Talk:No-communication theorem

Sorry, but...
I know, I Know, this is not a forum. But I honestly think this theorem is very, very broken. He shoves dozens of mathematical obscure formulas to try to furiously deny that it can happen something that is logically simple to analyze: We have two points, each with a photon "tied" to another by quantum entanglement. We know that if the spin of the photon "A" change, this change is reflected immediately in the photon "B". Knowing this, the observer "B" does _not_ need dozens of mathematical expressions to know that if the photon "B" spin change, was because the observer "A" changed the spin of the photon "A". How can this be so difficult? More valid questions would be: a) The change in spin is really instantaneous? This can be tested, although it is difficult; b) The change in spin can be caused by external interference? If yes, so which ones? c) The act of trying to measure the spin breaks, or can break, the entanglement? Less "Oh No!! It will break my preferred theory! heretic!", more "Let's see what happens" science please. 200.189.118.162 (talk) 17:11, 18 March 2013 (UTC) Changing the spin of system A does change the spin of system B, of course. The point of the theorem is that a scientist at lab A cannot control whether the system decoheres into spin up or spin down (in the entangled photon example- whatever analogue is applicable for other systems) along the axis of measurement. The result is that, regardless of anything the scientist in lab A does, system B has (not knowing the result of the measurement of system A) a 50/50 chance of observing spin up or spin down. A scientist at lab A can, after measuring her system, determine what system B will be measured as, but can in no way affect that measurement. Somephysicist (talk) 01:15, 16 February 2014 (UTC)

@Somephysicist that does not seem to be a general statement. That is a fact for that particular example but it doesn't seem translate to other experiments where there are more than two possibilities. Additionally more and more quantum eraser experiments (Kim et.al, Dopfer) are indicating our accepted understanding of the wave function is wrong.

Take the case of entangled photons in a momentum state such as in the Kim et.al. QE experiment, one photon that is "path-observed" changes the interference pattern of its partner. INSTANTLY. If this is true then the only thing that is missing to form an FTL communication device is being able to produce TONS of entangled photons at a time and instantly imaging the interference pattern. That should be a function of technology.

So what I'm saying is that one could in essence send an intense pulse of entangled photons where the partners after the non-linear (e.g. BBO) crystal are sent via fiber to e.g. NY and LA from Austin, TX. Although not shown to be necessary let's say that path lengths to each detector in NY and LA from the Austin source are exact. Then LA could observe "which path" information and affect INSTANTANEOUSLY the interference pattern of the clump of photons at NY. NY would not need a classical channel enabled coincidence counter to extract the interference pattern over time. They could just see it. Instantly. A little like shown in this video https://www.youtube.com/watch?v=wGkx1MUw2TU.

1's and 0's are transmitted by the change of the pattern. The pulse repetition rate would determine the bandwidth. But like I said this requires a source that produces a clump of momentum state entangled photons. This does NOT require a bunch of cloned photons. This probably is not the only way either. Even in the video above to really see the pattern they have to take several averaged frames but that is a technicality not a limit of physics.

It almost seems like the wavefunction is an object that propagates and evolves instantly, or superluminaly at least, while the transfer of energy (not information) or matter must happen at or below c. Ldussan (talk) 22:44, 2 April 2014 (UTC)

Untitled
Added conflict of interest label, clear dispute by the author(s)on the subject, weasel words and general disagreement. Certainly not an expert in the subject; suggest weaving opposing viewpoints into single article that presents both ideas before removing label. —Preceding unsigned comment added by 130.215.232.181 (talk) 03:24, 5 December 2009 (UTC)

It's going to be hard to find anyone competent to write on such a specialised topic who is not close to it. And because it is technical, what can usefully be said without the mathematics? Mike -- Netteville (talk) 11:08, 30 December 2009 (UTC)

Not sure if it's relevant but there was a semi-recent experiment that pretty much destroys this theorem. http://news.softpedia.com/news/Entangled-Light-Stored-in-Cooled-Atoms-80659.shtml --Mike reddog418

This proof only proves that it can't be done instantly since it assumes no time passes. This doesn't really seem to say anything about whether we can communicate faster than the speed of light, just that we can't do it in zero time flat. I keep hearing that the no communication theorem disproves superluminal communication completely- does this mean that there is a more complex proof somewhere that handles extremely brief time evolution? Particularly, the time frame that would be needed to surpass the speed of light? Even if it's too elaborate for the article, it should be described if the result exists. AllUltima (talk) 19:13, 27 March 2010 (UTC)


 * You have to realize that the distinction between "instantly" and "faster than the speed of light" disappears in relativity. "Simultaneity" loses its meaning. That is the significance of the boost transformations of the Lorentz group acting on Minkowski space. 178.38.115.176 (talk) 09:18, 5 May 2015 (UTC)

Not true. The no-communication and superluminal theories imply that the speed of light is a cosmological constant, and that with absolute certainty, cannot be violated. However, this assumes that particles have well-defined individual properties [hidden variables]; they do not. This was also the assumed hypothesis of the EPR experiment performed by John Bell-- if there was no communication going on that violates the speed of light, then John Bell's experiment would have resulted in locality, solidifying the cosmological constant of light, and proving that Einstein's objections of assumed realism were correct all along. It did not; information transmission between particles is instantaneous. Both this and the superluminal article approach quantum physics from the world-view of EPR, and not quantum physics in itself. Here's an article by Alain Aspect from a 1999 issue of Nature magazine that explains everything and cites further repeated experiments. — Preceding unsigned comment added by 209.105.184.93 (talk)


 * According to Special Relativity, for any two events which are connected by a super-luminal signal (which therefore are the endpoints of some space-like vector) there is a frame in which an observer would see the events as exactly simultaneous. In other words, for any space-like vector, there is a Lorentz transformation which makes the extremes to have the same time coordinate t (and vice-versa). Therefore, proving the theorem (in a Special Relativistic setup) for all simultaneous events means having proved it for any super-luminal speed (unless I'm missing something which goes beyond Special Relativity) 89.202.228.211 (talk) 14:06, 26 July 2010 (UTC)

Regarding erasers experiments. Do I understand correctly that the only reason they do not show FTL communication is because of the filtering? Is the filtering really needed? What if the source of the entangled particles already provides a well defined state - say, with a given polarization? If that is the situation, it would be better to return to the version were no final conclusion is made. Srjmas (talk) 10:28, 14 October 2010 (UTC)


 * The coincidence counters are what prevents FTL communication. Those counters are needed because a quantum eraser will actually produce two sets of interference patterns phase shifted from each other. So if you superimpose those interference patterns, you get essentially the same result as you would without an interference pattern. 184.100.111.110 (talk) 17:59, 15 November 2010 (UTC)

--

But this is what communication with Bell-pair is about: That Alice’s measuring apparatus interact spontaneous with Bob’s receiver system. The theorem only holds for the situation, where no successful connection is created between Alice’s system and Bob’s system. UChr (talk) 20:36, 6 January 2011 (UTC)

I think it is a no – proof – theorem. The proof is circular. It postulates: no connection / independency between Alice and Bob and uses this for later to show: no connection / independency between Alice and Bob. UChr (talk) 11:11, 15 January 2011 (UTC)

Opposing Viewpoint
The following is misleading in suggesting super luminal communication because it does not state all the conditions of the experiment.

"it is possible to cause or prohibit an ensemble of photons into making an interference pattern on a screen, by remotely manipulating their entangled twins"

There is no interference pattern without also the use of "Coincidence Logic" as shown in fig 3 of of the Zeilinger reference (Dopfer 1998 experiment). This coincidence logic requires a sub luminal communication from the non slit half before the interference pattern can be shown. Therefore there is no suggestion of super luminal communication and this section of the Opposing Viewpoint should be rewritten or removed. —Preceding unsigned comment added by 83.59.122.251 (talk) 00:23, 16 February 2011 (UTC)

I deleted a paragraph that claimed "As the no-communication theorem is a mathematical derivation on the Hilbert space of a single particle, its implications are not as clear for an ensemble of particles; where one is not attempting to transmit a single bit through a single particle, but instead a single bit through many particles (partial information through each particle)." This is wrong: there is no reference to the single- or multi-particle nature of the states involved in this proof. Of course, the cited Rev. Mod. Physics paper by Zeilinger says nothing of the sort claimed in this paragraph. --S. Hoyer (talk) 01:25, 16 May 2012 (UTC)

I removed the rest of this section after reading the cited references. Neither claims an opposing viewpoint to no-communication, but rather they present possible limitations of the proof. One, which I moved to the notes section, notes that the proof does not hold for non-local evolution. The other, "On the empirical foundations of the quantum no-signalling proofs," suggests that the no-signaling theorem relies on some additional formal assumptions (e.g., that states can be written as a sum of tensor product terms) not usually included in the axioms of quantum mechanics. These assumptions, however, are well established science, so I think this is a rather pedantic and meaningless dispute. (Note: I removed a long proof in this section of the Talk page alleging to show a counter-example to no-communication because this is not the place for original research.) --S. Hoyer (talk) 01:53, 16 May 2012 (UTC)

@shoyer you shouldn't have removed it because it is actually correct. That person is intelligently expressing what all of us QE physicists are wondering out loud so please put that statement back.Ldussan (talk) 22:51, 2 April 2014 (UTC)

What status should we attribute to the no-communication theorem ?

I don't understand why the no-communication theorem is considered as a theoretically exact proof of the impossibility of using quantum measurement statistics on systems A entangled with systems B to transmit classical information from Alice to Bob in a time independent of the distance between Alice and Bob. In my opinion, this theorem has only a "for all practical purpose" validity.

This remark is the same (it's not coincidental) than what can be told about the monotonic increase of Boltzmann entropy of isolated systems predicted by the second principle of statistical physics and derived, for perfect gazes, by Boltzmann's H-theorem (under the hidden assumption, contained in the chaos' dynamics hypothesis, that information about correlations between particles motion (induced by successive shocks between them) are irremediably irretrievable by a macroscopic observer).

Indeed, the proof of the no-communication theorem relies on the hypothesis that all information that can be gathered by quantum measurements on systems B is completely and perfectly embodied in the reduced density operator of systems B. This assumption implicitly rests on the hypothesis that irreversibility, more specifically the quantum measurement irreversibility (the erasure of the off-diagonal coefficients of the reduced density matrix of systems B thanks to the decoherence process) is objective and definitive. This is untrue in my opinion.

Indeed, the loss of information (Von Neumann entropy creation) that stems from the irreversible evolution of the observed system + measurement apparatus + environment is not a theoretical, but a practical one. From a theoretical point of view, decoherence is a reversible phenomenon. Macroscopic evolution irreversibility (quantum measurement irreversibility is not an exception) is only a statistical emergence (stemming from the lack of information of the macroscopic observer about the microscopic state of the observed systems) and not a fundamental law of nature.

This problem is not specifically a quantum mechanical one. The spin echo experiments exemplify in a startling manner the relative status of the notion of irreversible evolution. Consequently, a mixed state can evolve back toward a pure one in certain specific conditions. Information loss (thanks to the growth of EPR correlations of measurement apparatus degrees of freedom with environment's ones) is never absolute. The reduced density operator doesn't contain enough information to completely determine the mixed state future evolution.

Consequently, the no-communication theorem is, in my opinion, a for all practical purpose (presently correct) prediction. It rests on a confidence in the aptness of the reduced density operator to perfectly model the limitation of retrievable information (at the macroscopic scale) without any statistical imperfections (and without unknown hidden information retrievable by some unsuspected manipulation as is the case for spin echo experiments). This confidence is plagued with the same well known difficulties and discussions (summarized in the expression "the measurement problem") about the absence of objective criterion deciding when a quantum measurement is actually irremediably achieved (in fact never).

These considerations suggest, in my opinion, the physical hypothesis that Lorentz symmetry, simultaneity of relativity, relativistic locality and relativistic causality may be interpreted as a statistical emergence hiding a quantum preferred frame, a quantum preferred simultaneity and instantaneous hidden (instead of spooky) actions at a distance. — Preceding unsigned comment added by 86.219.40.214 (talk) 21:13, 9 March 2013 (UTC)
 * I found your comment rather intriguing. I haven't understood the full story of decoherence, irreversibility, and so forth. Perhaps your comment will bring me further in this.


 * You have called into question the convention that a measurement instantly modifies the quantum state by deleting off-diagonal terms — informally called "collapse". I agree that this is a kind of practical abbreviation, or idealization, of some deeper and truer story, and it may well be that this story is decoherence or many-worlds, suitably interpreted.


 * Yet all the results such as "no-communication", "no teleportation", and so forth, belong to the field of quantum computing. If you examine the premises of this field, you will find that they are very formal, even categorical in nature. There is a fixed vocabulary of "quantum operations" that are permitted.


 * "Collapse" is one of these operations. It is the only designated "output" mechanism (to classical bits), and like bidding in bridge or betting in poker, it has a double role as an "input" mechanism that modifies the system. "Collapse" is built in to the theory of quantum computing — it is everywhere in the theory. It's a kind of axiom. If a system admits the idealization called "collapse", then you can study it using the formalism of quantum computing. Otherwise, you should use a more fundamental theory.


 * A good analogy is "entropy". In classical thermodynamics, it is treated axiomatically. In statistical physics, the axiom is established as emerging from more fundamental laws. In the study of "thermal fluctuations", systems are studied where deviations from the axiom become important. And indeed, such systems exist — 10^23 does not trump every argument.


 * So I think your arguments are interesting (though I don't fully understand them), but they are at a more fundamental level than the field of quantum computing. 178.38.115.176 (talk) 09:59, 5 May 2015 (UTC)

New Attack Plan
This so called theorem has many problems on this wiki because there is more than one version of the quantum eraser experiment, at least three. Each of them cannot be analyzed for FTL communication (or lack thereof) in the exact same way.

I suggest we analyze each QE experiment first before we give reasons as to why there is or there is not a no communication theorem. I suggest we just state the no comm theorem in this page and give no reasoning behind it. We could even demote it to the no comm hypothesis.

Let's discuss the viability of the no comm hypothesis as we analyze all three versions of the quantum eraser experiments. Then once a consesus is reached we can come back and give reasoning for why the hypothesis is or is not valid. --97.104.194.126 (talk) 03:29, 5 July 2011 (UTC) ldussan

I have now made a critical comment UChr (talk) 17:54, 21 September 2011 (UTC)

Need for improvement
We should go back to what the intended purpose of Wikipedia is. It is not a forum to discuss the validity of some proposed theory or to introduce new ideas that have not been widely introduced before, specially through published articles or books. There is a note of this on the top of this talk page referring to this page itself, but it also applies (even more) to the main Article page. If there is such a No-Communication Theorem, who has stated or proven it? What does no-communication mean? If someone has published such a theorem, why is it that there is no mention of the physicist's name? Here "no-communication" is made to mean no-faster-than-light communication. If this where the prevalent meaning attached to "no-communication", we would have to accept this, even if it were not reasonable. But it would seem more reasonable to define no-communication in quantum mechanics as the impossibility of communication between two spacially separated (not necessarily space-like) points through the use of quantum entangled systems by measurement or modification of each of the systems but excluding any other classical communication channel. But of course, we could only define it this way if had been presented so before in published articles. If there is a published article (or several) that present a "No-communication theorem", then we would have to use the definition/s used by the author/s and mention the author/s. With respect to this topic, before reading it in Wikipedia I came across an article by Phillippe H. Eberhard and Ronald R. Ross titled "Quantum Field Theory Cannot Provide Faster-Than-Light Communication" The kind of proof that Eberhard and Ross present fits the meaning of the theorem described in the main article here, even though they dont call it just "no-communication". With respect to the various versions of the quantum eraser experiment, I think we would have to see if the authors, or anybody else who has analyzed these experiments has presented a "no-communication" theorem in connection with them. If what we are talking here does not refer to a particular theorem but to a family of theorems which is in the literature referred to as "no-communication" then we state it that way. For the moment, I would suggest that we start contributing in this talk page with some information about the different mentions of this in the literature. I also suggest that you take a look at Eberhard's article (which I think could be considered a theorem). Talking about theorems, There is a note on the top of this page that says that this article fits within Wiki Mathematics. I doubt it. I would classify it as only belonging to Physics. Alexepascual (talk) 17:36, 30 August 2012 (UTC)

Misc edits
I removed the following from the article:

Re operators S,T acting on hlibert spaces H_A, H_B:
 * which however need not be states on the subsystems (that is non-negative of trace 1)

because it's garbled. I changed it to the following text:
 * It is not required to assume that Ti and Si  are state projection operators: i.e. they need not necessarily be non-negative, nor have a trace of one.

However, this is a non-standard defintion of a density operator, so perhaps sigma should not be called that?? User:Linas (talk) 19:51, 17 November 2013 (UTC)

Issues in "Opposing Viewpoint"
The "Opposing Viewpoint" section of this article has multiple issues:


 * 1) The section sounds highly confrontational. The theorem, as stated in the article, is mathematically correct; therefore, the statement that "the proof is wrong" is not factually accurate.
 * 2) The viewpoint being expressed seems to be that certain assumptions of the proof, or their relation to terminology expressed in the informal statement of the proof, may not hold in certain experimental contexts. In this case, the author should elaborate on which specific assumptions are in question, the particular cases in which they do not hold, and the terminology they relate to.
 * 3) The example given has major stylistic issues and should be rewritten.

For the time being, I am tagging the section as misleading, for lack of a better label.

Ubershaman (talk) 16:03, 30 January 2014 (UTC)

The proof is based on some assumptions - among other: '… means that Alice's measurement apparatus does not interact with Bob's subsystem’. The little analysis of a part of a classic two channel experiments show that this assumption is wrong - and the proof is therefore wrong. UChr (talk) 16:31, 31 January 2014 (UTC)

Example: If one setting of Alice's polarizing beam splitter makes a distribution of Bob's polarizing beam splitter as (10% -90%) for the part of the beam that is being transmitted through Alice's splitter, then that part of the beam, who are reflected will create a distribution of (90% - 10%). Then Bob receives a random mix of (10% - 90%) and (90% -10 %). Another setting at Alice could give Bob a mix of (60% - 40%) and (40% - 60%). These are clear differences in Bob's system caused by changes at Alice’ place - and measurements of correlations between Alice and Bob have confirmed these differences. UChr (talk) 23:29, 31 January 2014 (UTC)

The whole point of this theorem is that, within the scope of a standard view of quantum mechanics, no such device as this polarizing filter describe in this section can be created. Frankly, there are enough issues with this section that I would advocate deleting it entirely. Somephysicist (talk) 02:09, 13 February 2014 (UTC)

The "Opposing Viewpoint" is not well-sourced and does not reflect any sort of consensus, or even well-regarded minority opinion, in physics on this issue. As such it has no place in this article.AvalonXQ (talk) 18:27, 27 March 2014 (UTC)

I hope that with my multiple links has clarified that the proof is wrong.UChr (talk) 14:39, 29 March 2014 (UTC)

Wikipedia is not the place to present fringe research contrary to the mainstream consensus. You don't get to just decide that a theorem generally accepted by the physics community is "wrong" and have Wikipedia reflect your fringe belief. AvalonXQ (talk) 20:23, 1 April 2014 (UTC)

Many believe - inspired by Einstein - that FTL communication is not possible. However, Einstein himself believed that entanglement opens up for FTL - communication.

Because they agree with the theorem, they do not read the proof critically enough. But when they now read the criticism, I expect, if they still think the proof is correct, some specific arguments.

Error in proves is not a matter of majority. UChr (talk) 15:45, 2 April 2014 (UTC)

So you admit that you are not documenting what is established science but are instead trying to publish your proof so more people will read and accept it. That is not what Wikipedia is for. Submit your proof to the physics community, have it become well-regarded and notable, and then this will be an appropriate place to add it. Not before. AvalonXQ (talk) 16:02, 2 April 2014 (UTC)

I'm not the only writer! But the third post is pure textbook material. I only observe the difference between the ordinary understanding of entanglement and assumptions of the proof. UChr (talk) 18:14, 2 April 2014 (UTC)

If you insist that this addition reflects a commonly-accepted opposition to the no-communication theorem, then cite your sources and the appropriate language. Don't write your own criticism, without any relevant citation past the second sentence, and call it science. AvalonXQ (talk) 18:53, 2 April 2014 (UTC)

You are not interested in or do not have skills for a serious discussion. UChr (talk) 19:39, 2 April 2014 (UTC)

I am both interested in, and have the skills for, a serious discussion. However, unless that discussion is about currently accepted science, this is not the place to have it. Unless you can support your material with appropriate sources and citations, stop trying to insert speculation into a science article. AvalonXQ (talk) 19:46, 2 April 2014 (UTC)

Are you scared? - Otherwise prove itUChr (talk) 20:22, 2 April 2014 (UTC)

@AvalonXQ I understand your point but UChr has a better point. This is the TALK page. Additionally Wikipedia is a NATURAL place to have these discussions. Lastly there are experiments coupled with LOGIC that defy this no-communication theorem proof as paradoxical. Many people have correctly stated that the proof is for single particle states and should be redefined as such. Ensembles states are not within the realm of the no-communication theorem.Ldussan (talk) 22:59, 2 April 2014 (UTC)

Just imagine this. If i wanted to send a data burst over an pulsed optical communication link I could send an ensemble state of photons over a fiber optic channel. Intensity would encode my 1's and 0's. My SNR would grow as the number of photons in my ensemble 1 state grew. BUT in principle I could send just one photon per "1" bit and use a sensitive detector to tell if I had a one or a zero. In an Ansible type of QE FTL link this could not happen because you can't SEE the signal (interference pattern) with a single photon (it's impossible). That is essentially the no-communication theorem in a round about way. With an ensemble of photons you can. Theory disproven.Ldussan (talk) 23:07, 2 April 2014 (UTC)

Yet again, this is not a physics conference. This is not the right venue to try to convince people that your original work (or the unacknowledged work of others) has overturned the consensus understanding of the physics community. Convince the physics community first; get your experiments published in respected journals and cited in appropriate reference books. Then come back and cite those references in the article. In the meantime, stop trying to argue for your fringe position on Wikipedia; that's not what it's for. AvalonXQ (talk) 19:51, 4 April 2014 (UTC)

A) Logic does not need referencing. B) This is a talk page  C) There is a consensus understanding??? D)The fact that you call it a fringe viewpoint means you're not qualified to talk intelligently on the subject and I suggest you step out of the way and let others modify this page that are qualified. E) This is not just my viewpoint. There are other reference-able sources that point out this issue.  The real question is why are they not properly referenced here.Ldussan (talk) 17:19, 8 April 2014 (UTC)

Why does it disallow all communication, even slower than light?
''The theorem disallows all communication, not just faster-than-light communication, by means of shared quantum states.

I don't see how the case of slower-than-light communication is even covered.

It seems to me that the theorem is a very formal one, heavily dependent on agreeing what algebraic operations are allowed. These in turn are motivated by the assumption that the points of measurement are spacelike separated.

Alice and Bob can each independently apply a trace operator (or some measurement) to the state. Since the points of application are spacelike separated, the operators have to commute, for the application even to make sense. And the fact is, they do commute. It doesn't matter who applies his operator "first", the operators algebraically combine so that they are truly applied in parallel. That is the setup of the problem. Then "no-communication" is the observation that the statistics experienced by Alice are independent of the operator applied by Bob, and vice versa.

But this really only makes sense if the two measurement events are spacelike separated. The formal algebraic notion of applying the operators "in parallel" doesn't even make sense if the two measurement events are timelike separated.

And furthermore, if the measurement events are timelike-separated, suppose Bob measures his particle first. Since there is a timelike separation, Bob's particle could travel to Alice and she could measure it and get information about what measurement he chose. (Or am I wrong here? Doesn't his measurement effect a form of preparation, which Alice can later detect at least statistically, according to the convention that measurement causes a collapse of the state?)

To rule this out, we must either constrain the lab setup in some way (for example, Bob's particle does not move toward Alice), or stick to the gentleman's agreement that Alice will only measure her own particle. But this is a bit artificial -- it's an extra rule -- and it only has to be introduced if the points are timelike separated.

In the spacelike case, she can't even perform the measurement of Bob's particle. This -- the spacelike separation -- is what motivates modeling Alice and Bob by separate measurement operators applied separately to the factors HA and HB in the first place. The correspondence between allowed Hilbert space operations and experimental setups, which is highly idealized at best, becomes very shaky indeed if we forget the underlying motivation.

What am I missing?

178.38.115.176 (talk) 01:01, 5 May 2015 (UTC)


 * I have to re-iterate my objections to the statements in the article:


 * ''The no-communication theorem states that, within the context of quantum mechanics, it is not possible to transmit classical bits of information by means of carefully prepared mixed or pure states, whether entangled or not. The theorem disallows all communication, not just faster-than-light communication, by means of shared quantum states.


 * Suppose that Alice prepares a state and sends it to Bob. It travels at less than lightspeed, of course. Bob measures the state. Obviously he thereby receives information from Alice, from a state she "shares" with him. This contradicts the statement made above, if you read it for its face value.


 * Of course, the technical meaning of a "shared quantum state" is a situation where a state is made available for measurement at two spacelike-separated points A and B. It is only under this condition that no bits can be communicated from A to B.


 * In fact, if the two points A and B are timelike-related, you don't have to call them Alice and Bob. You can call them Bob and Bob. Then the assertion becomes: Bob is unable to store classical bits by means of quantum states.


 * If this were true, it would be a new and novel no-go theorem called "Quantum Dementia" (quantum computers forget everything you tell them). Not a very compelling advertisement for quantum computing. And certainly false.


 * 178.38.115.176 (talk) 08:51, 5 May 2015 (UTC)

Remi Cornwall's disproofs
The work of Remi Cornwall seems to have dispelled the No-communication theorem. He shows that an interferometer which can work out mixed state (measurement) and pure (no-measurement) can lead to the system being factorised and is able to do this with a one (path entanglement) and two photon setup (HV entanglement). He also approaches the problem from a Decoherence Theory viewpoint. The basic tenants of his approach to proving the theorem wrong is: a) one takes the joint evolution of the system before taking the partial trace, b) the trace is a space-like operation, c) creation operators describing the beamsplitters are mapped outside the device where distance isn't an issue.

Ultimately people have to make their own minds up and an argument using standard analysis ought to worry the community. The single photon path entanglement version he lists is QM101 really, hard to argue against. I'm sure good people will pick up on Cornwall's work as it has the falsifiability criterion of true science (Karl Popper) and appeals to experiment.

Pure state
Quantum states that cannot be written as a mixture of other states are called pure quantum states, all other states are called mixed quantum states. What is this? --Дядько Ігор (talk) 17:50, 26 August 2017 (UTC)

The entanglement might actually be exploited for communication
This article made me rethink the no-communication theory. Even if not used on a couple of entangled particles, it can be used on a series of particles. Worth reading, in my opinion: https://arxiv.org/pdf/1704.02587.pdf Do you think it is worth adding inside the article? — Preceding unsigned comment added by Le petit poussin (talk • contribs) 23:28, 31 August 2018 (UTC)

Please edit the article to improve clarity
There is one edit that could dramatically improve the clarity of this article, IMO, and that was indicated above: Changing the spin of system A does change the spin of system B, of course. The point of the theorem is that a scientist at lab A cannot control whether the system decoheres into spin up or spin down (in the entangled photon example- whatever analogue is applicable for other systems) along the axis of measurement. The result is that, regardless of anything the scientist in lab A does, system B has (not knowing the result of the measurement of system A) a 50/50 chance of observing spin up or spin down. A scientist at lab A can, after measuring her system, determine what system B will be measured as, but can in no way affect that measurement. Can we have a vote as to whether to include this text, or something like it, please? It resolved my doubt about "spooky action at a distance", and may help others, too. I had imagined that a particle could be forced into a particular spin state, instead of just measured to see what its spin state was. David Spector (talk) 13:10, 6 December 2019 (UTC)