Talk:Quantum entanglement/Archive 7

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Archive 7

Subtleties regarding Non-locality, interpretational issues, and my recent edits

Hi. Ive made some edits to the non-locality section of this article reflecting the fact that some interpretations allow the non-local correlations to emerge from a description that is fundamentally local. This is most clearly the case with Many Worlds, where it is totally cut and dry, but in the mainstream interpretations like Copenhagen it is a little bit subtle, and can be called "local" depending on exactly how you define both "local" and the interpretation in question. In these cases, the issue is tricky for the point that I just made in the text: "If each distant observer regards the other as a quantum system, communication between the two must then be treated as a measurement process, and this communication is strictly local." So both Alice and Bob in these experiments can each formulate a story about what happened that is fully local and refers to "collapsing the wavefunction" of their partner, but doesn't recognize the agency of the partner. So clearly there is a tension with the "local" character of this description since we have no grounds for choosing one person over the other as "fundamental", but on the other hand you can see it is pretty close to being fully local too. At very least, from this perspective we clearly see that the non-locality of these interpretations is astronomically less glaring than in the hidden variable interpretations where there have to be huge non-local interactions all over the place.

Anyway, don't want to start a big discussion about foundations here, but this basic question clearly needs to be represented in this article, especially since the importance of this point has been recognized by many great thinkers in the field. Similar edits need to be made on the non-locality page too. __isocliff__ 04:40, 10 February 2013 (UTC) — Preceding unsigned comment added by Isocliff (talk • contribs)

True, this is subtle. But I'd say, such pictures as "I am an observer and you are just a quantum system" are... hmmm... less notable (or more marginal) than "we are both observers". The mainstream picture is "what we see is what we have" in the sense that the macro-world of our everyday life exists just as we see it. The subtlety is, what exactly is meant by "exists". Boris Tsirelson (talk) 06:43, 10 February 2013 (UTC)

speed of spooky action?

what about mentioning this (http://arxiv.org/abs/1303.0614 ) in the article? 201.171.102.182 (talk) 03:59, 10 March 2013 (UTC) Edit: full report here: http://arxiv.org/pdf/1303.0614v1.pdf — Preceding unsigned comment added by 201.171.102.182 (talk) 04:03, 10 March 2013 (UTC)

any reference that explains how it is known that the measured state doesn't occur until measurement?

Is there a layman's explanation that can be referenced, which discusses the concept of a state that isn't established until the measurement is made? With respect to the topic of quantum entanglement, I'm specifically looking for a discussion of why physicists conclude that the up/down state (or whatever characteristic is entangled) remains undefined until one photon is measured, rather than becoming defined at the instant they are separated, or at some other time between separation and measurement? (after entanglement, but earlier than measurement.) What experimental statistic would be different, depending on when the photon pair actually 'decays' into defined states, rather than remaining in the undefined state (where it isn't yet determined which photon is in which state)? ToolmakerSteve (talk) 04:02, 10 April 2013 (UTC)

The subject was long under debate. Bell's theorem shows how the statistics would be different. Brian Greene has a discussion in one of his popularizations of modern physics that makes it clear enough so that you can see how it might work out in a somewhat more complicated way in the real world.P0M (talk) 20:03, 25 April 2013 (UTC)

Decay of QE

Does quantum entanglement decay over time? If not, what mechanisms, if any, can break entanglement, do we even know the answer to this? Perhaps it would be useful to answer these questions in the article. This: http://www.scientificamerican.com/article.cfm?id=quantum-entanglement-sudden-death delves into the question a little but it's 4 years old.89.99.122.33 (talk) 02:06, 25 April 2013 (UTC)

Yes and no. It doesn't decay if an entangled pair is kept isolated from all interactions with other things. In practice it is a big job to keep a pair entangled. They become "disentangled" due to interactions with other things. They decohere. P0M (talk) 19:57, 25 April 2013 (UTC)

Ah, that's seems logical, do you think this should be in the article?195.169.213.92 (talk) 18:34, 28 April 2013 (UTC)

Maybe a link to quantum decoherence would be better. One problem with articles is that we need enough foundation to support the house, but we don't want too many semi-attached garages to lead people into confusion.P0M (talk) 19:13, 29 April 2013 (UTC)

Concept section

Someone needs to rewrite this section its too obscure. — Preceding unsigned comment added by 86.30.180.117 (talk) 09:47, 23 June 2013 (UTC)

Possible significant typo

I don't have the expertise to edit this article without review. The current text has "Indeed, without the above property, the von Neumann entropy would not be well-defined. In particular, U could be the time evolution operator of the system ..." I suspect that the intended wording was "U could not be the time evolution operator."

"U" typically is used to mean the time evolution operator. I do not know how this relates to the von Neumann entropy (although I wish I did know), but the current language is at least unclear, and possibly the opposite of what was intended. Thanks. 209.169.70.166 (talk) 01:25, 6 August 2013 (UTC)

I think, the intended wording really is "U could be the time evolution operator". The point probably is that the U-invariance of S holds for arbitrary U and in particular for the time evolution operator; and this special case associates the reversibility of a process with its resulting entropy change.

But indeed, this paragraph ("As an aside...") is very unclear. I am not quite understanding it. Boris Tsirelson (talk) 05:24, 6 August 2013 (UTC)

Can the introduction be improved?

I'd rather leave this to an expert to improve, but the second (i.e. main) paragraph of the introduction seems not really to get across what it is that's totally weird ("spooky") about this phenomenon. The fact that measurement results are correlated is not in itself surprising at all. I think it should say something about the possibility of getting sets of correlations that are not consistent with local realism, or something along those lines. W. P. Uzer (talk) 18:22, 8 January 2014 (UTC)

Indeed. At least, the following phrase could be added: "Entanglement cannot be reduced to shared randomness, and does not imply faster-than-light communication." Regretfully, we have no article on shared randomness. See for instance the book: "Shared Randomness and Entanglement in Communication Complexity" by Dmitry Gavinsky, University of Calgary (Canada), 2006, ISBN 0494191864, ISBN 9780494191866. Boris Tsirelson (talk) 19:50, 8 January 2014 (UTC)

Yes, the fact that we have as yet no information to offer on "shared randomness" would make such an addition not particularly useful at the present. The second part of your sentence could be added somewhere; but you see my point - there's nothing said at the moment (I'm talking just about the introduction, not the body of the article) that would even make anyone suspect that this implies faster-than-light communication - we just have this picture of two particles separating from each other and with their properties remaining correlated, which is not in itself in any way extraordinary. W. P. Uzer (talk) 20:39, 8 January 2014 (UTC)

Sure, sure, you are absolutely right. Regretfully, I am not enthusiastic to do the work myself. Boris Tsirelson (talk) 21:30, 8 January 2014 (UTC)

All right, I've tried my best (as a non-expert) to make better sense of this. My proposal for the introduction is here: User:W. P. Uzer/sandbox. (The editing history of that page shows how I got from the present version to that one, with some brief explanations of the changes in edit summaries.) I think my new version gives a fuller and clearer picture of the basic issues related to entanglement. I'd be grateful for any comments, especially from those who know the science better than I do. W. P. Uzer (talk) 13:31, 1 February 2014 (UTC)

Looks good. Probably still leaves the reader somewhat unsure "what it is that's totally weird"; but a lead cannot really explain this subtle matter anyway. Hopefully, in some future shared randomness will get its place here. Boris Tsirelson (talk) 17:00, 1 February 2014 (UTC)

Thanks - since there are no objections for now, I'm going to try putting the new introduction on the page. W. P. Uzer (talk) 08:40, 4 February 2014 (UTC)

Protons or photons?

Spontaneous parametric down-conversion process can split protons into type II photon pairs with mutually perpendicular polarization.

It should be 'split photons', not 'split protons'. — Preceding unsigned comment added by 106.217.145.179 (talk) 15:25, 20 February 2014 (UTC)

Indeed! Fixed. Thank you. Boris Tsirelson (talk) 16:55, 20 February 2014 (UTC)

General Relativity and a 5th dimension ("Wormhole")

The source for pretty much ALL of the Wormhole section is a pretty terrible article or two from ScienceDaily. They are printing misinformation, for example: General Relativity doesn't need a 5th dimension to bend around, it is a form of intrinsic curvature. Who knows how much else is wrong about that section... prasoc (talk) 19:45, 03 February 2014 (UTC)

I also feel like you. However, the "terrible article from ScienceDaily" refers to Julian Sonner, Phys. Rev. Lett. 111, 211603 (2013). There in the abstract I read: "the bulk dual of an Einstein-Podolsky-Rosen pair is a string with a wormhole on its world sheet. We suggest that this constitutes a holographically dual realization of the creation of a Wheeler wormhole". Is this also a terrible article? Boris Tsirelson (talk) 20:10, 3 February 2014 (UTC)

I am the author of the PRL in question - I have no wikipedia editing experience, but I think I should offer my opinion here. While I object to the previous statement that this is ‘a pretty terrible article’ ;), I do not think a section on the speculative relation between entanglement and wormholes should be in the main wikipedia article on quantum entanglement. The work referenced only deals with entanglement in theories with holographic duals and claims no wider applicability than this. Furthermore the work referenced is in the context of the so-called ER=EPR conjecture, which was proposed in a paper by Maldacena and Susskind, whose work came before the article that is cited here. Finally another paper, by Jensen & Karch takes precedence over my contribution. All that said, it is my personal opinion that there should not be a section on entanglement and wormholes as part of this wikipedia entry. Regards, Julian — Preceding unsigned comment added by 18.189.104.37 (talk) 20:45, 4 February 2014 (UTC)

Thank you very much, Julian! Your expertise is of great help, and very welcome. Maybe you'll have wikipedia editing experience in the future. :-) Boris Tsirelson (talk) 05:22, 5 February 2014 (UTC)

The holographic principle describes a mapping between two coordinate systems of different dimension. In this case one is four-dimensional and one is one dimensional. It is appropriate to talk about a "fifth dimension" only if you also talk about where the additional one-dimensional coordinate system arises. Similarly, it is inappropriate to talk about "the laws of gravity" not being "fundamental" unless you explain what that means. In this case, I think we're talking about the ability of a hypothetical quantum theory of gravity to reduce at a macroscopic scale to General Relativity. As the author of the paper in question himself noted, though, the relation between quantum entanglement and wormholes (which aren't even an observed phenomenon) is highly speculative, and is in no way notable enough to deserve mention in an article on quantum entanglement. Can we please delete this section? Quodfui (talk) 22:02, 28 February 2014 (UTC)

In accordance with the opinions expressed above, I deleted the section. If anyone wants to discuss further, feel free to revert me. W. P. Uzer (talk) 10:44, 1 March 2014 (UTC)

"a Chinese team of physicists disagrees"

"It was up until very recently not believed possible, however, to use this effect to transmit classical information at faster-than-light speeds. However, a Chinese team of physicists disagrees with this assessment. In Bounding the speed of `spooky action at a distance'[11] the team found that particles could be entangled at 4 orders of mangitude (or 10,000 times) faster than the speed of light."

No; it is still not believed possible. In [11] the point is not a transmission of information in our universe; it is rather a transmission needed to mimic quantum entanglement in a classical universe. More exactly, in a universe that satisfies the conditions of "realism" and "freedom" but violates "locality" (see Bell's theorem for detailed formulation of these conditions).

I revert. Boris Tsirelson (talk) 18:34, 10 March 2014 (UTC)

Now again: "It is thought that it is not possible, however, to use this effect to transmit classical information at faster-than-light speeds[10] (see Faster-than-light → Quantum mechanics). However, researchers at Los Alamos National Laboratory in the United States are working to develop a quantum network capable of transmitting communications[11]"

No; that work is about quantum cryptography; there information is transmitted no faster than light; entanglement is used for security, not for speed. I revert. Boris Tsirelson (talk) 07:04, 11 March 2014 (UTC)

No causal relation here

For example, assume that each of two hypothetical experimenters, Alice and Bob, measures the spin of one of a pair of entangled particles (with zero total spin). If Alice makes the measurement first, then her result will be entirely unpredictable, with a 50% probability of the spin being up or down. If Bob subsequently measures the spin of his particle, the measurement will be entirely predictable―always opposite to Alice's, hence perfectly anti-correlated.

The above sentences I plan to delete or change, but will wait a bit and listen to objections.

For one thing the experiment has been performed quite successfully and indeed with a spacelike interval of the mesurements. So in the first place it need not be described as a ″Gedankenexperiment″. Secondly the text ignores the fact that in this situation there is no absolute 'first' and 'subsequently'. Rather, depending on his inertial system, one observer will perceive Alice and another Bob as measuring first. The concept of the experiment puts the two on equal footing. That, in my opinion, completely rules out the idea of one measurement determining the outcome of the other by any physical influence. A different matter it is that the measurement I'm first told of tells me, 'determines', what the other value is, since I know of their complementarity. But that cannot be meant by the above statement which stresses the temporal order of the measurments.

For me a good analog is an image doubled by a beam splitter. The two images produced are perfectly correlated, but neither can be said to determine the other by way of causality.

Comments?-- Binse (talk) 00:46, 11 March 2014 (UTC)

In the Cosmic Wheeler experiment, one interpretation says that when two telescopes are flipped into place, one aimed at each virtual star, the photon-splits (which had happily been traveling down both arms of the cosmic interferometer) realize that they can't both manifest a photon, that one of them is going to have to show up on the CCD at the end of one telescope, so they go back in time a billion years or so and "decide" to either go one way or the other and to do so as a particle. Another interpretation says that the two wavefunctions continue just as they had been doing for the last billion plus years, each goes down the barrel of its own telescope, and then without regard to the barriers to any kind of known communication they decide among themselves which will wink out and which one will scintillate as a single photon on the CCD. So there is either a time paradox or a space paradox. The problem is, of course, that humans are trying to make a model that can work well enough to tell what nature will pull out of its black box. P0M (talk) 04:01, 11 March 2014 (UTC)

I support Binse. Boris Tsirelson (talk) 06:45, 11 March 2014 (UTC)

Hallo POM! What you write looks interesting, imaginative, and nothing against it. But it doesn't hit the point. I'm not concerned with the problem of how nature does produce the correlation of Alice's and Bob's masurements. That is a riddle to me as to anyone. What I'm saying is only (please read again) that the pattern of causality, meaning physical determination of a later measurement by an earlier one, cannot be applied here, simply because with two events at a spacelike interval there is no 'earlier' and 'later'. And therefore the article shouldn't use such words. Why formulate a paradox where there is no need to? My analog you must not take further than it is meant to go: just to remind you that we may well observe correlated events without forcing them into the pattern of cause and effect.

Perhaps my intention becomes clearer if I just try formulating an alternative.

For example consider an experiment that has actually been performed. Two experimenters, Alice and Bob, each measure the spin of one electron of a sequence of entangled pairs. The pairs are in the singlett state just defined. The measurements are taken along the same axis, distance and timing chosen so as to make the interval between the two measurements spacelike.

Both experimenters find quite unpredictable random sequences of 'up' und 'down' where both values appear with probability 50%. Alice's and Bob's measurement, despite their distance, turn out to be complementary for each pair. However, because of the distance they are unable to notice the correlation. To observe it it is necessary to transfer both measuring reports to a common site for comparison. And that, according to present knowledge requires at least the time that light needs to travel there.

Note, that for two events at a spacelike interval there is no universal 'earlier' and 'later'. Rather it depends on the inertial system of an observer in which temporal relation he sees the events. Therefore the correspondence between Alice's and Bob's measurements cannot appropriatly be described as one measurement determining the other: Different observers would disagree about the role of cause and effect.

It's become a bit lengthy. But again: Comments? -- Binse (talk) 01:07, 12 March 2014 (UTC)

Nothng special. I was agreeing with you, but at the same time pointing to another kind of unobserved human bifurcation, a conceptualization as two separate "anomalies" of what may need to stay a unity and isn't an anomaly but just something that everyday experience doesn't prepare us well to digest. P0M (talk) 03:19, 12 March 2014 (UTC)

Thank you POM for clarifying that. I got the impression you were opposing.-- Binse (talk) 01:50, 14 March 2014 (UTC)

I think the added text places too much emphasis on the 50% same-axis case, which as is noted, can be simulated deterministically anyway, unlike the cases that produce Bell's inequality violations. Also it's not true to say they are unable to notice the correlation - just that some minimum time must elapse before they can notice it, but that isn't really the point. W. P. Uzer (talk) 08:42, 15 March 2014 (UTC)

Indeed, same-axis case is of little interest. Boris Tsirelson (talk) 06:54, 17 March 2014 (UTC)

Concept

"Entanglement can be broken when a measurement is made AND the entangled particles decohere through interaction with the environment (measurement device)." — why "and"? It can be broken when the entangled subsystems decohere through interaction with the environment (generally not at all a "measurement device"; for instance, a gas). By a measurement, entanglement can be broken; does this include decoherence? Usually it does; but some authors admit also a measurement that can be undone, in which case entanglement is not quite broken, rather it is extended to the meter, and can be restored. Entanglement tends to be broken by a measurement over one subsystem (particle), or both subsystems separately. However, a joint measurement (for example, of the total spin, or total moment, or the distance between) can even create entanglement between subsystems that were uncorrelated. Boris Tsirelson (talk) 15:40, 21 March 2014 (UTC)

I agree; I tried to improve the sentence in question, but you may be able to do better. I also looked at the source that is cited for this statement (on the Google Books preview) and it appears that nothing is said on this particular topic on the page given. Possibly the intention was that the whole chapter which begins on that page is supposed to serve as a reference for the whole paragraph. W. P. Uzer (talk) 20:01, 21 March 2014 (UTC)

Something more-or-less relevant: entanglement-by-measurement. Boris Tsirelson (talk) 21:00, 21 March 2014 (UTC)

Faster than light communication

Given that quantum entanglement is often represented in science fiction as enabling faster-than-light transmission of information, shouldn't this be addressed, at least in passing?

It's a huge can of worms, some of which are thoroughly metaphysical if not mystical.

Maybe what needs to be said is that our ordinary understanding of "communication" is that somebody does something on one end of a long-distance apparatus, and a corresponding something happens on the other end. So, for instance, a computer will send a blip that represents a 1 or another kind of blip that represents a 0, and a matching 1 or 0 will show up in the reception apparatus on the other end of the telephone line or fiber optic cable. However, we cannot command a quantum object to take on any value that would serve as a communication token. We do something on this end that makes a quantum random event occur. It might be a quantum state that we would conventionally assign the value 1 or it might be a quantum state that we would conventionally assign the value 0, but we don't know which one we will get on this end. All we know is that the person who holds the entangled mate of the quantum object that we just forced to make up its mind whether it is a 1 or a 0 will discover, upon making a similar measurement, that if it was a 1 on this end it will be a 1 on that end. That like saying that I could draw a random slip of paper from a hat that had some letter written on it, and somebody in the neighborhood of Alpha Centauri could draw a slip of paper and it would have to be the same letter. But so what? We pull an "R" and send a light-speed signal to Alpha Centauri and eight years letter we get back a message, "Hey! I pulled an "R" too. Is that supposed to mean something?" "I don't know, it's just what came up by chance. Neat experiment, huh?"P0M (talk) 06:02, 13 January 2014 (UTC)

Seems to me (this is what I was alluding to in the thread above) that the weirdness of entanglement becomes most visible not when the same measurement is made at the two separate locations (since in that case, as you say, you just get predictably identical or predictably opposite results, and so what?), but when sets of different but related measurements are made, such as measurements of spin in three different directions. That's when you get violations of Bell's law and the sense that something akin to (but not quite) "information" must be traveling instantaneously between the locations - as if one of the particles "knows" in which direction the spin of the other particle was measured, and lines up its own spin accordingly in that direction, so that the other measurements can correlate appropriately. W. P. Uzer (talk) 07:42, 13 January 2014 (UTC)

Can you see any way in which that kind of weirdness can be used to make an ansible?P0M (talk) 13:54, 13 January 2014 (UTC)

No, I believe it can't, but it still seems weird. W. P. Uzer (talk) 14:17, 13 January 2014 (UTC)

I can't help hoping that Cramer is right and that an ansible will emerge, somehow, by some clever use of the weirdness.

Somewhat to my regret I got involved in trying to figure out what the article on Counterfactual definiteness is supposed to be about. In the beginning I thought the article was just wrong, but later I began to think that there is something there that the article wasn't succeeding in communicating. I've tried to draft a better beginning to the article on the Discussion page. Interest in the subject seems to be motivated by a desire on the part of some to challenge Bell's law by saying that (somehow) the reason that so-called entanglement phenomena occur is that Bell used counterfactual measurements (measurements of what I expect I would have had to have gotten?) in his calculations, so his theory is flaky. If his theory is flaky then he can't prove anything about the impossibility of there being hidden variables involved. Something like that, I guess, but I really am pulling left and right for anything I can get Gestalt closure on, and I'm doubtful that I've got it right. Could you have a look? P0M (talk) 18:53, 10 March 2014 (UTC)

I don't understand. A couple of entangled particles are produced. The spin of the first particle is clockwise (though we have no way of knowing this as of yet, the choice is totally random), the spin of the second particle is counterclockwise (though we have no way of knowing this as of yet, the choice is totally random), the total spin is zero (we already have a way of knowing this, this is not random). Why can't they have this very arrangement right at the start, when the entanglement was formed, and not when one of these spins was measured? Why do they need to communicate anything at the point of measurement so that to demonstrate that the total spin is indeed zero? I admit that I don't understand a lot, but this question is natural and does not seem to be addressed explicitly. Thank you anyone who reacts. - 89.110.3.223 (talk) 11:48, 8 May 2014 (UTC)

Okay. I see that this topic is something beyond a “normal human”'s capability to understand. ;) What the question I have posted has to do with the “hidden variables” (why hidden? just the spin, nothing else), I have no idea; in what ways it was answered by Bell, the same. But somehow it apparently was. - 89.110.3.223 (talk) 12:42, 8 May 2014 (UTC)

Yes, it may seem to be just shared randomness. Until you know Bell's theorem that shows that it is not shared randomness! Just the spin, nothing else... nice... now try to calculate correlations as a function of angles, and you will never get the right answer! You see, making "this very arrangement right at the start, when the entanglement was formed" the particles cannot know which projections of their spins will be measured. Boris Tsirelson (talk) 12:45, 8 May 2014 (UTC)

Thank you. Food for thought. As far as I understand, the next step in such an “intuitive” explanation is that, unlike in classical mechanics, in QM one could not have a single vector value that would be accountable for all possible projections at once, so this knowledge is indeed necessary… Or I am entirely mistaken, but anyway, thanks. As for the article and especially the lede: maybe it is better to edit the lede so that it leaves no doubt that no way of considering prearrangement leads to coherent conclusions? So far, the article seems to reject outright only the deterministic interpretation (like, there are some variables that are not treated by QM, but determine decisively the effects and measurements that we observe). - 89.110.3.223 (talk) 13:34, 8 May 2014 (UTC)

Really, I agree with you, and this is already written by me at the end of #No causal relation here: "Indeed, same-axis case is of little interest." But this may lead to problems; you cannot really explain the main point without turning this article into another article on Bell theorem. Boris Tsirelson (talk) 15:21, 8 May 2014 (UTC)

Faster-than-light transmission of data now possible?

According to this New York Times article: http://www.nytimes.com/2014/05/30/science/scientists-report-finding-reliable-way-to-teleport-data.html?smid=tw-nytimes&_r=0

Should we include these new findings in this article? windwaker (talk) 21:49, 4 June 2014 (UTC)

No, it is not written there that faster-than-light transmission happened. Look again, closely. Still, "Because it depends on classical communication, which can proceed no faster than the speed of light, it cannot be used for superluminal transport or communication of classical bits." (Quoted from "Quantum teleportation".) Boris Tsirelson (talk) 05:40, 5 June 2014 (UTC)

Time - Quantum entanglement

Time is an emergent phenomenon that is a side effect of quantum entanglement, say physicists. And they have the first experimental results to prove it...

https://medium.com/the-physics-arxiv-blog/quantum-experiment-shows-how-time-emerges-from-entanglement-d5d3dc850933

http://arxiv.org/abs/1310.4691

Kartasto (talk) 11:03, 23 July 2014 (UTC)

Quantum entanglement - source for the arrow of time

"Quantum uncertainty then gives rise to entanglement, the putative source of the arrow of time..." Here is more:[1]

Kartasto (talk) 12:27, 12 August 2014 (UTC)

“Other types of experiment”: paragraph discarded

In Quantum entanglement#Other types of experiment a paragraph was discarded in https://en.wikipedia.org/w/index.php?title=Quantum_entanglement&curid=25336&diff=626785176&oldid=625926388 by User:Ruibjr without justification. Though their contribution history shows one only other edit, a not very helpful one in 2006(!), their addition seems genuine. I have therefore restored the discarded paragraph but not reverted the edit. PJTraill (talk) 10:42, 24 September 2014 (UTC)

Material needs clarification

In the section "Apparent paradox" it reads: "The seeming paradox here is that a measurement made on either of the particles apparently collapses the state of the entire entangled system—and does so instantaneously, before any information about the measurement could have reached the other particle (assuming that information cannot travel faster than light). In the quantum formalism, the result of a spin measurement on one of the particles is a collapse into a state in which each particle has a definite spin (either up or down) along the axis of measurement. The outcome is taken to be random, with each possibility having a probability of 50%."

I fail to see how when the determination of one of the entangled particle determines the other is a paradox. If they were entangled in such a way as to have complimentary properties then they do so regardless of where they are in the universe.

"However, if both spins are measured along the same axis, they are found to be anti-correlated."

OK, they start out anti-correleated and remain so. Nothing paradoxical about that.

"This means that the random outcome of the measurement made on one particle seems to have been transmitted to the other, so that it can make the "right choice" when it is measured."

Again, I see nothing paradoxical about that as they were paired from the start and I see no reason for transmission of its "choice" of spin state needing to be transmitted. If you know one is spin-up then it follows the other is spin-down.

"The distance and timing of the measurements can be chosen so as to make the interval between the two measurements spacelike, i.e. from any of the two measuring events to the other a message would have to travel faster than light. Then, according to the principles of special relativity, it is not in fact possible for any information to travel between two such measuring events—it is not even possible to say which of the measurements came first, as this would depend on the inertial system of the observer. Therefore the correlation between the two measurements cannot appropriately be explained as one measurement determining the other: different observers would disagree about the role of cause and effect."

Either I am slow or there is something left out of the above paragraph? If the measurement of one of the entangled particles causes a perturbation of that particle and results in that perturbation (whatever that may have been) being instantaneously imposed on the other, that would be paradoxical. If my interpretation is correct, mention should be made about the measurement causing some perturbation, and that effect is apparently then transmitted instantaneously. Zedshort (talk) 00:59, 19 November 2014 (UTC)

Surely, measurement is a perturbation! This is well-known to be inevitable in quantum mechanics. Measuring this projection of spin we get a state with a certain value of this projection and therefore (taking into account properties of spin) any other projection uncertain. It is this "perturbation" seemingly/paradoxically "transmitted". Boris Tsirelson (talk) 06:34, 19 November 2014 (UTC)

Yes it is, and that is my point. The entire paragraph fails to state what should be made explicit: that the measurement creates a perturbation that is then imposed on the other particle with which it is entangled. I would change it but I don't know enough about the subject to say with certainty that I am correct in that, hence I pointed out what I believe to be a flaw in the writing. If you feel certain that that point is correct then I request you to make the changes. Thanks. Zedshort (talk) 15:29, 19 November 2014 (UTC)

No, sorry. I did my best there, but it does not fit Wikipedia. Boris Tsirelson (talk) 06:44, 20 November 2014 (UTC)

Articles about entangled partcles (generation, observation, and characterization)?

Wikipedia need articles about entangled partcles, explaining generation, observation, and characterization:

• Photon entanglement: a begining, but a bad quality article.
• Electron entanglement (need rewrite redirection): no article. For stub see ex. 1.
• etc.

There are some people working with it? There are articles with another names (ex. "fermion entanglement"[2])? --Krauss (talk) 19:49, 14 December 2014 (UTC)

Apparent paradox vs paradox

Stating that laws of nature are paradoxical as opposed to apparently paradoxical is going too far. I dare say that most physicists believe that nature houses no true paradoxes. It is just that nature is not fully understood. The formulation I reverted to is perhaps not the best, but is acceptable in comparison. YohanN7 (talk) 08:22, 27 December 2014 (UTC)

Concern about the "Special Theory of Relativity" section

New user here, I'm not sure if this is how to bring up concerns but that "Special Theory of Relativity" seems dubious, or at least not to contribute anything new. I'm not a physicist so if there's some way one could check I'd be interested but it would seem that it would imply (1) a static interpretation of quantum mechanical particles (2) reverse causality. Finally the whole view is based on the assumption that we can treat all particle, relativistic or not, as not experiencing internal time simply because the photon does. I feel like this is in conflict with really any modern view of the phenomenology of quantum physics and so shouldn't really be reported as a solution to the problem. Are there any guide lines for figuring out if a view can be reported as fact? Or could a physicist take a look and make a call on this?

Indeed, the article by J.Lee (the only source of this section) is very problematic. Far from being well-accepted. Unconvincing zigzags between FTL and causality. By the way, the speed of a photon is not c, it is a bit smaller. The speed of a bare photon is c. But a dressed photon involves virtual electron-positron pairs; not quite massless; and not quite "timeless". But there are a lot of other, more important objections. For now, an encyclopedia should at best note "see also a discussion in ..." rather than "Another theory explains quantum entanglement..." (highly overstated). Boris Tsirelson (talk) 06:50, 19 December 2014 (UTC)

I really think the theory should not be in Wikipedia until it has some acceptance. 178.38.107.210 (talk) 17:06, 30 April 2015 (UTC)

What is wrong?

In the section "Ensembles", I tried to put in what I thought is the correct general form of a density matrix (mixed state) for a composite system. But then it turns out to be equivalent to the definition given just below that, for a separable state. What gives?

178.38.123.1 (talk) 02:37, 1 May 2015 (UTC)

Visibility of the section "Reduced density matrices"

I have seen that reduced density matrices are everywhere -- after all, they just express the "mixed state" that each system is, considered spearately. Therefore, they rightfully have an important role, and indeed I wish that this section, which explains them mathematically, were more visible in Wikipedia. Indeed, I have seen them implicitly referred to in many articles on entanglement, and I wasn't able to understand these articles until I discovered the formal definition here.

But immediately after defining "reduced density matrices" we have the following two paragraphs:

Reduced density matrices were explicitly calculated in different spin chains with unique ground state. An example is the one-dimensional AKLT spin chain:[2] the ground state can be divided into a block and an environment. The reduced density matrix of the block is proportional to a projector to a degenerate ground state of another Hamiltonian.

The reduced density matrix also was evaluated for XY spin chains, where it has full rank. It was proved that in the thermodynamic limit, the spectrum of the reduced density matrix of a large block of spins is an exact geometric sequence[3] in this case.

My objections to these two paragraphs:

(1) They are incomprehensible at this point in the article.

(2) They seem to be too specialized to be useful in this rather general pedagogical section. Why mention just these two obscure papers in this particular moment?

So I put them in a separate section. But maybe they could go.

so there are 2 states to everything in the universe so when we smash atoms and do finally see the inside out of an atom will we be able to communicate to ourselves in another reality or state? It must be there are 2 of each of us maybe more if i see this quantum properly. We will be able to send signals through time itself not just along it... Edward Russell — Preceding unsigned comment added by 216.223.138.85 (talk) 23:15, 3 July 2015 (UTC)

178.38.123.1 (talk) 03:01, 1 May 2015 (UTC)

Aug 2015 experiment Experimental loophole-free violation of a Bell inequality using entangled electron spins separated by 1.3 km

New experiment, results to be peer reviewed. May be a future addition after peer review agrees with the results. http://www.nature.com/news/quantum-spookiness-passes-toughest-test-yet-1.18255?WT.mc_id=TWT_NatureNews http://arxiv.org/abs/1508.05949 JScience 18:45, 29 August 2015 (UTC)jcardazzi

Experiments

It's not clear which "experiments" are thought experiments and which actually take place in a laboratory. Kortoso (talk) 19:01, 17 November 2015 (UTC)

This article suffers from abstractitis

"Abstractitis" is Ernest Gowers' term for writing that is so abstract that it cannot be understood. This article suffers from clear-cut case of it. In it, em-dashes and semicolons incorrectly connect sentence fragments, and this appears deliberately designed to mask the confusion of the author. Whereas the first sentence could almost pass as coherent, the article quickly becomes unreadable. It could very well be that quantum entanglement is simply a confused subject, and that nobody really knows what it's all about. But I challenge someone to edit this article (or rewrite it) such that it can be understood by a general audience. — Preceding unsigned comment added by 76.10.134.7 (talk) 16:41, 21 November 2015 (UTC)

Hypothesis that quantum entanglement is the source of space–time

This article ([3]) should be incorporated into this Wikipedia page. — Preceding unsigned comment added by 24.196.131.25 (talk) 07:33, 22 December 2015 (UTC)

I suspected crack-pottery, but it is an article in Nature. Could perhaps go as an "external reading" link. The paper is non-technical, and could have been written under the influence of mushrooms or something (well, that is the nature of entanglement phenomena), but could equally well be totally serious. Someone with deeper understanding will have to judge. YohanN7 (talk) 14:45, 28 December 2015 (UTC)

Regretfully, I am not "someone with deeper understanding"; and anyway, Wikipedia editors are not supposed to decide... I'd say, the paper is as serious as every paper in "Nature"; however, (unlike most other papers) it does not state any scientific truth (yet?); rather it states that this direction of thinking is notable, and for now no one knows what will result from it. Probably, the same can be said about recent ideas of t' Hooft in physics and Univalent foundations in mathematics. Boris Tsirelson (talk) 16:35, 28 December 2015 (UTC)

According to our policy (WP:Fringe theories, WP:Neutral point of view/FAQ#Pseudoscience), these "Alternative theoretical formulations" which have a following within the scientific community are not pseudoscience, but part of the scientific process. They should not be classified as pseudoscience but should still be put into context with respect to the mainstream perspective. Boris Tsirelson (talk) 07:50, 29 December 2015 (UTC)

There is already an article ER=EPR started since December 14, 2014. Is it something related to the article mentioned by the unsigned editor?--LaoChen (talk)21:55, 30 December 2015 (UTC)

Yes, it is. In fact, that paper is discussed in the paper in Nature. Boris Tsirelson (talk) 05:57, 31 December 2015 (UTC)

"Special Theory of Relativity" should be removed

This was brought up by someone else in the archives, but I think the theory referenced in this section is highly problematic. I suggest removing this section entirely. It's author is not a physicist and appears to publish only on sites such as vixra.org (http://vixra.org/author/justin_lee) where in addition to this paper he has papers "demonstrating" that Einstein was wrong and we can detect an absolute rest frame and that the speed of light isn't the same for all observers (http://vixra.org/pdf/1212.0043v4.pdf).

The paper referenced on Wikipedia makes use of a dubious retro-causality model that actually doesn't even solve anything in the case of his electron-positron example since, even if it is right, the particles don't get their properties (from the future) until long after they were created. Einniv (talk) 19:23, 3 January 2016 (UTC)

I agree. Moreover, such concern was already discussed, see here (but nothing happened). Boris Tsirelson (talk) 19:51, 3 January 2016 (UTC)

I got bold and deleted that subsection. Boris Tsirelson (talk) 19:56, 3 January 2016 (UTC)

The example does not seem to require entanglement

The initial section says the fact that if two particles resulting from a single event are required to have opposite spins, measurement will show that they do indeed have opposite spins. I do not see why that requires entanglement. You can obtain the same result with billiard balls. My understanding is that it requires a series of two interactions to establish entanglement. Fairandbalanced (talk) 21:56, 26 September 2015 (UTC)

Welcome in the reality of today's physics. A physicist after the end of his education has no more time to think in this way :-) But you are right. But since this is an encyclopaedia for well established science, there is nothing to change. HolgerFiedler (talk) 08:43, 1 January 2016 (UTC)

It does need entanglement. The point is your freedom to choose the axis of the spin measurement. More details in Bell's theorem. --2003:57:E679:8B01:D6C:2352:5241:B44C (talk) 22:41, 23 January 2016 (UTC)

Loophole?

Hensen's 2015 Nature article (ref 17) claimed that the example of electron spin entanglement was loophole free. Is this not worth noting in the article, or is it insufficiently confirmed? 212.183.128.219 (talk) 11:47, 7 March 2016 (UTC)

Wiseman and Nature thought 2S was noteworthy enough for publication.[4] Cpsoper (talk) 12:10, 7 March 2016 (UTC)

Relativity of simultaneity

This needs to be addressed better in the article. Saying something occurs "instantaneously" makes no sense unless the frame of reference used is specified. Urhixidur (talk) 17:31, 6 April 2016 (UTC)

Meaning of entanglement

I think that the last paragraph of the section Meaning of entanglement contains some flaws:

... the quantum system considered here seems to acquire a probability distribution for the outcome of a measurement ..

We are talking of a single measurement on a single system here, but a single measurement cannot yield a distribution but just a single value.

This probability distribution is in general different from what it would be without measurement of the first particle.

I think that's wrong. The probability distribution is always the same, e.g. 50:50 up/down. If it would not be the same, we could transfer information faster than light by choosing the measurement setup for each "first particle" of a batch immediately before measuring the "second particles". --PM3 (talk) 16:36, 15 June 2016 (UTC)

Testing a system for entanglement

I think there needs to be a review of this section, but I'm not too familiar with this subject. The first paragraph section ends with this weird sentence:

An implementation of the algorithm (including a built in Peres-Horodecki criterion testing) is brought in the "StateSeparator" web-app.

I cannot find anything about a "StateSeparator" web-app (website / web application?), I don't understand how it relates to the rest. Why would a web application help here?

Also, the second paragraph

China launched the world’s first quantum communications satellite

is just a single sentence and then a larger quote taken from another website. It's an interesting fact, but does not explain anything on the topic of the section. --92.73.219.52 (talk) 04:44, 26 September 2016 (UTC)

Experimental data finding that needs verification

I am suggesting to add this section here because I have analyzed data from a recent entanglement experiment that gives somewhat odd indication. This will help to bring it to people who may be interested in verifying similar analysis in the data they may have. My analysis can be read at http://vixra.org/abs/1609.0237 Please let me know if it is ok to add this section.

This can be a useful feature on wikipedia in general where the moderators can review this kind of findings and add to scientific topics for further verification.

(Moved from the article; was added thereto by User:Kpvats. Boris Tsirelson (talk) 04:57, 9 October 2016 (UTC))

Now User:Kpvats added it again, with the phrase "Please provide some reason/comments for removing." It seems he/she does not realize that ANY such reasons, comments etc are ABSOLUTELY unacceptable in encyclopedic articles (did you ever see such phrases in any encyclopedia??); their place is here, on the talk page. I revert. Boris Tsirelson (talk) 20:48, 9 October 2016 (UTC)

199.190.211.3 (talk) 20:34, 10 October 2016 (UTC) OK, Hope this is the right place (I clicked on the link you provided. Now, could you please let me know what is the reason for not having the new section where people can post things to be verified, which can be added there post moderator review. I have one such analysis that I want verified by others having similar data.

That is, you want to use Wikipedia as a forum. No, this is not allowed. Being an encyclopedia, Wikipedia accepts only information from reliable sources. A self-published article (in arXiv, the more so in viXra) is generally not a reliable source. Sometimes it may be, but only if the author is already well-established, well-reputed scientist. Boris Tsirelson (talk) 21:09, 10 October 2016 (UTC)

Incorrect example should be corrected or removed

"a spin-zero particle could decay into a pair of spin-½ particles. Since the total spin before and after this decay must be zero (conservation of angular momentum), whenever the first particle is measured to be spin up on some axis, the other, when measured on the same axis, is always found to be spin down. (This is called the spin anti-correlated case; and if the prior probabilities for measuring each spin are equal, the pair is said to be in the singlet state.)​"

If the spin axis of the original particle was the same as the measurement axis, this would be true (ie, prepare vertical and measure vertical), but if you measure on a different axis, you only have probabilities of seeing this based on cos^2 of the difference between the measurement axis and the original particle axis. Many of the particles prepared in this manner, will not be anti-correlated if measured off of the original particles spin axis. Something in addition to what is stated here would have to be done to ensure entanglement. This example is not worded properly. 2001:56A:F006:DE00:89A4:C3BD:383D:3975 (talk) 17:54, 10 March 2017 (UTC)

No! An important non-classical feature of the spin is, that there is no "spin axis of the original particle". The original particle is of spin zero, and its (spin component of) state vector is rotation invariant (that is, spherically symmetric). The decay does not break this symmetry; but now the symmetry applies to the pair, not to each particle separately. No cos^2; all axes have the same property, perfect correlation. It may sound incredible, but it is a fact (confirmed by numerous experiments). Boris Tsirelson (talk)

quantum entanglement in time

however,quantum entanglement theory based on intraction between two particle or single particle and a wave but what abut entanglement between two particle that occurs in different time ? — Preceding unsigned comment added by 5.216.107.43 (talk) 23:24, 21 March 2017 (UTC)

Glove analogy

Why no reference to the glove analogy resolution to the paradox, originated I believe by Dr. Brian Greene? Even if this has been refuted, it is surely notable. For those not familiar with it, here is the thought-experiment: Suppose I take a pair of gloves and place each in a box without looking. A friend takes one box to a distance. At a signal (compensated for light speed) we open our boxes simultaneously. Mine contains a left glove. His contains a right glove. But we don't consider this a paradox! It does, however, raise interesting questions about the nature of an "object" and space "between". D Anthony Patriarche (talk) 02:04, 1 August 2017 (UTC)

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Aspect article, the Question of Loopholes

In the paragraph after that photons-affect-generators loophole he talks about the free will loophole ie superdeterminism, which is unclosable. Can we please at least restore some wording that explains what Alain means; at the moment the article has multiple discussions about how loophole-free bell tests, and another sentence about how they're not loophole free w/ no explanation. Porphyro (talk) 20:21, 5 March 2018 (UTC)

Aspect's article is perfectly clear. We don't need people writing into Wikipedia what they think Aspect meant. 47.201.178.44 (talk) 14:13, 21 March 2018 (UTC)

I agree, it is clear. He talks about the free will loophole, which is superdeterminism. The two are literally synonymous. Porphyro (talk) 14:36, 21 March 2018 (UTC)

Aspect is also talking about the Setting Independence loophole, see Aspect's reference 30, which is perhaps not exactly the same thing as the Superdeterminism loophole. At any rate, yes, we must let Aspect's words speak for themselves.47.201.178.44 (talk) 14:50, 21 March 2018 (UTC)

The following sentence with published references should be included in Wikipedia where treating the question of loopholes: An ensemble interpretation of quantum mechanics explains quantum correlations without any instantaneous action-at-a-distance and completely avoids any question of loopholes. Published References: http://www.worldscientific.com/doi/abs/10.1142/S0217979206034078 https://arxiv.org/ftp/quant-ph/papers/0404/0404011.pdf http://physicsessays.org/browse-journal-2/product/47-3-dean-l-mamas-an-intrinsic-quantum-state-interpretation-of-quantum-mechanics.html — Preceding unsigned comment added by 47.201.178.44 (talk) 13:09, 6 March 2018 (UTC)

Two of those references are uncited by anyone. I'm not saying no one has that view but it is a fringe view and doesn't deserve to be presented as absolute fact on wikipedia. Porphyro (talk) 13:29, 6 March 2018 (UTC)

But it certainly is satisfying is it not, to have such a simple resolution of the question. I like it very much. — Preceding unsigned comment added by 47.201.178.44 (talk) 13:43, 6 March 2018 (UTC)

I would agree, but I'm not sure the first source at least justifies the claim. Even among supporters of Ensemble interpretations, the general consensus is that for realism we need nonlocality. As per Wikipedia:Fringe theories, the article cannot make such a theory appear more notable or widely accepted than it is. I don't think we have the sources here to justify an inclusion in the article, much less a prominent one. Porphyro (talk) 13:47, 6 March 2018 (UTC)

There are numerous variations of the ensemble interpretation. This one solves the loophole question. — Preceding unsigned comment added by 47.201.178.44 (talk) 13:57, 6 March 2018 (UTC)

Copied to the Quantum Mind page

The following section was copied to Quantum Mind on 9 Feb 2018 by [[user: wcrea6}}:

Quantum entanglement is a physical phenomenon which occurs when pairs or groups of particles are generated or interact in ways such that the quantum state of each particle cannot be described independently of the state of the other(s), even when the particles are separated by a large distance—instead, a quantum state must be described for the system as a whole. Measurements of physical properties such as position, momentum, spin, and polarization, performed on entangled particles are found to be correlated. — Preceding unsigned comment added by Wcrea6 (talk • contribs) 03:36, 3 July 2018 (UTC)

EPR and locality

I tried to correct this statement: "Bell proved ... the principle of locality ... was mathematically inconsistent with the predictions of quantum theory." No, he only proved this when locality is combined with a hidden variable theory. Some people do subscribe to nonlocal interpretations of quantum theory, but it is not true that locality is inconsistent with the predictions. Roger (talk) 05:36, 3 July 2018 (UTC)

My point was that there is no point writing something like "locality.. combined with a hidden variable theory". The wavefunction of quantum mechanics is nonlocal. If you want to have a theory that is local then you *must* be introducing some sort of ontological overhead to the system that comes under the heading of what I think you are calling "hidden variables". Perhaps you are using a slightly different notion of locality, or of hidden variables, but maybe it would be instructive if you could give me an example of a local theory that doesn't make use of hidden variables? As far as I am aware, the only truly local models for quantum theory are Everettian viewpoints that do carry some extra ontological baggage, but it's the abandonment of the single world assumption and the ability to counterfactually reason that provides most of the power. Porphyro (talk) 10:44, 3 July 2018 (UTC)

Quantum field theory is a truly local theory in that there is no action-at-a-distance. See this recent paper [5] for a good explanation. But this is not the place to argue about what Bell proved anyway. There is a WP article on Bell's theorem that explains it in considerable detail, and it says "No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics." That is, Bell assumed locality and hidden variables to get his conclusion. If you don't agree with that statement of Bell's theorem, you could raise the issue on that page. This page just needs to summarize it in several sentences, but not use some philosophical argument to state it in a very different way. Roger (talk) 16:28, 3 July 2018 (UTC)

I believe you are misreading the logical intent of that sentence. It is clear to talk about "local hidden variables", because not all hidden variables are local. But it does not make sense to talk about the combination of locality and hidden variables, because all attempts to make quantum theory (or subtheories) local MUST have hidden variables. Saying that Bell's proof only applies when you have both locality and "hidden variables" implies you can have locality without hidden variables. I don't think that the quantum field theoretic version is quite as cut-and-dry as you are implying. I also would suggest that the author of the paper you linked has missed the point somewhat. Perhaps some other editors will put their hats into the ring on this issue. Perhaps you would be okay with wording that sentence something along the lines of

".. one of their key assumptions, the principle of locality, as applied to the kind of hidden variables interpretation hoped for by EPR, was mathematically inconsistent with the predictions of quantum theory." Porphyro (talk) 16:39, 3 July 2018 (UTC)

Your proposal is an improvement, but it is incorrect to say: "all attempts to make quantum theory (or subtheories) local MUST have hidden variables." Quantum field theory, as it is usually presented in textbooks, is a local theory with no hidden variables.

It is true that Bell sometimes used the word "local" to include an assumption of hidden variables. However, his definition is not accepted. The article links to principle of locality, so we should use the definition given there. Roger (talk) 19:56, 3 July 2018 (UTC)

it’s local in the sense that interactions are local, but that’s not what people mean by local when they’re talking about entanglement. Porphyro (talk) 20:50, 3 July 2018 (UTC)

There is no sense in us arguing about what "local" ought to mean. The article refers to another article defining the term. If you don't agree with that definition, go fix that page, Principle of locality. It says that Bell proved that "quantum mechanics violates either locality or realism". Here, "realism" is another word for hidden variables. It is inconsistent with common usage to say that quantum mechanics violates locality. Roger (talk) 21:34, 3 July 2018 (UTC)

Your point is inconsistent with what you’re tying to make it say, though. If I add a hidden variable to a theory you consider “local” that has no relevance to the scenario we’re considering, it doesn’t mean that theory is ruled out by Bell’s theorem. It’s just not correct to say it’s the conjunction of “locality” and “hidden variables” that’s problematic. Also, the article you’re citing says Bell’s theorem rules out the intersection of Locality and Realism. You can edit this article to a similar wording if you wish; otherwise I suggest we wait for a third party to weigh in. Porphyro (talk) 21:42, 3 July 2018 (UTC)

Yes, the conjunction of “locality” and “hidden variables” is problematic. That is the same as saying that the intersection of Locality and Realism is problematic. You're right, you don't ruin a local theory by adding an extraneous hidden variable. But it if is a local hidden variable, and it is being used to predict quantum phenomena that would be subject to Heisenberg uncertainty, then you have a problem. As you suggest, I will wait for others to weigh in, but I am really just trying to make the text conform to what the WP articles on locality and Bell say. Roger (talk) 00:52, 4 July 2018 (UTC)

would you be amenable to a statement about “local realism” or mentioning realism rather than “hidden variables” (or just keep hidden variables in the phrase “local hidden variables” like the Bell article). By your previous statement it seems as though that is what you mean by the concept, since you only mind hidden variables that give definite value to non-commensurable quantities (ie realism). This brings the article into line with the Principle of locality page. Porphyro (talk) 07:14, 4 July 2018 (UTC)

Realism is just another euphemism for hidden variables. I don't think the term is helpful, unless it is defined. The point here is that the EPR paper was shooting for a "complete" theory, which seemed to mean a combination of features (locality and hidden variables) that Bell's theorem and subsequent experiments proved to be impossible. Phrase it however you want. Roger (talk) 02:50, 6 July 2018 (UTC)

Non-locality and entanglement

To a layperson, this section appears to contradict itself, although it may be the case that further explanation could eliminate that impression:

"While it is true that a pure bipartite quantum state must be entangled in order for it to produce non-local correlations, there exist entangled states that do not produce such correlations, and there exist non-entangled (separable) quantum states that present some non-local behaviour. A well-known example of the first case is the Werner state that is entangled for certain values of $p_{sym}$, but can always be described using local hidden variables. In short, entanglement of a state shared by two parties is necessary but not sufficient for that state to be non-local."

This is a contradiction unless there are forms of non-local behavior that are not non-local correlations (if so, an example of this would help, as the example of Werner states serves to further clarify another logical possibility). As a layperson, I read "non-local" in the context of the rest of this article as a stand-in for "non-local correlations"; note that the last sentence quoted above also appears to use the term in this way.

If that usage is indeed correct, then the contradiction appears as follows:

1. Entanglement is necessary but not sufficient for non-locality => All non-local phenomena result from entangled states
2. "[T]here exist non-entangled (separable) quantum states that present some non-local behaviour" => Some non-local phenomena do not result from entangled states --Joel (talk) 18:22, 8 August 2018 (UTC)

To my knowledge, the 2nd statement is wrong: according to quantum mechanics, non-entangled states never show non-local behavior. A non-entangled state can be written as a mixture of product states. Therefore, its behavior can be described by a "local hidden variable model" (the hidden variable determines which state of the mixture is realized in any given experiment, given that variable, all possible measurements on the system can be reproduced by a local theory (since the system is in an uncorrelated state (product state). Maybe the origin of this statement is the paper "Quantum nonlocality without entanglement". Phys. Rev. A. 59: 1070. 1999. arXiv:quant-ph/9804053. doi:10.1103/PhysRevA.59.1070. {{cite journal}}: Cite uses deprecated parameter |authors= (help) but this is a different notion of non-locality, unrelated to Bell inequalities (and, to my knowledge, not used very much). (In concerns whether states can be reliably distinguished, cloned etc by local operations.) Unless some other citation is provided, I would remove the misleading statement. --Qcomp (talk) 11:27, 9 August 2018 (UTC)

Thanks for that -- the point about local hidden variables directly addresses the question I was hoping this section would clarify. The Bennett et al. paper is also very helpful, and an interesting kind of non-locality even if it deviates from the standard usage of the term. To this reader's mind, all of the above information would make a great addition to the article -- perhaps with a bit of further context about how the two notions of non-locality differ. But I appreciate the information regardless! --Joel (talk) 18:25, 11 August 2018 (UTC)

thanks for the feedback; I've reformulated the paragraph on non-locality accordingly; if you think it is no longer confusing, you might want to remove the marker; if it's still unclear, please elaborate. --Qcomp (talk) 20:31, 12 August 2018 (UTC)

Thanks, I've removed the tag. --Joel (talk) 02:12, 15 August 2018 (UTC)

Lede needs trimming

The lede badly needs trimming. I'm trying out a rather brutal chopping out of what I consider to be excessive detail, placing the chopped out sections here for discussion.

Given that the statistics of these measurements cannot be replicated by models in which each particle has its own state independent of the other, it appears that one particle of an entangled pair "knows" what measurement has been performed on the other, and with what outcome, even though there is no known means for such information to be communicated between the particles, which at the time of measurement may be separated by arbitrarily large distances.

demonstrating that the classical conception of "local realism" cannot be correct

Since faster-than-light signaling is impossible according to the special theory of relativity, any doubts about fake entanglements due to such a loophole have reduced to effectively zero.

After all, if the separation between two events is spacelike, then observers in different inertial frames will disagree about the order of events. 

Joe will see that the detection at point A occurred first, and could not have been caused by the measurement at point B, while Mary (moving at a different velocity) will be certain that the measurement at point B occurred first and could not have been caused by the A measurement. Of course both Joe and Mary are correct: there is no demonstrable cause and effect.

Entanglement is considered fundamental to quantum mechanics, even though it wasn't recognized in the beginning. 

Prokaryotic Caspase Homolog (talk) 19:14, 23 January 2019 (UTC)

More trimming

Bayesian Network representation of a classical CMI entanglement model of three random variables $\varrho _{A,B,\Lambda }$.

—instead, a quantum state must be described for the system as a whole

Prokaryotic Caspase Homolog (talk) 21:14, 23 January 2019 (UTC)

 (or which test was being performed)

Prokaryotic Caspase Homolog (talk) 21:18, 23 January 2019 (UTC)

Intro's paradox description seems broken

"However, this behavior gives rise to seemingly paradoxical effects:" ... which is the remarkable aspect of entanglement, so the completion of this thought is important...

"any measurement of a property of a particle performs an irreversible collapse on that particle" What? The particle collapses? I thought the idea was that the particle's wave function "collapses", which is to say that the collection of states that the wave function delineates, in which the particle is said to be simultaneously, reduces to a single state. That particle itself doesn't "collapse".

..."and will change the original quantum state"... What is the original quantum state? And how does this change it? If "wave function collapse" has some meaning, then surely the "original" state-of-affairs is that the particle is in multiple states simultaneously, from which post-observation state is selected?

And so far this description has focused on wave-function-plus-observation, not the entanglement paradox... still waiting...

"In the case of entangled particles, such a measurement will be on the entangled system as a whole." OK... that sounds reasonable... where's the paradox? Gwideman (talk) 22:34, 7 February 2019 (UTC)

Angles between axes

• "The hidden variables theory fails, however, when we consider measurements of the spin of entangled particles along different axes (for example, along any of three axes which make angles of 120 degrees)." (W. P. Uzer 20:48, 22 March 2014)
• "This is incorrect. The 3 axes are orthogonal, which means they make 90 degree angles (up-down, left-right, forwards-backwards)" (46.69.245.167, 17:56, 13 October 2019)‎

Really, the optimal choice of the axes depends on spins of the particles and the Bell inequality used. For photons (spin 1), angles 22.5° (times 1,2,3) are used often; for spin 1 particles — angles 45° (times 1,2,3), in a plane. Three orthogonal axes? probably never. Anyway, this detail is unneeded here. I delete it. Boris Tsirelson (talk) 19:30, 13 October 2019 (UTC)

Quantum entanglement of electrons (Cooper pair) with a small disturbance in the trajectory

Hi, until now I do not see any discussions or thoughts about this idea: Imagine you have a Cooper pair of 2 electrons. They are maximal entangled. What happens, if you change the way of flight of ONE of its electrons a little? Will the other entangled electron follow this small disturbance at once (in the opposite direction)? This may decide, if a Cooper pair of electrons is a SINGLE particle (just separeted from the rest of the world, with own spin, angular momentum, momentum, charge conservation) or still 2 electrons. Their behavior can help understanding Quantum Mechanics a lot Dietmar — Preceding unsigned comment added by 84.153.119.218 (talk) 14:48, 31 October 2019 (UTC)

References 8 and 9 (However, all interpretations agree that entanglement produces correlation between the measurements and that the mutual information between the entangled particles can be exploited, but that any transmission of information at faster-than-light speeds is impossible.) are purported to say the opposite of references 33-35. (A minority opinion holds that although quantum mechanics is correct, there is no superluminal instantaneous action-at-a-distance between entangled particles once the particles are separated.) I am missing something or is this a mistake. — Preceding unsigned comment added by 129.7.0.184 (talk) 20:59, 4 November 2019 (UTC)

Quantum cognition and quantum entanglement

About the controversial section "Human cognition" (copied hereto for now, see below) I wonder, if indeed "quantum cognition" "applies the mathematical formalism of quantum theory", then, how can it violate Tsirelson bound? If it can, then it is about a "superquantum entanglement", beyond "quantum entanglement", not describable by the mathematical formalism of quantum theory. Boris Tsirelson (talk) 13:11, 17 October 2019 (UTC)

Disputants, please discuss, do not editwar. Boris Tsirelson (talk) 13:52, 17 October 2019 (UTC)

Entanglement in human cognition has been studied in the domain of quantum cognition by means of cognitive experiments testing the CHSH inequality, one of the versions of Bell inequalities. Violations were recorded and even violations stronger than Tsirelson bound are possible in human cognition. .

• Quantum entanglement effect on consciousness as the fables & parareligious cultural touchstones of "revelation" vs "taboo" of such ideas as psychic driving; philosophical zombies; the violence inherent in the system; Scientology; Psychic Vampires; Mysticism; "Kill the head vampire" trope; Channeling; DMT elves; "logos as particles vs spirit waters waves chaos"; prayer; transcendental meditation; outer body experience; heaven & hell; &c. Text mdnp (talk) 22:32, 10 November 2019 (UTC)

need a detailed explanation on the density matrix paragraph

As a non-specialist, I'm looking at the example given in the reduced density matrix paragraph and I would appreciate very much if the details of the computation could be given to get from the entangled state to the reduced density matrix of A, using the definition in the same paragraph. I tried doing it myself but could not get the result.

EDIT : I had some discussions over quantum computing stack exchange on this subject, and I receive a nice answer about it. Maybe it could be used to make this subsection clearer ?

Update : I

 — Preceding unsigned comment added by Stilgarnat (talk • contribs) 15:56, 12 March 2020 (UTC) 

Need to distinguish entanglement from classical correlation

There are a few places where the text describes entanglement in ways that also apply to classical correlation. e.g. in the intro "any measurement of a property of a particle results in an irreversible wave function collapse of that particle and will change the original quantum state. In the case of entangled particles, such a measurement will affect the entangled system as a whole. " This text applies equally well to a classical probability distribution, e.g. if I have two envelopes both containing papers with the number 0 or with the number 1, then opening one envelope will tell me instantly what is in the other envelope even if it is far away. So things have to be written carefully to distinguish quantum entanglement from classical correlation. I will take a stab at this when I have time but does anyone else have ideas for references we can borrow from here? Aram.harrow (talk) 23:25, 11 June 2020 (UTC)

Quantum Woowoo

Right this intro ends with this uncited bit of woo: "The utilization of entanglement in communication, computation and quantum radar is a very active area of research and development." Note under "applications," the complete lack of known utility as a communication device? There is a good reason for that; it is known to be theoretically impossible! Quantum entanglement doesn't break the laws of physics, if you try to use it to create a paradox you simply fail. Entanglement doesn't survive your experiment. This can't be used to do useful things you otherwise couldn't do. The existence of the effect relies on not trying to do anything with it. Unfortunately, there is the additional false and unsourced claims in the intro to Applications which says, "With the aid of entanglement, otherwise impossible tasks may be achieved." Bzzzzt, wrong. Nothing otherwise impossible may be achieved.71.63.160.210 (talk) 17:44, 27 December 2019 (UTC)

"Nothing otherwise impossible may be achieved"?? Quantum teleportation is achieved. Quantum pseudo-telepathy is achieved. Any transmission of information at faster-than-light speeds cannot be achieved, indeed, which is stated there. Boris Tsirelson (talk) 20:29, 27 December 2019 (UTC)

In addition to Tsirelson's answer (great to see the legend commenting here, btw), entanglement allows information-theoretically secure private communication which is otherwise impossible. Still it's worth being clear that it can't send a message. Aram.harrow (talk) 10:32, 15 June 2020 (UTC)

Telepathy

If two people had entangled particles in the regions of their brain responsible for communication, couldn't this allow for telepathy? 139.138.6.121 (talk) 07:29, 27 July 2020 (UTC)

Photosynthesis

Second sentence: "Without such a process, the efficient conversion of light into chemical energy cannot be explained." This is a very strong statement that is not currently supported by reference or consensus. Please comment before I delete it. Charles Juvon (talk) 01:58, 2 August 2020 (UTC)

Sorry, given this reference that cites Fleming, I think the sentence can stay and it simply needs more referencing: https://phys.org/news/2010-05-untangling-quantum-entanglement-photosynthesis.html#:~:text=When%20two%20quantum%2Dsized%20particles,act%20as%20a%20single%20entity. Charles Juvon (talk) 13:17, 2 August 2020 (UTC)

I'd say this ...

If you can't explain it simple, you can't explain it. — Preceding unsigned comment added by Koitus~nlwiki (talk • contribs) 22:27, 29 January 2021 (UTC)

Unsolved problem?

Why is this article listed in the category Unsolved problems in physics? In what way is quantum entanglement considered to be an unsolved problem? —Kri (talk) 15:03, 19 February 2021 (UTC)

Because somebody added the page. You're right, it shouldn't be there, I removed it. Tercer (talk) 15:14, 19 February 2021 (UTC)

Synthesis, undue, etc.

The following discussion is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

---

This is not a good addition. In addition to the fact that one of the sources is just an arXiv preprint, the others are talking about different things, so combining them like this would be synthesis even if it were warranted to describe a self-declared "minority opinion" in a broad overview like this. For example, Khrennikov is talking about his "Växjö interpretation", which is not the same the ensemble interpretation espoused by, e.g., Leslie Ballentine. And whether they're taken separately or together, these just aren't significant enough to talk about in this article. XOR'easter (talk) 15:49, 25 April 2021 (UTC)

Furthermore, saying that "a minority opinion holds that although quantum mechanics is correct, there is no superluminal instantaneous action-at-a-distance between entangled particles once the particles are separated" is just bizarre, this is the majority opinion. It doesn't describe at all the contents of the papers being referenced, though. The sentence "no hidden variables and using the statistical ensemble interpretation" is an oxymoron, the statistical ensemble interpretation is a hidden-variable interpretation (or a hidden-hidden-variable interpretation, because you have hidden variables but are not allowed to talk about them). Tercer (talk) 18:32, 25 April 2021 (UTC)

I simplied the statement to avoid your concerns. Not all researchers believe there can be any communication once the particles are separated. No nonlocality. 47.201.194.211 (talk) 18:55, 25 April 2021 (UTC)

You removed the part about hidden variables, the rest is still there. The idea that there can be communication through entanglement is completely fringe, very few researchers would defend that. Tercer (talk) 19:13, 25 April 2021 (UTC)

I wrote actually "no superluminal instantaneous action-at-a-distance", which is the common central theme of the references cited. 47.201.194.211 (talk) 19:26, 25 April 2021 (UTC)

That is also a very unpopular position; a couple of researchers do believe in Bohmian mechanics or collapse models, but they are emphatically not mainstream. Tercer (talk) 19:33, 25 April 2021 (UTC)

I wrote "A minority opinion". 47.201.194.211 (talk) 19:34, 25 April 2021 (UTC)

But it is the majority opinion. Tercer (talk) 19:47, 25 April 2021 (UTC)

Wikipedia accepts minority opinions when well sourced. 47.201.194.211 (talk) 01:35, 26 April 2021 (UTC)

On a side note, as to where Bell went completely wrong in his work, Bell used "superposition" which is a false concept as Ballentine shows in his Chapter 9, and as Schrodinger also showed with his Cat. It is because Bell used fairytale "superposition" that he derived fairytale "entanglement" with superluminal instantaneous action-at-a-distance which is nonsense right on the face of it. 47.201.194.211 (talk) 02:01, 26 April 2021 (UTC).

That's enough for me, I give up. Tercer (talk) 12:43, 26 April 2021 (UTC)

If you give up, then put it back into the article and don't censor it. There are other Physics Fairytales like "entanglement" or the "big bang" fairytale not to mention "black hole" fairytales, for none of these fairytales does Wikipedia allow a word of criticism.47.201.194.211 (talk) 13:39, 26 April 2021 (UTC)

"I give up" means giving up trying to explain to you why are wrong, not surrendering and letting you rule the article. --Hob Gadling (talk) 13:49, 26 April 2021 (UTC)

Then explain how what I said about Bell is anywhere wrong. 47.201.194.211 (talk) 14:26, 26 April 2021 (UTC)

Entanglement is not a "false concept" or a "fairy tale", and Schrödinger's cat wasn't a demonstration that it was. Nor does Ballentine say that superposition is a "false concept", in chapter 9 of his textbook or anywhere else. What he rejects (pages 234 ff.) is the idea that a pure quantum state provides a complete and exhaustive description of an individual system. Instead, for him, A pure state describes the statistical properties of an ensemble of similarly prepared systems. A superposition of two pure states is another perfectly valid pure state describing (in Ballentine's view) the statistical properties of a different ensemble. No fairy tales. XOR'easter (talk) 16:10, 26 April 2021 (UTC)

To say as Bell that a pair of entangled particles exists in a superposition until observed, and that then there is an action-at-a-distance between them, is all fairytale, and Balletine doesn't buy it. That's where Bell went all wrong. And those references show that the ensemble interpretation explains observations with no superluminal action-at-a-distance at all, so Quantum computers will never work. Those references definitely belong here in Wikipedia. Please restore them.47.201.194.211 (talk) 19:21, 26 April 2021 (UTC)

Bell actually said that "observable" was so vague a word that it has no place in a formulation with any pretension to physical precision. You seem to be attributing to him a position very nearly the opposite of what he would have found reasonable. The references were to a smattering of different views, not a single cohesive interpretation. And the leap to "quantum computers will never work" is completely unwarranted. So, no, nothing is going to be restored. XOR'easter (talk) 19:57, 26 April 2021 (UTC)

You are wrong on all counts, the five references are not a "smattering of different views", you just cannot read them. They all say the same thing, that the observations can be explained by a statistical ensemble interpretation with no superluminal action-at-a-distance. The five references have been in Wikipedia for six months and still absolutely belong there and should not be pulled by a layman like you. Restore them.47.201.194.211 (talk) 01:18, 27 April 2021 (UTC)

XOR'easter is actually a theoretical physicist, unlike you, Randy in Clearwater. You really don't know when to quit do you? Hemiauchenia (talk) 17:56, 29 April 2021 (UTC)

Entanglement is a fairytale because there is no such thing as superluminal instantaneous action-at-a-distance. And Schrodinger came up with his Cat to show that Superposition is a fairytale, because a Cat is certainly not both alive and dead at the same time. Now, Quantum computer schemes rely on both fairytale Superposition and fairytale superluminal instantaneous action-at-a-distance, and so Quantum computers are fairytales that can never work. Those five references should be restored because they show how observations can be explained by the ensemble interpretation without any of the above fairytales, which is extremely important. Restore them. 47.201.194.211 (talk) 03:08, 29 April 2021 (UTC)

Quantum computers have already been built. they work. - MrOllie (talk) 03:21, 29 April 2021 (UTC)

Your article says "the promise of quantum computers". They are not computers and they don't work.47.201.194.211 (talk) 03:29, 29 April 2021 (UTC)

I would suggest reading more than the first line of the abstract. MrOllie (talk) 03:33, 29 April 2021 (UTC)

I would suggest you not believe people who have placed all their eggs in the basket of "quantum computers", and do your own thinking. Restore those five references. 47.201.194.211 (talk) 03:37, 29 April 2021 (UTC)

Only the 3rd reference (which is identical to the 4th) even begins to come close to the wikipedia definition of a reliable source as arXiv is generally not considered a reliable source. It is not yet clear if the 3rd source is sufficient to say that this is a significant-minority position, which might satisfy WP:PROFRINGE. There is also a lack of consensus as now multiple editors have pointed out, and this is quickly becoming tendetious. --FyzixFighter (talk) 04:35, 29 April 2021 (UTC)

It's tendentious for sure, but it's also quite entertaining. I mean, Ballentine — the world's most prominent advocate of the ensemble interpretation — has a whole section of his textbook on quantum computing. XOR'easter (talk) 18:10, 29 April 2021 (UTC)

I've escalated to ANI, see Wikipedia:Administrators'_noticeboard/Incidents#IP_disruptively_editing_at_Quantum_entanglement. Hemiauchenia (talk) 17:53, 29 April 2021 (UTC)

All 5 references should be restored. They are all published in excellent journals and they all say the same thing, that a statistical ensemble interpretation can explain the observed correlations with no superluminal instantaneous action-at-a distance. That is extremely important and a valid minority opinion. As for Ballentine, I have his 1998 edition and he did not have that section on quantum computing which he added to the 2015 Second Edition, but that does not matter because other researchers and specialists in the ensemble interpretation may see it differently from Ballentine. So Please restore those 5 references they are well published. 47.201.194.211 (talk) 02:41, 1 May 2021 (UTC) The Bryan Sanctuary references 28 and 29 are excellent, and should be restored. See him explain it here: https://www.youtube.com/channel/UCeV4pJeQ-tiVGG3-OT6iGvw 47.201.194.211 (talk) 03:27, 1 May 2021 (UTC)

No, they weren't all "published in excellent journals". One of them wasn't even published in a journal at all. This was explained at the very beginning of this section. You've already been blocked once; if you persist without learning anything, you'll only get blocked again. XOR'easter (talk) 13:50, 1 May 2021 (UTC)

The purpose of Arxiv is to get preliminary results out there safeguarded and accessible, it warrants inclusion. 47.201.194.211 (talk) 14:21, 1 May 2021 (UTC)

Preprints on the arXiv are not peer-reviewed. Consequently, they are not reliable sources for Wikipedia's purposes. XOR'easter (talk) 17:55, 1 May 2021 (UTC)

Arxiv papers are often included in Wikipedia as initial reports which are followed up by published articles.47.201.194.211 (talk) 19:36, 1 May 2021 (UTC)

Wikipedia policy regarding arXiv explicitly says: "...There is consensus that arXiv is a self-published source, and is generally unreliable with the exception of papers authored by established subject-matter experts. Verify whether a paper on arXiv is also published in a peer-reviewed academic journal; in these cases, cite the more reliable journal and provide an open access link to the paper (which may be hosted on arXiv)." So, no WP does not include initial reports from arXiv until they are followed up by published articles. --FyzixFighter (talk) 22:39, 1 May 2021 (UTC)

I agree, and the Arxiv reference I cite is indeed followed up by a published article, so include it. 47.201.194.211 (talk) 13:46, 2 May 2021 (UTC)

"Followed up by" isn't the same as "is". You'll win no friends by making demands. XOR'easter (talk) 15:42, 2 May 2021 (UTC)

The reason you want to censor the following sentence that I wrote into wikipedia is that it is an existential threat to everything you have been pushing for years, but how can science progress with minority opinions censored ?

A minority opinion holds that although quantum mechanics is correct, there is no superluminal instantaneous action-at-a-distance between entangled particles once the particles are separated.[28][29][30][31][32]. — Preceding unsigned comment added by 47.201.194.211 (talk • contribs)

Those five references must be restored. They show that serious researchers believe that the statistical ensemble interpretation does explain observed correlations of so-called entangled particles with no superluminal instantaneous action-at-a-distance. This minority opinion is extremely important and definitely should be restored. 47.201.194.211 (talk) 14:22, 4 May 2021 (UTC)

As Alice asked after she went through the looking-glass, "But how can you talk with a person if they always say the same thing?" XOR'easter (talk) 17:38, 4 May 2021 (UTC)

How do you talk with someone who wants to censor opposing opinions? 47.201.194.211 (talk) 19:32, 4 May 2021 (UTC)

Professor Bryan Sanctuary explains his articles in his seminar video series how a statistical ensemble treatment explains the observed correlations with no superluminal action-at-a-distance. The five references should definitely be restored. See https://quantummechanics.mchmultimedia.com/toc/ 47.201.194.211 (talk) 01:10, 5 May 2021 (UTC)

Additional unreliable sources do not bolster the reliability of an arXiv preprint, and accusations of censorship are not an argument. XOR'easter (talk) 17:59, 5 May 2021 (UTC)

The Arxiv is just a prelude to his main article which is published in 2006 in the International Journal of Modern Physics B, a perfectly good source. Please restore the five references. 47.201.194.211 (talk) 01:14, 6 May 2021 (UTC)

As was already explained, being followed by a journal article doesn't make a preprint reliable. Since you seem to be emphasizing the writings of Sanctuary in particular, I'm curious: do you know him personally? If so, Wikipedia's policy on Conflicts of Interest would be relevant. XOR'easter (talk) 15:04, 6 May 2021 (UTC)

I have never met any of the authors of those five references, but I do believe in what they are saying, that a statistical ensemble interpretation can explain correlations with no superluminal instantaneous action-at-a-distance, because instantaneous action-at-a-distance is pure Voodoo, and most physicists would agree with me in private. That's why those five references deserve at least a one line mention in Wikipedia the way I had added it. Please restore it. 47.201.194.211 (talk) 15:42, 6 May 2021 (UTC)

As Tercer pointed out at the beginning of this discussion, the idea that there could be superluminal communication through entanglement is the fringe position. The text you are insisting upon is hopelessly confused about what the majority and minority opinions are. XOR'easter (talk) 17:00, 6 May 2021 (UTC)

You are not understanding what the five references are saying, they are all saying there is no superluminal communication nor superluminal instantaneous action-at-a-distance, of any sort, between so-called entangled particles once separated. Restore those five references. 47.201.194.211 (talk) 19:09, 6 May 2021 (UTC)

Your 'five references' are restating the majority opinion in a slightly different way. They're not in the minority, so it is not appropriate to put a statement in the Wikipedia article claiming that they're in disagreement with the mainstream. - MrOllie (talk) 19:20, 6 May 2021 (UTC)

Are you kidding ? they are not in agreement with the usual instantaneous action-at-a-distance at all ! 47.201.194.211 (talk) 19:27, 6 May 2021 (UTC)

Just about everyone agrees with the No-communication theorem. - MrOllie (talk) 19:33, 6 May 2021 (UTC)

Of course, and no one is contesting that, so what's your point? 47.201.194.211 (talk) 01:13, 7 May 2021 (UTC)

Yep. And, on top of that, this article needs less reliance on primary sources, not more. XOR'easter (talk) 19:37, 6 May 2021 (UTC)

Well sourced primary sources are perfectly acceptable. You guys stop making lame excuses and restore those five references. 47.201.194.211 (talk) 01:04, 7 May 2021 (UTC)

The policy that I linked explains quite clearly why that is untrue. XOR'easter (talk) 16:25, 7 May 2021 (UTC)

Quite to the Contrary! You must be one of those 'entanglement' freaks and I hate to tell you that your baby is ugly, there is no such thing as superluminal instantaneous action-at-a-distance, it is all Voodoo, and a fairytale. Restore those five references because they set the record straight. 47.201.194.211 (talk) 17:06, 7 May 2021 (UTC)

Hi 47.201.194.211. The change you request has been refused. Wikipedia operates by Wikipedia:Consensus among editors and you do not have consensus here. Do not take this personally. We are very careful about physics articles. High ranking and much awarded professors have found that out when their more speculative work has been removed for lack of independent analysis in the physics literature. StarryGrandma (talk) 17:53, 7 May 2021 (UTC)

superluminal instantaneous action-at-a-distance, it is all Voodoo, and a fairytale. Well, the vast majority of physicists happen to disagree with you. Wikipedia is a mainstream encyclopedia, and not a soapbox for fringe cranks like yourself. Hemiauchenia (talk) 03:11, 9 May 2021 (UTC)

All physicists agree with me in private, even you. 47.201.194.211 (talk) 19:29, 13 May 2021 (UTC)

The discussion above is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

Einstein's opinion

"Einstein and others considered such behavior impossible". There is a significant school of thought which believes that this was not the case so far as Einstein was concerned, see for example Saturday, May 08 here. Gog the Mild (talk) 19:39, 27 May 2021 (UTC)

Photons and kinetic energy

Interferometrist, kinetic energy is the energy of matter in motion. Photons don't have mass and aren't matter. Their energy is proportional to their frequency, not their velocity since it is the same for all of them. The interaction of photons with matter that produces radiation pressure is due to the transfer of momentum to an object. Note that Wikipedia's radiation pressure article talks only about momentum. People are confused by this because photons interacting with matter can seem like balls bouncing off a surface. There are discussions on Quora where many people talk about the kinetic energy of photons and others try to explain why photons do not have kinetic energy. It is certainly possible to say the energy of a photon is totally kinetic energy, but that is not consistent with the definition of kinetic energy in physics. In an article such as this which is about a different topic, it is better to just leave it as photon energy. StarryGrandma (talk) 18:04, 29 October 2021 (UTC)

Traditionally energy was described as either kinetic or potential. Yes, there are a lot of reasons that this distinction isn't totally justified. You measure the mass of something and then say it has a "potential energy" of 1kg * c^2. Then you look inside the black box and see that part of that mass is due to motion of matter within the box, so some of it would have to be classified as kinetic energy. So yes, those distinctions are not clear and absolute. But clearly the 18th century definition of kinetic energy you refer to is much too deficient, now that we know that many other things (in fact EVERYTHING that can be measured!) also has mass, including the "photon," just that it doesn't have rest mass. And saying that photons' "energy is proportional to their frequency, not their velocity" (velocity squared, you mean) isn't exactly right either, since once you get something sped up almost to c, doubling its kinetic energy hardly changes its velocity, likewise for EM waves. Rather for matter you multiply its momentum by v/2 to get energy, and for photons multiply by v=c, very similar but no 1/2.

In any case, I do not think there's any way you could call electromagnetic energy of a plane wave "potential energy" so I think it is clearly kinetic energy, unless you know of a third classification of energy (which I'd love to hear about!). If you must talk about particles, then I guess you would then have to say that that kinetic energy is contained in "photons" which you admit have a momentum which you multiply by c to get kinetic energy. And BTW, saying that "photons interacting with matter can seem like balls bouncing off a surface" is a not-untrue picture in the case of photons that are reflected/scattered without loss (without adding to the internal energy of the matter it was reflected from), not that it's a good picture (there is no good picture of the photon) but because all of the conservation laws governing elastic collisions are followed exactly when EM radiation is reflected. And also, although it is often written this way (and sometimes I catch myself using such terminology!) a photon does not have a "frequency" or "wavelength". Those are wave concepts that have no meaning in relation to particles; the best you can properly say is that a photon is the minimal unit of energy corresponding to an EM wave described in terms of frequency and k-vector.Interferometrist (talk) 21:44, 29 October 2021 (UTC)

You may be thinking too classically. This is an article about quantum things after all, not the classical limit.

It may often be a useful simplification to think in terms of potential and kinetic energy in a system, but there are many kinds of energy in physics. See Feynman's lecture on conservation of energy here where near the end of section 4.1 (What is energy?) he says: Second, the energy has a large number of different forms, and there is a formula for each one. These are: gravitational energy, kinetic energy, heat energy, elastic energy, electrical energy, chemical energy, radiant energy, nuclear energy, mass energy. See our articles on Radiant energy and Radiation pressure#Radiation pressure in terms of photons.

Light has both wave and particle characteristics, but it is neither one. Photons are quantum-mechanical things and a photon has a well-defined frequency. Quantum mechanics was born with the discovery that light comes in discrete units, later called photons, such that E = hν where E is the energy, h is Plank's constant and ν is the frequency. Plank got the Nobel Prize for this in 1918. Photons have many properties, as described in Photon#Physical properties, but the section "Relativistic energy and momentum" makes no mention of kinetic energy. StarryGrandma (talk) 22:51, 29 October 2021 (UTC)

If we use the relativistic relation of kinetic energy and momentum, $E_{\text{k}}={\sqrt {p^{2}c^{2}+m^{2}c^{4}}}-mc^{2}$, and set the rest mass equal to zero, then it can make sense imo to say that photon's have kinetic energy. However, wikipedia articles should not be based exclusively on editors' understanding of the subject - what do reliable sources (which does not include discussions on Quora) say? Is it common for reliable sources to talk about a photon's kinetic energy? Or, in this instance, do the sources for this statement regarding quantum entanglement and photosynthesis talk about a photon's kinetic energy or just its energy? --FyzixFighter (talk) 01:00, 30 October 2021 (UTC)

It's probably more common just to say "energy", because all the energy is kinetic. A photon's kinetic energy is its total energy [6]. The phrase "photon kinetic energy" does show up (e.g., [7][8][9]); it's just a wordier way of saying the same thing. Life is too short to read Quora. XOR'easter (talk) 05:57, 30 October 2021 (UTC)

I agree with User:XOR'easter: it's easier (and much more common) to refer to "the energy of a photon" rather than to specify "kinetic energy" as if there were many different contributions to the photon's energy. --Qcomp (talk) 15:08, 30 October 2021 (UTC)

I'm not going to make a big deal out of what amounts to a linguistic issue. In my mind, saying that kinetic energy is transferred connotes the immediate transfer of energy, as in an elastic collision or an electron raised to a higher energy level the instant a photon is received (or visa-versa). As opposed to the transfer of energy when you transfer gasoline to a gas tank, or wind a watch. But never mind.

And calling all energy either potential or kinetic (already a flawed distinction as I conceded) is "classical" not in the sense of classical physics but as we had been taught in previous years, but if Feynman didn't accept that distinction 50 years ago then I guess it isn't "classical" any longer but "antiquated" and I'll avoid thinking in those terms. However I do think FyzixFighter was thinking in the same terms as me when he calculates the kinetic energy as the difference between a particle or system's total energy and its rest energy (which in the case of light means the limit of doppler reddening when you're moving away from it, trying to reach its rest frame) which is zero.

But on a different subject, I contest the statement of @StarryGrandma: that "a photon has a well-defined frequency". At most you could mean that the energy of light, even a single photon, can be measured with a precision of up to h/T using an apparatus that takes at least T long to determine it. We have a laser downstairs that emits pulses that are 10 or 20fs long. Do the photons coming from that laser have a well defined "frequency"? Before you passed them through a spectrometer? And what does it even mean (beyond being the f in E=hf)? Or wavelength? No, those only have to do with the wave that it is part of, and in this case that wave has a frequency spread of many THz (and these photons are all part of the same wave, thus indistinguishable, at least before you see where each happens to land in a spectrometer). Where is it written (among well-respected theoretical physicists) that ν and λ have any meaning in relation to a particle without referring to its wavefunction (EM field for a photon)? I'd be interested in seeing that. As far as I'm concerned, saying (as I would avoid!) that a photon has a frequency and wavelength is simply a statement about the wave that it is part of. But please, prove me wrong! Interferometrist (talk) 21:49, 31 October 2021 (UTC)

Spooky Action at a Distance

It is questionable whether Einstein was talking specifically about entanglement when he used this expression. Here is the relevant passage from his letter to Max Born (3 December 1947):

Meine physikalische Haltung kann ich Dir nicht so begründen, dass Du sie irgendwie vernünftig finden würdest. Ich sehe natürlich ein, dass die principiell statistische Behandlungsweise, deren Notwendigkeit im Rahmen des bestehenden Formalismus ja zuerst von Dir klar erkannt wurde, einen bedeutenden Wahrheitsgehalt hat. Ich kann aber deshalb nicht ernsthaft daran glauben, weil die Theorie mit dem Grundsatz unvereinbar ist, dass die Physik eine Wirklichkeit in Zeit und Raum darstellen soll, ohne spukhafte Fernwirkungen.... Davon bin ich fest überzeugt, dass man schliesslich bei einer Theorie landen wird, deren gesetzmässig verbundene Dinge nicht Wahrscheinlichkeiten, sondern gedachte Tatbestände sind, wie man es bis vor kurzem als selbstverständlich betrachtet hat. Zur Begründung dieser Überzeugung kann ich aber nicht logische Gründe, sondern nur meinen kleinen Finger als Zeugen beibringen, also keine Autorität, die ausserhalb meiner Haut irgendwelchen Respekt einflössen kann.

I cannot substantiate my attitude to physics in such a manner that you would find it in any way rational. I see of course that the statistical interpretation (the necessity of which in the frame of the existing formalism has been first clearly recognized by yourself) has a considerable content of truth. Yet I cannot seriously believe it because the theory is inconsistent with the principle that physics has to represent a reality in space and time without spooky action at a distance [phantom actions over distances] ... I am absolutely convinced that one will eventually arrive at a theory in which the objects connected by laws are not probabilities, but conceived facts, as one took for granted only a short time ago. However, I cannot provide logical arguments for my conviction, but can only call on my little finger as a witness, which cannot claim any authority to be respected outside my own skin.

Max Born - Eroica (talk) 10:43, 30 December 2021 (UTC)

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This article confuses entanglement with interference

"Quantum entanglement has been demonstrated experimentally with photons, neutrinos, electrons, molecules as large as buckyballs, and even small diamonds. The utilization of entanglement in communication, computation and quantum radar is a very active area of research and development."

The references cited for electrons and buckyballs do not experimentally demonstrate entanglement, only interference. As such this is overstating the demonstrated entanglement for massive particles. The vast majority of experiments, and the ones that are most informative, have all used photons. Unless there are citations for experiments that demonstrate ENTANGLEMENT of massive particles (electrons, or molecules/buckyballs/diamonds) I suggest that the first sentence be edited or deleted. Ralivingston1952 (talk) 20:11, 9 February 2023 (UTC)

The reference about electrons indeed demonstrated entanglement, but you are right that with buckyballs only superposition was demonstrated. Also, the experiment with neutrino oscillations also only demonstrated superpositions. I removed them. Tercer (talk) 10:29, 10 February 2023 (UTC)

I believe that the electron and diamond experiments also do not demonstrate entanglement of massive objects since the connection between them is via photons, not massive particles. However, since the articles claim entanglement and it is only my opinion that they do not, it is appropriate that my opinion not go into the Wikipedia article. Thank you for your prompt response. Ralivingston1952 (talk) 14:33, 10 February 2023 (UTC)

I think that massive objects are made of particles (known nowadays as fields). Kartasto (talk) 14:40, 10 February 2023 (UTC)

I have a problem with the first sentence of the article.

The article begins with these words: "Quantum entanglement is the physical phenomenon ... " My problem is with the word "physical." In one way of looking at it, physical properties are involved. But there appears to be a metaphysical aspect to quantum entanglement that cannot be explained by ordinary principles of cause and effect. Cause and effect happens when things bump into each other, figuratively speaking. But quantum entanglement occurs when particles are separated by such extremes than they could not have a direct effect on each other. 2600:8801:BE24:1A00:78CA:B2D3:C552:115A (talk) 19:48, 14 October 2022 (UTC)

It can be observed in an experiment, so it is physical. Metaphysics is the study of the unobservable. Cause and effect have nothing to do with the definitions of these terms - and I think a lot of people would disagree with the definition you gave here anyway. MrOllie (talk) 20:00, 14 October 2022 (UTC)

I agree with the objection. Entanglement is then defined in terms of how states are described, and not directly as something physical. There is no experiment to determine whether a particular particle is entangled. 04:00, 9 November 2022 (UTC)

I also have a problem with the first sentence, though for a different reason. I don't claim the sentence is false but that is it unclear (at least to me) due to the words "share spacial proximity" followed by the words "separated by a large distance." Perhaps it could be rewritten without that first phrase? In other words: "Quantum entanglement is the phenomenon that occurs when particles of a group are generated or interact in a way such that the quantum state of each particle of the group cannot be described independently of the state of the others, including when the particles are separated by a large distance." This version seems much more straightforward to me.Renejs (talk) 22:29, 12 March 2023 (UTC)