Talk:Quantum entanglement

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Where are 3-particle experiments !?!?!?

There are 3-particle versions of the 2-particle entanglements. Yet I see no mention of them here! Surely you all know whereof I speak -- I came here to find the name.  :smack: Jamesdowallen (talk) 17:55, 4 January 2012 (UTC)

There is a wikipedia page for Multipartite Entanglement, albeit a very messy one - http://en.wikipedia.org/wiki/Multipartite_entanglement --Sabri Al-Safi (talk) 18:25, 11 January 2012 (UTC)

A clarification (?)

The following sentence has been added:

Although the "effect" of quantum entanglement appears to exceed the speed of light, there is no violation of special relativity or causality which declares that information cannot be transferred faster than the speed of light.

How can an "effect" not be paired with a cause? Is the word put in scare quotes in the quoted sentence because of this very question?

How can causality get violated? A description, even a highly abstract and tightly organized description such as a theory, cannot be violated. A description is not a law of heavenly commandment. Einstein postulated that the speed of light is the highest speed that can be achieved by anything and that it has a constant value. It follows from that postulate that anything that moves through the space-time continuum will not exceed c. So any transmission of a force from one place to another will not exceed that speed. If "information" means a physical sign of some state of affairs such that a decision on what to do about the state of affairs can be validly based on the presence of the sign, then the assertion is that such a physical state of affairs (a morse-code radio transmission for instance) cannot reach its recipient at any speed faster than c. Isn't that all that is at stake here? It is known (1) that one kind of thing that happens in the universe is that forces move through space at c, (2) that a change in something at one place in space-time will transmit a change to something at some other place in space-time, (3) so a change will always occur at a location d distance away at a time equal or greater to t=c/d. If the first event is called a cause and the second event is called an effect, then the effect will always occur at a time t=c/d or later than the cause.

As for entanglement, either nothing at all is happening to the quantum states of entangled entities after they have separated, or something very interesting is happening. First, if causation involves priority in space-time, then there is no causation involved between the entangled pairs. The change in state of one member of an entangled pair involves a change in state of the other member at the same time. If we look at the interaction between the measuring apparatus and the entangled twin particles, then the idea of causation is preserved if the measuring apparatus goes into operation at some time t and the entangled particles change state at some time t+1.

If we conceptualize the experiment in somewhat different terms, then the measuring apparatus goes into operation at time t=1, the measuring apparatus acts only on one of the entangled twin particles, and the fact that the first of the entangled twin particles changes state somehow determines how the second of the twin particles does or will at the time of measurement change state in a way that is "correlated" to that of the first.

If it is true that in the beginning entangled particles each have a superposition of states, then, whether you call it causation or something analogous to causation, it becomes necessary to understand how the two particles coordinate their changes of phase even across astronomical distances.P0M (talk) 21:20, 25 November 2011 (UTC)

"It follows from that postulate that anything that moves through the space-time continuum will not exceed c." Language must be used carefully so we understand what is being said and so that the tool of mathematics (which is precise) can be used. There are lots of things that move through space and time that exceed a velocity of c. The only speed limitation imposed by nature is on information and matter, and that limitation applies to observations in an inertial frame of reference.

Superpositions of allowed (quantum) states exist at specified times and places. They refer to tiny phenomena, such as elementary particles, or to larger phenomena that share a quantum state, such as superfluid helium or superconductivity of electricity. There is no evidence that particles (which can be observed) coordinate anything faster than c. Such evidence would contradict the speed limit c on information propagation. However, wave functions, which describe the way that superposition states change in space and/or time, can extend throughout a region of space-time and appear to reflect sophisticated coordination between actions happening at far-away times and places. Wave functions may appear to violate causality, but such apparent violations are illusions appearing only in observable values, not in the wave functions themselves, which are always consistent with respect to the entire system involved.

QM does not violate causality. However, it may predict that there is a finite possibility that the fluid in a cup may overcome gravity and leave the cup. In real life, the only way that happens is if the fluid is in a macroscopic quantum state, such as can happen with liquid helium. It doesn't happen with water, so our common experience is not violated by QM.

Many QM experiments challenge us, and seem like magic tricks, because the rules of quantum reality are not the rules we are familiar with in daily life. We live in an intermediate scale of size, temperature, time intervals, etc., where Newtonian mechanics applies very accurately. Consequently, we live by "common sense" physics. It is no surprise that we try to coerce the reality revealed by experiment in other size and temperature (etc.) regimes to "make sense" to us. However, physics objectively reveals the truth of various regimes, fearless and independently of our common sense physics. Even Einstein seemed to find QM unnatural--he felt that QM must be very incomplete; it could not be true as it stands. Yet, subsequent theory and experiment has upheld even the earliest results in QM, such as double-slit interference, strange though they seem to us. David Spector (talk) 15:37, 2 December 2011 (UTC)

The word "effect" was put in scare quotes in the article, which makes it "o.k." I guess. But it is also true, as you say, that "language must be used carefully so we understand what is being said." That is why I objected to the supposed clarification.P0M (talk) 16:28, 2 December 2011 (UTC)

I see your point about this small issue, but all these QM articles are just a starting point, particularly for the general public who don't realize how their very limited experience in time, temperature, pressure, gravity, electrical current, etc. ill prepares them to understand the remarkably dramatic range of the description of reality that is being developed by science. Even scientists find it difficult to expand their common sense beyond our familiar classical regime, as you can see in the inconsistent or argumentative comments they make on any physics forum when articles are published on results in QM, cosmology, special relativity, etc.

And all this tumult is as nothing compared to the coming insights when philosophy has its turn, and the Absolute, changeless Self is finally identified subjectively through the experimental mental procedure known as transcending (about which there is as yet no WP article due to the near-monopoly on this knowledge held by the Transcendental Meditation organization). We are about to discover that we, as the human race, are missing an entire necessary state of consciousness (in addition to waking, dreaming, and deep sleep). I'm thinking this paradigm transformation may begin happening as early as five or ten years from now. The importance to society will be undeniable from the standpoint of the beneficial health effects alone. Wait until you see the difficulty the general public will have getting used to such ideas, again seeming to be in contradiction with common sense. Having been prepared by the mind-expanding vistas opened by QM, physicists may be, as a group, alert early adopters of transcending. Stranger things have happened. Sorry for the digression. David Spector (talk) 18:22, 2 December 2011 (UTC)

Entangling macroscopic diamonds at room temperature

Editors here will want to use:

• K. C. Lee, M. R. Sprague, B. J. Sussman, J. Nunn, N. K. Langford, X.-M. Jin, T. Champion, P. Michelberger, K. F. Reim, D. England, D. Jaksch, I. A. Walmsley (2 December 2011). "Entangling macroscopic diamonds at room temperature". Science 334 (6060): 1253-1256. doi:10.1126/science.1211914. http://www.sciencemag.org/content/334/6060/1253.full. Lay summary. 
• http://www.sciencemag.org/content/334/6060/1253/suppl/DC1 supplementary materials]

Enjoy!LeadSongDog come howl! 16:44, 2 December 2011 (UTC)

Yes, I enjoy! Boris Tsirelson (talk) 06:38, 13 December 2011 (UTC)

Confusing sentence

Somebody tried to improve the following sentence by adding "i.e. one" in parentheses. The sentence didn't make sense before, and the addition only makes more obvious how strange it is:

In general, a bipartite pure state ρ is entangled if and only if one, meaning both (i.e. one), of its reduced states are mixed states.

If anybody understands what the intended meaning is, please reword.P0M (talk) 21:17, 12 December 2011 (UTC)

It should mean that the following three conditions (on a bipartite pure state) are equivalent: (a) it is entangled; (b) at least one of its reduced states is mixed; (c) both of its reduced states are mixed. Let a native English speaker formulate it nicely... Boris Tsirelson (talk) 06:33, 13 December 2011 (UTC)

I think the English is fine, but I am having trouble with seeing how to put this in formal logic because it appears to be tautological in that both its reduced states being mixed makes it impossible that at least one of its reduced states is not mixed. So what is the use or function of (b)?

To me it looks like:

"A bipartite pure state ρ is entangled." ↔ ("At least one of its reduced states is mixed."∨"Both of its reduced states are mixed.")

Maybe you are saying:

"A bipartite pure state ρ is entangled."&("At least one of its reduced states is mixed."∨"Both of its reduced states are mixed.")

I guess the word "equivalent" is puzzling to me in your formulation. If you replace ∨ with & above, then (b) is unneeded because (c) means: (b) & "Any other of its reduced states must be mixed."ll

So I still need some help.P0M (talk) 16:01, 14 December 2011 (UTC)

I know that you are not a mathematician. :-) Condition (c) is (seemingly) stronger than Condition (b). That is, (c)→(b) evidently. But it does not mean that (b)→(c) evidently! Thus, it does not mean that (b)↔(c) evidently. Nevertheless I claim that (b)↔(c); this is not evident (at least for non-experts) but true. And so, finally, the claim is (a)↔(b)↔(c). Boris Tsirelson (talk) 18:47, 14 December 2011 (UTC)

Thank you. I think I now know what needs to be said. In this case, formal logic is of greater use than ordinary English.P0M (talk) 19:43, 14 December 2011 (UTC)

Formal logic, really? For a wide audience? Or maybe just say: ..."in fact, it cannot happen that one of the two reduced states is mixed while the other is pure" (or something like that). (I mean, it cannot happen for a bipartite pure state; for a bipartite mixed state it can happen, of course.) Boris Tsirelson (talk) 21:21, 14 December 2011 (UTC)

So it is true that you really do read minds! I was about to ask you for an instance of a false case.

Above, I proposed:

Both its reduced states being mixed makes it impossible that at least one of its reduced states is not mixed,

and you just said:

It cannot happen that one of the two reduced states is mixed while the other is pure"

I fail to see how these two statements are different. "Pure" means "not mixed," no?

Maybe I am missing something in the context, either the experimental context or the theoretical context, that separates the following two statements temporally: "(b) at least one of its reduced states is mixed; (c) both of its reduced states are mixed." I can understand that one might determine the status of two reduced states in sequence, but does that have some significance in the way that the order of operations in matrix multiplication has a significance not relevant to ordinary multiplication?P0M (talk) 04:55, 15 December 2011 (UTC)

No, it is about a single bipartite state, at a single instant; the time is not relevant.

Yes, "pure" means "not mixed".

Sure, "Both its reduced states being mixed makes it impossible that at least one of its reduced states is not mixed" (which is a bit of pure logic, irrespective of any physics).

But the trivial statement above does not exclude the (seemingly possible) case of one pure and one mixed. Right? Both the assumption and the conclusion are violated in this case; so what? Boris Tsirelson (talk) 06:41, 15 December 2011 (UTC)

Indeed, so what?

Above you said: "Condition (c) is (seemingly) stronger than Condition (b). That is, (c)→(b) evidently. But it does not mean that (b)→(c) evidently! Thus, it does not mean that (b)↔(c) evidently. Nevertheless I claim that (b)↔(c); this is not evident (at least for non-experts) but true. And so, finally, the claim is (a)↔(b)↔(c)."

(c)→(b) is o.k., logically at least. In fact it is a tautology. We need the facts, however.
(c)→(b) does not mean that (b)→(c), but on the other hand it does not exclude the possibility either. We need the facts to evaluate further.
Neither of those sentences mean that (b)↔(c), but on the other hand if it turns out that (c)→(b) and (b)→(c) are both true representations (not destroyed theories or old wives tales), then (b)↔(c). At this point we are only considering how things would work out just looking at the functions of logical connectives. Saying (b)↔(c) means that at least one of its reduced states is mixed if and only if both of its reduced states are mixed. And then working backwards you want to work in the statement "The bipartite pure state is entangled," making the whole thing:
"The bipartate pure state is entangled if and only if at least one of its reduced states is mixed if and only if both of its reduced states are mixed."
That sentence is incompatible with the statement that "One reduced state of the bipartate pure state is pure and another reduced state of the bipartate pure state is mixed."
So far we are only looking at empty schemata. We may have a clearer idea than before what what empirical evidence we would need to further evaluate these assertions, but that has to come from the laboratory. We might find a case of some bipartate pure state with a pure reduced state and a mixed reduced state. Logic does not tell us whether in that case entanglement would be exhibited. But if it turned out that way, then the earlier assertion that both reduced states have to be mixed would be fatally challenged, no?
Maybe we need a simple list of what the conditions are that are consistent with entanglement, and what the conditions are that are not consistent with entanglement.P0M (talk) 09:16, 15 December 2011 (UTC)

I am afraid I miss your point (after all, the whole matter is very simple), but I try to reply. The mathematical theory of pure bipartite states proves that it cannot happen that one reduced state is pure and the other mixed. The same theory proves that in the case of pure reduced states entanglement is absent, while in the case of mixed reduced states entanglement is present. (Do not ask me what happens in the case of one pure and one mixed, or else I'll ask you whether or not 4<5 in the case of 2+2=7.) All that holds irrespective of any experiments (except for the fact that the whole theory is ultimately experiments-based, of course).

Moreover, the notion of entanglement (as presented here, unlike my article on CZ) is not formulated on the empirical level. It is about theory, not experiment. True, this entanglement sometimes has (very important) empirical implications. But it is not defined in empirical terms. Boris Tsirelson (talk) 10:38, 15 December 2011 (UTC)

And here is an elementary counterpart.

The following three conditions on a parallelogram ABCD are equivalent:

(a) its area equals AB times BC;

(b) at least one of its four angles is right;

(c) all the four angles are right.

Yes, as simple as this is! Boris Tsirelson (talk) 11:25, 15 December 2011 (UTC)

Thank you.P0M (talk) 16:49, 15 December 2011 (UTC)

entanglement and physical interaction

I suggest to change the introductory sentence, in order to define the entanglement without mentioning the physical interaction. I say this in the light of measurement-based entanglement protocols, which use path-erasure. Now I don't have time, but if no one objects, I will proceed with this idea Oakwood (talk) 22:25, 23 December 2011 (UTC)

"Attempts to talk around the phenomenon" section

The "Attempts to talk around the phenomenon" section has no references and reads like original research. Should probably either be removed or reworded (with citations added). I'm not sure the section adds any real value to the page so unless people care terribly or are willing to fix it I vote for just deleting it. Medlefsen (talk) 20:35, 25 January 2012 (UTC)