that the singlet state, which existed for t 1 < 0, collapsed into an eigenstate of S 1 x and of S 2x , for t1 > 0. In the same vein, the second observer may say that the singlet state held for t2 < 0, and thereafter collapsed into an eigenstate of S 1 y and of S 2y , as a result of her test. Statements like those of our fictitious observers are not only contradictory—they are utterly meaningless. There is no disagreement about what was actually observed. Johnjbarton (talk) 01:56, 17 November 2024 (UTC)

Another reference discusses the exact scenario described in the article:

• Quantum Paradoxes: Quantum Theory for the Perplexed Yakir Aharonov, Daniel Rohrlich ISBN: 978-3-527-40391-2

On page 194, the discussion begins:

• "Another version of this paradox involves two distinguishable particles in an entangled state..."

The analysis tries to make sense of collapse in the distant Alice/Bob measurements. Part way through:

• "The two observers cannot agree on who collapsed the state."

In the conclusion of chapter 14 they say:

• "The paradoxes of Sect. 14.1 suggest that there is no Lorentz invariant account of collapse..."
• "Thus observers in different frames disagree about collapse."

The simplest way to fix the article is to remove the content about collapse as I did in my edit. Johnjbarton (talk) 23:41, 19 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)

Aspects of current text that I object to include "Entanglement is broken". Entanglement is not a stick: it can't be "broken". There is no physical mechanism in QM to break entanglement. Furthermore the claim bundles decoherence implicitly with measurement, an unproved and controversial connection. Finally, the claim is sourced to a reference which says nothing of the kind.

My replacement was the absolute minimum I could do to repair the content. The paragraph I used from pg 117 was:

• "The superposition principle asserts that Ψ in Eq. (5.1) is a pure state. This means that there exists a maximal test (having four distinct outcomes) such that a photon prepared in state Ψ will always yield the same, predictable outcome. As we shall presently see, such a test may include either a mirror to reflect the photon through the crystal, or a second crystal which is the mirror image of the first and recombines the two beams. The state of the photon is thereby decorrelated—it again is a direct product—and its polarization can then be tested as usual."

The sentence I added is not the content we should have either. I suggest a section called "Measurement" with two paragraphs. The first would be on how measurement selects a single product from a superposition, leading to the correlated results. The second would be about microscopic models of the measurement process which start with state entanglement with the apparatus. Both topics are vast and speculative; both topics should be discussed. So I suggest these two paragraphs be minimal, focused on entanglement, and frank about the lack of conclusions. Johnjbarton (talk) 18:24, 25 November 2024 (UTC)

Proposal to rewrite the History section.

The History section relies almost exclusively on primary sources. It lacks sourced perspective and some of the content belongs in Bell's theorem if backed by secondary sources. Secondary sources for the history of entanglement include:

• Last chapter in Gilder, L. (2008). The Age of Entanglement. United Kingdom: Knopf Doubleday Publishing Group.
• Appendix to Schroeder, D. V. (2017). Entanglement isn't just for spin. American Journal of Physics, 85(11), 812-820.
• Esp. for Stapp's role spreading Bell's ideas (pg 56) in D. Kaiser, How the Hippies Saved Physics (Norton, New York, 2011).

I think the section would be clearer if it were chronological, omitted some of the Bell's theorem details (loopholes), and added content based on Schroeder's history. Johnjbarton (talk) 17:08, 15 November 2024 (UTC)

I concur with trimming the details about loopholes (niche interest, not really pertinent for an overview at this level). XOR'easter (talk) 20:58, 15 November 2024 (UTC)

Thanks for your improvements! Three issues remain in my mind:

• "Bell's work raised the possibility of using these entanglement correlations as a resource for communication." We need a source that explains who made the connection, "work" doesn't happen by itself.
• We need a source for Kocher's work. He is cited by Clauser but "first case of entangled visible light" should be cited.
• A sentence for Zeilinger.

Johnjbarton (talk) 23:45, 15 November 2024 (UTC)

Quantum teleportation shouldn't be credited to Zeilinger alone. Bennett et al. introduced the theory, and an independent experimental confirmation was also done by Popescu's group. XOR'easter (talk) 00:35, 16 November 2024 (UTC)

Ok I fixed this. I didn't go into names since the Bennett work was joint rather than mainly a professor/student thing and its covered in the main article. Johnjbarton (talk) 02:06, 16 November 2024 (UTC)

Issues in "Notable experimental results proving quantum entanglement"

I'm trying to sort out the section "Notable experimental results proving quantum entanglement". I started by deleting the content that duplicates our new and improved History section. Similarly I deleted the loophole free test sentence per our discussion on the History section.

That leave two items, one is content based on entanglement swapping and this then this sentence:

• "In three independent experiments in 2013, it was shown that classically communicated separable quantum states can be used to carry entangled states."

with a ref to Physics World. The first primary ref discussed in the magazine is Cubitt et al which says:

• "Note that this does not imply entanglement can be created by LOCC since a quantum channel was used." LOCC=local operations and classical communication. So something does not add up. I think the issues discussed in the primary articles are sufficiently complex that we should not rely on Physics World. I was unable to find a physics journal review. These are not discussed by Horodeki review. The theory paper has 190 cites, the two experiments have 90ish.

I think the effect is interesting for the article but I don't think our content is correct and we don't have a way to fix it.

So I propose to replace this section with a summary of quantum entanglement swapping which matches the middle of the section and would be more coherent with the concept section. Johnjbarton (talk) 01:55, 21 November 2024 (UTC)

The "classical communication" part is simply wrong. It is quantum communication. I support removing this experiment, which is in no way noteworthy. I also support removing the entire section, as a section about "notable experiments" is inherently problematic. It is hard to determine what are the notable ones, and it is a magnet for self-promotion. Tercer (talk) 17:58, 21 November 2024 (UTC)

I replaced the section with two paragraphs on teleportation and entanglement swapping. My descriptions are very short, trying to give the flavor of the ideas and focusing on entanglement. I omitted any history and equations. In theory the linked articles have more. Johnjbarton (talk) 01:10, 22 November 2024 (UTC)

I don't like your figure. It suggests that classical communication is necessary between Alice and Bob, and between Bob and Carol. This is not the case. You only need it either between Alice and Bob or Bob and Carol. Tercer (talk) 11:06, 22 November 2024 (UTC)

Sorry I don't know where your Alice, Bob, and Carol are in the diagram.

In the W diagram, if we have Alice on the left and Bob on the right, we can put Carol in the middle in charge of the Bell state measurement. Alice and Bob have no way to discover their state's entanglement unless they communicate with Carol. Thus they have no way to discover that entanglement was swapped.

If on the other hand we want to compare entanglement swapping to quantum teleportation, then we could choose classical communications between Alice and Carol followed by Alice calling Bob or Carol talks to Bob and then Bob talks to Alice.

In quantum teleportation the Bell state measurement is performed by either Alice or by Bob: communications with Carol are not needed. If Alice performs the measurement, she calls Bob; if Bob performs the measurement he calls Alice. I believe this is the form that you are describing. I agree that if our goal is to teleport the unknown state on the left to the final measurement on the right we only need classical communications of the Bell state result to the right. But if our goal is to verify entanglement swapping, then the knowledge in the Bell state measurement need to be communicated on both directions.

Do you have a suggestion on how to improve the diagram? Johnjbarton (talk) 17:19, 22 November 2024 (UTC)

Alice is on the left, Bob is in the middle, responsible for the Bell state measurement (not "Bell state analysis"), and Carol is on the right. Bob teleports his half the entangled state he shares with Alice to Carol. This requires him to send classical information to Carol. That's it. He doesn't need to send anything to Alice. She will share the desired entangled state with Carol, whether she knows it or not. Unlike quantum teleportation, she doesn't need to apply any corrections. Telling her that the protocol succeeded is not necessary, and is not part of the protocol. To fix your image simply erase the classical communication between Alice and Bob. Tercer (talk) 08:14, 23 November 2024 (UTC)

I have seen sources use "Bell state measurement" and others that use "Bell state analysis". Thus I don't care either way.

My figure is directly supported by the Horodecki review I cited below. Johnjbarton (talk) 19:09, 23 November 2024 (UTC)

Of course the review doesn't support your figure. They talk about a local rotation done by Alice and Bob, not Alice and Dave. No classical communication to Dave is done.

Instead of trying to get an exact quotation from a source you should spend a couple of minutes actually doing the calculations for entanglement swapping, and then you will understand that communication is only needed on one side. I'm not asking too much, the calculations are absolutely trivial. Tercer (talk) 21:00, 23 November 2024 (UTC)

You know that was a misprint. Bob, in the ABCD form of the experiment, has no reason to perform any rotations. Obviously it was David not Bob that was meant as it is Alice and David who will "obtain the entangled state AD".

The way I understand this is, just as you say, classical information does not need to go to Alice if we are expecting to teleport the unknown state labeled "Alice" to David. But that is quantum teleportation. In quantum teleportation, Alice's state remains unknown. For entanglement swapping, the experiment is to show entanglement of the particle Alice find with the one David finds. As in the standard EPR, until Alice and David compare results they don't know their particles are entangled. So Alice needs to know that her particle is involved.

I'll look for some more sources on this. Johnjbarton (talk) 23:40, 23 November 2024 (UTC)

In section 10.2.2. "Basic entanglement swapping", Figure 10.3 on page 201 of this book:

• Van Meter, R. (2014). Quantum networking. John Wiley & Sons.

has as item 3, "B communicates measurement results and new entanglement status to A and C." (The diagram uses ABC, with the Bell state measurement at B.) This diagram is logically equivalent to the one I drew.

However, a paragraph on the previous page says

• "In theory, A never needs to be told that the operation has occurred. Although C must apply corrective operations to complete the reconstruction, A is entirely passive, merely storing its half of the Bell pair in a buffer memory. However, A is very likely waiting on the completion of the swapping operation in order to perform some other action; at the very least, an application at node A is waiting to use the end-to-end Bell pair."

Of course this is all in the lingo of networking but I think this sheds light on our different points of view. In my point of view, Alice is waiting to complete the operation "demonstrate entanglement swapping". Johnjbarton (talk) 00:17, 24 November 2024 (UTC)

So you found a source explicitly telling you that communication is needed on only one side, but you're going to be stubborn and refuse to fix your figure. Tercer (talk) 22:50, 24 November 2024 (UTC)

No, that is not what I said. The figure is fine for the goal of entanglement swapping, which is an experiment involving all four particles.

All of these scenarios are about information and most of the confusion about them relates to who-knows-what-when. Does the EPR experiment involved classical communications? I say yes, because until Alice compares to Bob there is no correlation. No correlation means no experiment. The issue with entanglement swapping is the same. Until the two end points are compared, the experiment is not complete.

Furthermore the "Progress in quantum teleportation." supports my version. And you have provided evidence beyond your personal opinion. Johnjbarton (talk) 23:29, 24 November 2024 (UTC)

This is not a matter of personal opinion, this is a mathematical fact. One that you have already confirmed yourself, so I don't know what further "evidence" you can possibly want. Tercer (talk) 07:44, 25 November 2024 (UTC)

The following description supports my version of the diagram.

• Let Alice share a maximally entangled state...AC with Clare, and Bob share the same state with David... Now, Clare and Bob perform a joint measurement in the Bell basis....If Alice and Bob will get to know the outcome, they can perform local rotation, to obtain the entangled state AD. ...

See page 876

• Horodecki, R., Horodecki, P., Horodecki, M., & Horodecki, K. (2009). Quantum entanglement. Reviews of modern physics, 81(2), 865-942.

Johnjbarton (talk) 17:38, 22 November 2024 (UTC)

Another source that shows a figure almost identical to the one in the article appears in Figure 2b page 341 of

• Hu, XM., Guo, Y., Liu, BH. et al. Progress in quantum teleportation. Nat Rev Phys 5, 339–353 (2023). https://doi.org/10.1038/s42254-023-00588-x

The caption describes part b as Quantum entanglement swapping Johnjbarton (talk) 18:41, 24 November 2024 (UTC)

Complex edits that need to be discussed.

I reverted this change as it contains many parts I disagree with. We should discuss the issues and see which parts we agree and which parts need addition input. Johnjbarton (talk) 16:55, 21 November 2024 (UTC)

My revert was undone with no discussion. Johnjbarton (talk) 17:16, 21 November 2024 (UTC)

Request for consensus to replace first sections of article with draft.

Update: We're going to try section by section. Johnjbarton (talk) 00:00, 27 November 2024 (UTC)

Quantum entanglement is poorly sourced and out-dated. It is an important area of quantum mechanics that has seen significant new work, has been featured in 2022 Nobel prize, and the basis of many parts of quantum computing. I have been trying to add references and modernize this article but @Tercer has been reverted my edits. Since large parts of the article has no sources, multiple edits are involved. To avoid seven different discussions and to ensure we have a coherent article I have created a Draft to replace the current content of three major sections of the article. This will undo the following reverts:

1. revert discussion
2. revert discussion
3. revert discussion
4. revert discussion
5. revert discussion
6. revert discussion
7. revert discussion

Please reply Replace to use my draft or Keep to leave the current content. Johnjbarton (talk) 18:39, 23 November 2024 (UTC)

Replace The draft has reliable mainstream sources that can be verified and used to extend the article. The current content includes unsourced incorrect editorial opinions. Tercer's reverts were all inappropriate as the edits were sourced and no reply from Tercer discussed sources. This is basic wikipedia stuff. Johnjbarton (talk) 18:42, 23 November 2024 (UTC)

Not everything is a WP:VOTE. I will try to provide feedback on your draft during the week if this can wait (my activity is reduced this weekend).--ReyHahn (talk) 20:52, 23 November 2024 (UTC)

What else can I do? Johnjbarton (talk) 23:18, 23 November 2024 (UTC)

I've no objection in principle to the general approach of proposing a draft that replaces one or more sections of the article. (I've done similar things in the past.) I just don't think that this draft is ready to do that yet. XOR'easter (talk) 06:56, 24 November 2024 (UTC)

I agree with ReyHahn. Also, the mechanism (replacing sections) may not be any better. I suspect that major contributors will need to be in a more cooperative frame of mind first. —Quondum 21:34, 23 November 2024 (UTC)

I welcome any suggestions on how I might be more cooperative. I thought the draft would be a way to make progress. Johnjbarton (talk) 23:18, 23 November 2024 (UTC)

I was not implying that you were being uncooperative, only that there does not seem to be an atmosphere of cooperation at the moment, judging from ANI. A start would be to slow things down. From my side, I should familiarize myself better with the discussions above. —Quondum 23:48, 23 November 2024 (UTC)

I don't understand what's going on with the "Wavefunctions" subsection. It starts by saying The wavefunction for two non-interacting particles can be written as the product of two one-particle wavefunctions, but a joint state can fail to factor even if the interaction was in the past. Then it starts talking about energy for some reason without introducing a Hamiltonian or saying what the energy levels are. (What if the a, b, c, and d wavefunctions are all degenerate?) The next subsection begins, Entanglement is often introduced using spin states... So, why not just start there? XOR'easter (talk) 04:51, 24 November 2024 (UTC)

Thanks, I can see that is confusing. I had a couple of goals with this section. First I really like how Peres uses a function of positions to explicitly highlight that the three parts of the product are in different locations. I think we internalize this point but readers coming in may not notice this key aspect if they start by seeing bra-ket notations.

I pasted on Griffiths example to give a concrete example that readers only familiar with the first bits of QM might recognize, energy quantization. I'll rethink this and see if it can be salvaged.

The other way to start this section would be much more modern: start with (abstract) qubit notation and then relate the notation to wavefunctions, spin, polarization. Do you think that would be better? Johnjbarton (talk) 05:07, 24 November 2024 (UTC)

I'm staring at the table and can't tell what it is trying to say. It's just numbers without meaning, and the whole concept of energy comes in from nowhere. I'm away from my copy of Griffiths and Schroeter so I can't check right now, but I assume that example makes more sense in its original context.

I think that by now, the idea that quantum mechanics involves something called "entanglement" is at least as widely spread as the idea that it involves discretized energy levels. XOR'easter (talk) 05:51, 24 November 2024 (UTC)

Thanks for your feedback, very helpful. I've tried to salvage the section by adding context to describe the states and I walked through the table in the text. The idea of the table is to give readers a different perspective. I think in our heads we read a superposition formula and understand that each term gives a probability. Griffiths and Schroeter use the energy measurement to get this across. Please take a refresh look; maybe others will chime in.

Yes, I suppose by now entanglement/energy has flipped among many readers. Still I think we should have a wavefunction example and Griffiths and Schroeter has a compact section.

I added a new section "Representations" that adopts the modern qubit form. Does it help? Maybe spin should come next. Johnjbarton (talk) 17:01, 24 November 2024 (UTC)

• Where to start?

  • Edit 1: seems to be a good revert, the phrasing was odd. Also I do not see why the notes are needed.
  • Edit 2: seems to be a good revert. I am sure we can find a quote for that.
  • Edit 3: seems to be a good edit.
  • Edit 4: no preference over either version. One should be careful on interpretation bias.
  • Edit 5: I do not like "Its as if the second qubit know".
  • Edit 6: Both seems weird on teleportation but in the new edit it is unclear if a classical channel is even neeed. I prefer the original version of swapping.
  • Can you explain edit 7?

  Cheers--ReyHahn (talk) 15:41, 25 November 2024 (UTC)

  This approach of referring to all 7 topics by number makes replying very challenging. Each revert has a corresponding Talk page Topic. Would you consider moving the discussion to the Topics? Johnjbarton (talk) 17:36, 25 November 2024 (UTC)

  I started a new Topic for edit 7 Talk:Quantum_entanglement#Source_for_"Alice's_original_state_is_necessarily_destroyed" Johnjbarton (talk) 18:45, 25 November 2024 (UTC)

• In big lines the draft seems ok but I worry on the technicalities. Maybe we should approach it section by section? History, then definitions and so on?--ReyHahn (talk) 15:44, 25 November 2024 (UTC)

  If you are worried about technicalities, then why aren't you worried about the existing content? We can't discuss the technicalities in the current article because these personal opinions cannot be verified.

  I tried section by section and in return I got a barrage of childish insults. Nevertheless, if you think this is the best approach, please be specific since any action on the page I take will undo reverts. You listed "History" so are you proposing that I open a Topic on History to discuss the draft version vs current? Johnjbarton (talk) 17:55, 25 November 2024 (UTC)

  Sorry if this has been harsh maybe the issue should have been notified to WP:PHYSICS sooner. Yes we can check history first.--ReyHahn (talk) 07:24, 26 November 2024 (UTC)

Source for "Alice's original state is necessarily destroyed"

The current article says

• "Since Alice's original state is necessarily destroyed during the process, it is said to be "quantum teleported" to Bob's laboratory..."

with with reference that discuss quantum teleportation, but don't say anything about "necessarily destroyed", which in my opinion is an important condition that should be verifiable. I attempted to add a reference by my change was reverted with an edit summary:

• "incorrect an ungrammatical. Alice's state might as well be known, it doesn't change anything."

Here is the text I summarized:

• "First, in quantum mechanics a state cannot be cloned (the famous 1982 no-cloning theorem), so what is going on? The answer is that the Bell-state measurement does not reveal any information about the properties of the individual photons and the initial state of Alice’s photon can be unknown to her or anyone else. During the Bell-state measurement Alice’s quantum state loses its identity and is eventually destroyed, so Bob’s final quantum state is not a clone."

Johnjbarton (talk) 18:44, 25 November 2024 (UTC)

I do not understand your claim here. Alice has some unknown state, it is "lost" after the process and Bob retrieves it. Why does this need more explanation? Why is the no cloning theorem explained here?--ReyHahn (talk) 07:28, 26 November 2024 (UTC)

Alice's subsequent state presumably no longer can be used to obtain information about her past state, but Bob's state can. ReyHahn's statement makes reasonable sense to me (albeit "lost" needs definition). To say "Alice’s quantum state loses its identity and is eventually destroyed" makes little sense to me: what is a state's "identity"?, and "eventually" is unsuitable: the connection with her initial state superposition is lost immediately during the process. The term "destroyed" similarly needs definition and is not a great term; more precise language might improve understandability of the existing description in the article. —Quondum 14:29, 26 November 2024 (UTC)

Destruction is the technical term, we do not have to define everything. I fear that to define that we would have to use "reside", "lost" or "no longer there", but I am open to ideas.--ReyHahn (talk) 14:42, 26 November 2024 (UTC)

I seem to be running into this a bit – Tercer also pointed out to me that other terms also are "technical" in this sense. All disciplines tend to repurpose terminology in this way, which I find unfortunate (I think it creates a barrier to entry, which slows the field). In WP we cannot assume that the reader is familiar with these terms in the way that a textbook would, and many readers will make inappropriate associations. Maybe the solution would be to have an immediate link to a clear definition inside WP of each such term. —Quondum 15:02, 26 November 2024 (UTC)

I'm confused by both replies here. You seem to be arguing that the overview in a written by the Chief Editor of Nature Review Physics is incorrect and not notable. It is not a deep technical work so it does not make sense to worry about technical definitions.

The no-cloning theorem is routinely discussed in the context of quantum teleportation. See for example

• Horodecki; others (2009). "Quantum entanglement". Reviews of Modern Physics. 81 (2): 865–942. arXiv:quant-ph/0702225. doi:10.1103/RevModPhys.81.865.

Page 875

• At the same time, Alice has just one of the Bell states: the systems A and A' become entangled after measurement, and no information about the state q is left with her. That is, the no-cloning principle is observed, while the state was transferred to Bob.

I'm fine if you don't care for the way I summarized Georgescu or you have a better source or you want just remove the claim in the article. But I don't understand how you can argue against the Georgescu source solely based on your own opinions. Johnjbarton (talk) 17:14, 26 November 2024 (UTC)

If you allow me to reformulate my comment here: Nobody is questioning the sources. My problem here is related to the original article saying nothing about the no cloning theorem and then in your version a whole discussion appears without proper connectivity as if there is some sort of surprise. I think that your version is either unnecesary or lacks introduction. Also it is not a direct replacement of the original phrase you change "destroyed" per "loses its identity and is eventually destroyed" what is implied here?--ReyHahn (talk) 17:32, 26 November 2024 (UTC)

Ok thanks. I agree that no-cloning appears with no context.

Let's back up. The original/current content is

• Since Alice's original state is necessarily destroyed during the process, it is said to be "quantum teleported" to Bob's laboratory through this protocol.

To me this sentence reads as condition-implies-result: the destruction-implies-teleportation. I expected to read more about how the destruction implies teleportation when I looked in the reference. The reference says nothing about destruction.

How should this be repaired? Johnjbarton (talk) 17:47, 26 November 2024 (UTC)

If you insist on removing that phrase and want to keep it short, you can just say 'it is said to be "quantum teleported" to Bob's laboratory through this protocol.' The WP:SCAREQUOTES are doing all the work here.--ReyHahn (talk) 17:52, 26 November 2024 (UTC)

I do not insist on removing the phrase. I only insist that if the phrase is attributed to a reference, then the reference should discuss it. All I was trying to accomplish was to elaborate on the phrase according to a ref that actually verifies. I did not do a great job but I don't understand why my effort is rejected rather than the basis for further improvements. Johnjbarton (talk) 19:07, 26 November 2024 (UTC)

Non-locality and entanglement

The current section "Non-locality and entanglement" starts with this unsourced claim:

• In the media and popular science, quantum non-locality is often portrayed as being equivalent to entanglement.

The section then heads off to discuss only the complex and unsettled half of the equivalence. It ends with some hidden content claiming that a highly cited paper used non-standard terminology.

I think a much simpler paragraph is called for. The key takeaway from the paragraph should be the half of the equivalence that is not complex: nonlocal correlations require entanglement. We should start with that and try to make it clear. Then discuss the reverse. Pure states that are entangled lead to nonlocality. Then the exceptions can be mentioned as not fully understood. This matches the Brunner reference treatment on page 437.

The primary refs to proofs and the claim about the Bennett paper would not survive. Johnjbarton (talk) 23:25, 25 November 2024 (UTC)

Is it necessary to address the purported claim of "equivalence" at all in the article? It would seem reasonable to discuss what is, rather than what is not: WP articles are meant to be neither pedagogical nor should they dissect the media. This seems roughly to be in line with what you are suggesting, though I am not really comfortable with the whole framing: non-locality seems to me to be one of those concepts that is very dependent on the model (e.g. MWI vs. collapse interpretations: MWI is described as fully local). —Quondum 23:57, 25 November 2024 (UTC)

I agree that we can quickly get lost in nonlocality. However I think a short section based on a Brunner's Review of Modern Physics article with 3000+ cites and a section named "Nonlocality versus entanglement" is something we must have in an article on entanglement.

I wrote a quick draft of a section on Nonlocality. Can you take a look? Is there enough to make the idea clear? Johnjbarton (talk) 00:16, 26 November 2024 (UTC)

Coming at it from an outsider perspective, I find that section (I presume you mean this, since your link does not quite work) inscrutable. Following the first link there, Principle of locality, I find that not much better. I would have described locality as that the only influences at any point in spacetime are those that have propagated there, implying that influences lie wholly within the the past light cone. I don't see probability as having any role at this level of description (even though it is useful in designing experiments to test it), aside from that it demands a familiarity with probability theory, and would seek to eliminate any reference to it here. —Quondum 00:52, 26 November 2024 (UTC)

Ok, thanks your perspective is very helpful. I've changed the content to introduce the reason for probability. The results in QM are random so "influences" can only be determined by comparing two random results via correlations. Please take another look: any better? Other ideas?

If you know of any source on "classical locality" please let me know and I will try to update principle of locality. As far as know locality was just a built-in assumption which is why EPR did not go into detail about it. Johnjbarton (talk) 18:50, 26 November 2024 (UTC)

Looking at the cited Brunner paper, the interpretation of the argument through probabilities becomes a little clearer, but that paper takes time to build the background to the interpretation (but is still challenging to the reader who is not versed in reading between the lines). Even then, it is difficult to frame it appropriately: the paper somewhat naïvely defines locality in such a way that MWI QM is inherently "nonlocal". This introduces inherent bias into the perspective. Notwithstanding number of citations, I would not build a description from one description. —Quondum 01:19, 26 November 2024 (UTC)

If you have another reliable source for nonlocality and entanglement that you think is less "biased" we could include it as well. However, Bell's definition is very widely used as the citation list makes clear. Johnjbarton (talk) 19:01, 26 November 2024 (UTC)

Actually after re-reading this ref, page 104:

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

I think you reading something into the content that I did not write and which does not follow. The Bell definition of locality and the observed quantum correlations mean that no classical local variable model is feasible. It does not require that QM is "nonlocal". It is not a classical theory. Peres/Terno point that relativistic quantum field theory is local.

Nonlocality is inherently an issue of space and time; non-relativistic QM is, well, non-relativistic. As Peres/Terno and Horodecki et al. discuss, relativistic treatments of scenarios like EPR are quite disappointing: the experiments are actually invalid or undefined. Johnjbarton (talk) 19:56, 26 November 2024 (UTC)

I think we are getting stuck on conflicting use of terminology within the field. The term "locality" seems to be an example. Is MWI local or not? It depends on the definition of locality. —Quondum 22:40, 26 November 2024 (UTC)

That's a question to consider on those articles. In this article we only need to discuss the relationship between entanglement and nonlocal correlations. Johnjbarton (talk) 23:58, 26 November 2024 (UTC)

The article title is much broader than the idea that you have just stated; besides, the reader is given no clue that a definition given here is not a universal definition for a term in QM, which is a recipe for confusing readers.

I think I should get out of the way. We seem to keep talking past each other. —Quondum 00:36, 27 November 2024 (UTC)

I'm fine with qualifying the defn. I changed the text slightly as a result. Does that help? I don't know what else to do. Johnjbarton (talk) 01:40, 27 November 2024 (UTC)

It is an improvement in that it alerts the reader that locality has varying definitions. —Quondum 02:32, 27 November 2024 (UTC)

Proposal to replace the History section

I propose to replace the History section with the content of User:Johnjbarton/sandbox/quantum_entanglement_concept#History. The significant changes are

1. FROM

• Their concept had two systems interact, then separate, and they showed that afterwards quantum mechanics cannot describe the two systems individually.

TO

• Their concept had two systems interact, then separate, and they showed that quantum mechanics predicts the measurements on those systems are related but provides no mechanism for the relation.<ref name="MerminMoon-1985"/>{{rp|38}}

BECAUSE: The end of the existing sentence is ambiguous; the new sentence is extracted from Mermin ref and relates to EPR incompleteness.

2. DROPPED FROM

• Like Einstein, Schrödinger was dissatisfied with the concept of entanglement, because it seemed to violate the speed limit on the transmission of information implicit in the theory of relativity.

TO

• Like Einstein, Schrödinger was dissatisfied with status of quantum mechanics, reasoning that "we do not have a quantum mechanics that takes into account relativity theory".

BECAUSE: Schrödinger says "caught the dogmatic quantum mechanics by the collar" and "we do not have a quantum mechanics that takes into account relativity theory, that is among other things, that respects the finite speed of propagation of all effects". The speed limit is just one issue and both men are taking aim at QM because of the prediction of entanglement rather than at entanglement itself.

3. DONE FROM

• Einstein later referred to this as "spukhafte Fernwirkung" or "spooky action at a distance".

TO

• Einstein described the effects of entanglement as "spukhafte Fernwirkung" or "spooky action at a distance", meaning the acquisition of a value of a property at one location resulting from a measurement at a distant location.

BECAUSE: "this" is ambiguous. In the current article "this" seems to refer to 'limit on transmission of information', but how can it then be "action"?

4. FROM

• In 1964 John S. Bell demonstrated an upper limit, seen in Bell's inequality, regarding the strength of correlations that can be produced in any theory obeying local realism, and showed that quantum theory predicts violations of this limit for certain entangled systems.

TO

• In 1964 John S. Bell showed that quantum entanglement predicted a different kind of correlation, one not discussed in the EPR paper, and this correlation cannot be reproduced by any local hidden variable theory alternative to quantum mechanics.

BECAUSE: As noted in the Mermin ref cited, the key was Bell's focus on other correlations. The correlations in EPR were not quantitative so strength is a distraction for the purpose of history.

5. Revised A section on Kocher concerning early EPR experiments. I moved this from a different part of the article. I checked the Clauser ref and they say "The experimental arrangement was similar to that of Kocher and Commins". There are probably more refs on Kocher. Sentence in the paragraph on early experiments demonstrating entanglement, with ref to Clauser's paper where the abstract says "A proposed extension of the experiment of Kocher and Commins, on the polarization correlation of a pair of optical photons,"

ADDITION:

• In his 1967 PhD work, Carl Kocher[15][16] presented results from apparatus in which two visible light photons successively emitted from a calcium atom were shown to be correlated.

6. FROM

• a student named Artur Ekert suggested that these super-strong correlations could be used as a resource for communication

TO

• a student named Artur Ekert suggested that these entanglement correlations could be used as a resource for communication

BECAUSE: the word "super-strong" is extraneous, ambiguous, and not used in Ekert paper.

I would normally put this 6 edits in separately to so the edit summary can help track the reasoning.

References

Johnjbarton (talk) 01:30, 27 November 2024 (UTC)

@ReyHahn Is this what you had in mind? Johnjbarton (talk) 01:31, 27 November 2024 (UTC)

1. Oppose. This is a terrible attempt to explain the EPR paradox. The sentence should be just erased instead.

2. Oppose. The proposed change removes the simple, clear explanation and inserts instead an unexplained reference to relativity. The original should be retained.

3. Support. This is correct and a helpful clarification.

4. Oppose. The EPR paper doesn't discuss correlations, but rather post-measurement states. Instead of trying to explain EPR badly, the original one is a simple summary of Bell's results and should be retained.

5. Oppose. WP:UNDUE. We're not here to WP:RIGHTGREATWRONGS. For better or worse Freedman and Clauser's work is the notable one, not Kocher's. Moreover the subject here is nonlocality, mentioning Kocher requires a break in the flow to talk about entanglement only.

6. Oppose. The proposed change is not even grammatical. "Entanglement" is a noun, not an adjective. In any case, Ekert was not proposing to use any correlation produced by entangled states, but specifically one that violated a Bell inequality. We are free to paraphrase our sources, we don't have to copy-paste what they say. Tercer (talk) 11:13, 27 November 2024 (UTC)

Regarding #5, I tried to find a source for "the first case of entangled visible light" from older versions of the article. The only one I could find was written by Kocher. On the other hand Clauser et al prominently mentions Kocher's work in their short abstract so they clearly considered the work vital to theirs. I scaled down the Kocher sentence and moved it up to the paragraph where the first entanglement experiments are discussed. Johnjbarton (talk) 17:49, 27 November 2024 (UTC)

1. The proposed change makes the text longer but not more clear. I do not like the existing sentence either. "Their concept had" reads as non-idiomatic.

2. This change makes the text less specific, for no apparent reason. If Schrödinger was dissatisfied with QM because of entanglement, he was dissatisfied with entanglement.

3. I like this change. It's a good clarification.

4. I prefer the original text to the proposed modification, but I'm not entirely happy with the original either. The focus on "the strength of correlations" ignores/obscures the essential point that Bell's theorem requires considering multiple different experiments and comparing the correlations obtained in different configurations. This is what Bell himself emphasized with the Bertlmann's socks story. Perfect correlation, or perfect anti-correlation, in a single experiment is just not interesting.

5. This sounds WP:UNDUE.

6. I don't like "super-strong". Not only is it vague, but also, trying to make it more precise doesn't help. Perfect correlations in a classical theory (Bertlmann's socks) are "super-strong", aren't they? What's special about entanglement here? XOR'easter (talk) 18:50, 27 November 2024 (UTC)

I think it's natural to say that a correlation that achieves a higher probability of victory in a nonlocal game is stronger than one that achieves a lower probability of victory. But I think you have a point, the reader is more likely to understand strength in terms of perfect correlation/anti-correlation.

Nevertheless, I don't like saying that Ekert proposed using entanglement; his idea was to use nonlocality. Tercer (talk) 19:05, 27 November 2024 (UTC)

My text was based on Horodecki Rev.Mod.Physics

• The first discovery within quantum information theory, which involves entanglement, is due to Ekert (1991).

Do you have a different reference? Johnjbarton (talk) 19:16, 27 November 2024 (UTC)

I see what you mean. My feeling right now is that it's OK, since we have two sentences that go from the more general to the more specific. I'm not passionately attached to the current phrasing, though. XOR'easter (talk) 20:39, 27 November 2024 (UTC)

I changed it then. While not wrong, I think it was misleading the way it was. Tercer (talk) 08:38, 28 November 2024 (UTC)

Looks like a good change. XOR'easter (talk) 21:47, 29 November 2024 (UTC)

Thanks. Regarding #2, "If Schrödinger was dissatisfied with QM because of entanglement," is not the case as far as I can tell. That is my point. He indicts QM for not being relativistic. Maybe we need to get rid of this bit.

Regarding #4 I agree with your point of view so maybe we can come up with a better way to say it. Johnjbarton (talk) 19:08, 27 November 2024 (UTC)

But it was the tension he perceived between entanglement and relativity that he found problematic: The imagined act of combining two or more systems into a single one leads to great problems as soon as one tries to introduce the special principle of relativity into Q.M. XOR'easter (talk) 20:28, 27 November 2024 (UTC)

Ok, the bit I don't like in the current text is "...it seemed to violate the speed limit on the transmission of information..." because this does not sound like anything I've read from that era. How about

• Like Einstein, Schrödinger was dissatisfied with the concept of entanglement because it seemed to violate the theory of relativity.

or

• Like Einstein, Schrödinger was dissatisfied with the concept of entanglement because it seemed to violate the finite speed of propagation required by relativity.

Johnjbarton (talk) 23:05, 27 November 2024 (UTC)

Sorry, but I don't understand the difference in substance between the current text that you don't like and your suggestion. Could you elaborate on what exactly you find unsatisfying? XOR'easter (talk) 00:17, 28 November 2024 (UTC)

Oh, just the focus on "information". Einstein, also mentioned in the paragraph, was still talking about 'particles'; the debates about the nature of QM were in early stages and would fill books over time. I think the focus on 'information' is a modern one. I think the sentence imagines what they would say today. Johnjbarton (talk) 00:24, 28 November 2024 (UTC)

Ah, I understand now. I'm not too concerned about that; the joke about the Physical Review expanding faster than light "but this is consistent with relativity because no information is transmitted" is decades old. Mermin says he heard it from Peierls in 1961. I suspect that older sources might be more likely to talk about signals rather than information, like Kennard did in 1934 and Eddington did in 1922. But it's the same idea. XOR'easter (talk) 01:11, 28 November 2024 (UTC)

Reprise We are making some progress and I've learned some thing about the outlook of other editors. I will make one more pitch for revised versions of remaining items (numbered as above) then give up.

1. The current text has no reference and it says "...afterwards quantum mechanics cannot describe the two systems individually." To me this is puzzling: in QM if I measure an singlet state (which will be "afterwards") I get two individual answers. Thus I want to read a reference that backs the text claim or to change the text.

4. How about:

• In 1964 John S. Bell published Bell's inequality, showing that certain entangled systems have a different kind of correlation, a correlation stronger in quantum theory than any correlation predicted by a theory obeying local realism.

5. I agree that the old text was over the top, but Kocher's 1967 work was a historic advance in experimental entanglement per Clauser's abstract; the technique as also used by Aspect. How about this sentence right after the Wu/Shaknov sentence:

• In his 1967 PhD work, Carl Kocher presented results from apparatus in which two visible light photons successively emitted from a calcium atom were shown to be correlated.

6. The current text says

• Artur Ekert suggested that the violation of a Bell inequality could be used as a resource for communication.

and it cites Gilder, Louisa "The age of entanglement: when quantum physics was reborn." Pages 314-315 discusses Ekert meeting Bll but it says nothing violation of Bell inequality. The closest is :

• "If the Clauser-Horne-Shimony-Holt inequalities are violated" (meaning that Alice and Bob's photons remain entangled),

The section says 'entangled' multiple times and for the history section I think we should say "effects of entanglement" and leave the details for direct discussion of Ekert's work.

References

Johnjbarton (talk) 19:27, 28 November 2024 (UTC)

As an example of entanglement

The paragraph beginning "As an example of entanglement..." discusses decay of a subatomic particle but I have not been able to find a reference that discusses this example in this way.

On pg 32 of

• Aharonov, Y., & Rohrlich, D. (2008). Quantum paradoxes: quantum theory for the perplexed. John Wiley & Sons.

they talk about photons from pion or positronium decay and about photons from atomic cascade. Peres on 155 in his QM book uses photons from atomic cascade. Both books are using this example is roughly the same way as the article, as an intro example to discuss quantum correlations. Peres' book also has the classical example of a bomb fragmenting into two parts with angular momentum. Clauser and Aspect use atomic cascade and their short experimental descriptions are easy to read.

So unless some knows of a reference for the current content I suggest we switch to atomic radiative cascade / photon polarization for the example. Any example we pick will necessarily require some background the reader may not have. Abner Shimony's Scientific American article uses "a sheet of polarizing film" which I think is about as good as we can do.

Other suggestions? Johnjbarton (talk) 01:37, 1 December 2024 (UTC)