Talk:Quantum entanglement/Archive 4

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Faster than light / instantaneous decoherence aspect enhancement.

LoneRubberDragon (talk) 09:29, 20 June 2008 (UTC)

I understand the aspects of not being able to transmit *information* faster than light, regarding the [no-communication-theorem].

Part 1

1.0 But could there be a couple sentences more of foundation of the *instantaneous* wave function decoherence aspect of widely separated entangled entities? For example, do experiments show the effect is "functionally, always instantaneous", or, for example, that it is "superluminally context(x) times faster than the speed of light"?

It would be good to look for documents to cite in this regard. It is my impression that the current understanding is that nothing propagates from entangled entity one to entangled entity two. For one thing, if that were the case then lots of the retrocausality arguments would fall because they assume instantaneous production of a change in the more remote of the entangled pair members. The understanding I have is that the two members of the entangled pair are, in effect, the same thing, so that if something is done to one "member" of the pair then it is equally being done to the other "member." One of the reasons for entanglement experiments to use long lengths of optic fiber cable is to get enough distance between the entangled pair members to be able to measure any difference in time of action.

I really like this section you wrote. By considering the wavefunction of, say, two polarized photons as one entity of probability wavefunction, that can partially decohere as a whole, makes more mathematical sense when considering an expanding Fourier window for analysis, that intersects experimental equipment at the edge of the conventional light cone. And quantum mathematical collapse operator speaking, as instantaneously operating, that it makes perfect sense that the effect is always perfectly instantaneous, as the compound object wavefunction entity is one thing, mathematically, and on the material experimental plane(!). LoneRubberDragon 2008 06 21 A 0204

I read the related Bohm inequality language, referred in your introductory material, but the other articles' nonlocal effects descriptions, were more instantaneous metaphorical than your convention in this response. LoneRubberDragon 2008 06 21 A 0204

I think I remember reading that one of the surprising findings in regard to quantum tunneling is that it does not take longer for one photon to be registered at the far side of the wall than at the near side. I do not see why that should be, so perhaps I have misremembered.

If there were a multiple of the speed of light involved in explaining the decoherences, then that fact would create a difficulty for physicists to explain: What model will allow the prediction of these multiples of the speed of light? Everything else we know indicates that there is one speed of light, and everything we know about wave motion in homogeneous media is that the velocity of the wave front movement is a function of the rigidity or elasticity of the medium. There are difficulties with talking about such a "luminous aether," but at least it gives some intuitive grasp of some of the known features of light propagation. If there were several speeds of light, the conceptual scheme needed to talk about light transmission, even in a figurative way, would become much more complex. But I have seen no instances of such a discussion.P0M (talk) 07:29, 21 June 2008 (UTC)

Part 2

2.0 And, helpful, would be a touch more foundation *why there is* an *instantaneous* wave function decoherence of widely separated entangled entities. I can wrap my technical-layman head around other articles' probabilistic wave functions, and accept measurements as a necessary defineable mystery, but the *instantaneous at great distance* alteration of a second entities' probabilities, by measurement of a first entities' probabilities, lacks something. I understand that you may allude to just this foundation point, in the [retrocausality] sentence, but it is a stretch for laymen to follow this explanation, a little open ended foundation, but a good link, nevertheless.

This line of questioning keeps reminding me of some of the philosophical writing of the great mathematician Leibniz. He was a very logical thinker, and he attempted to create a coherent system of thought that would put for an examination of fundamental categories like space and time. If I remember correctly, one of his points was that if there were two entities that were exactly the same then it would be difficult to say what we meant by "two entities" if we could not identify each of them with separate space and/or time coordinates. But he also concluded that space and time are only relations. My point is just that we have to think very carefully about what we mean by words like "instantaneous," "non-instantaneous" (time consuming), "same entity," etc.

I always find that perfect wording is a necessary evil, that takes numerous revisions, at times, to refine introductory and intermediate materials, so that the language is as self consistent as possible, and best scaffolds understanding into the deeper materials. Subtleties arise, when the majority reader assume nothing faster than light, then hear of "instantaneous spooky effects at a distance", then look over articles and papers to sort through what the real truth is behind the words. Your treating the compound object probability wave function as a single entity, makes perfect wording for the mathematician using the wave function collapse operator. So even if, physically, it is still a touch difficult to comprehend a wavefunction, say, 10 light years wide partially decohering instantaneously, mathematically, it makes perfect sense by QP mathematics definitions. It is odd to think, as most experiments have the core of the group wave function all in a local setup, and EPR stretches fourier windows to the other extreme by putting all the group wave function at both ends of a light cone. It is very metaphorically and very not metaphorically like how the DC term also shows up at the highest frequency end of a periodic signal window fourier transform, but only metaphorically. LoneRubberDragon 2008 06 21 A 0204

True. I remember reading lots of stuff about electricity and electronics starting when I was in junior high school that really messed me up. One of the hopes I have for Wikipedia is that kids stuck in small towns in the hinterlands can look for information here and not find a bunch of misleading nonsense that they will have to root out later on. P0M (talk) 01:50, 22 June 2008 (UTC)

The idea of entanglement, as it comes up in historical process, presupposes things that are not ordinarily entangled. Starting from our experience as human beings, it seems almost perfectly clear that things are not entangled. Ideas to the contrary, e.g., instantaneous mental telepathy, always have the aura of mystery religions hanging about them, and no wonder -- we do not find reliable proofs of these things in our own experience. So our overwhelming prejudice when we come to think about entanglement is that if something is here and something is there then they cannot be the same thing. Quine has some pre-entanglement thoughts about that kind of reasoning in his main book on logic, but most people probably read those ideas and think that he was simply being "philosophical" and that the ideas had no practical merit.

But let's look at things from the other end of the telescope. Suppose that one event triggers another. We can look to Feynman for a definition of a single event that seems to have salience in the current situation. If an electronic device is arranged so that a single electron changes its orbital from a high energy one to a low energy one, the difference in energies will then appear as a moving something-or-other that we cannot see and that (according to Feynman's way of describing things) goes forward by all possible paths. Then at some later time an electron circling an atom somewhere else is boosted to a higher orbital, and the whatever-it-was-that-travels travels no more. So we have a beginning and an end and all we are really very sure of is that energy gets transferred across space somehow and that the transfer occurs at the speed of light. All that just to say that we have a single event going here.

Good points on the all-paths-simultaneously probability-phasor integrals. I remember working one partial problem of a photon traveling the speed of light from a light source to a detector, by integrating ellipsoidal shell surfaces of a second internediate point reachable at the speed of light, from both foci to the shell points. Each shell oscillated phases cancelling each other out for the most part leaving an oscilating ellipsoidal light surface term, and leaving a steady state straight line path between the light source and detector contributing to the solution, the classical answer (the hard way *grins*). I'm sure if I took every intermediate light speed path of the photon integral, that the oscillating term would have disappeared, too, leaving only the classical straight line path from light source to detector. LoneRubberDragon 2008 06 21 A 0204

Now let's put a certain kind of crystal in the path along which the vast majority of photons have been detected when fired from our special apparatus. It is the kind of crystal that lets an incoming photon boost an electron as usual, but the electron quickly drops out of its higher orbit and resumes a lower orbit, and when it does another event occurs -- only this time two photons get fired off in different directions and each having part of the energy of the original photon.

Note that by the curious definition of event we accepted above, we now have a single event that is characterized by a single x,y,z,t origin, but the two photons that go off will end up at x',y',z',t' and x,y,z, and t. That description does not tally with our ordinary idea of what "an event" is supposed to be like, but we are stuck with it because that is the way that the universe works.

This one never bothered me. Energy is conserved, and spherical or wavelength resonant axial electron orbitals, can act like a mathematical bifurcation saddle point, on the incoming photon energy, to permit two half, or less than half, energy photons to spread in both directions by the wave function entering the saddle point. Which may explain why they are one entity to begin with, bifurcation saddle point mathematically speaking. LoneRubberDragon 2008 06 21 A 0204

Our sense of propriety is not so greatly insulted if the experimenter makes an experiment with a short leg and a long leg and then does something to the photon that is moving down the short leg. If the experimenter demands of the photon that it manifest according to its wave nature or according to its particle nature, then we are not too much bent out of shape if the other photon turns out minutes, or millenia, later to have a complementary state. We might imagine that when the first photon is affected by the actions of the experimenter, some signal indicating that change goes back along the original line of progress and then follows the other fork and "catches up with" the second photon. But of course it will have to have gone faster than the speed of light to make up for lost time.

Yeah, it is not a "signal", but yet the probability wave function entity collapses instantaneously, understandable by math, but unintuitive by mechanism / analogy. (pure pun) The tails / tales of fourier spectrum are quite dangerous ideas! 09:08, 21 June 2008 (UTC)

It's even worse when the photon on the short leg of the experiment is allowed to be detected without anything having been done to influence it, and, much later, the second photon, the one on the long leg of the experiment, is subjected to some manipulation that forces it to manifest according to its wave nature or according to its particle nature. The "free" outcome turns out to have been in accord with the "forced" outcome that occurred afterwards. What this appears to mean is that the determination is not just "instantaneous" but "retrocausal."

So it may be that the straightforward way to conceptualize this kind of entanglement is that the event occurs "out of time." Another way to say it would be that the single event is not over until it is all over. To me, that does not really seem to help. Whether going faster than light or going backwards in time, it all seems quite strange and impossible. Saying that an event occurs out of space and time is not very cool either.

Part 3 was archived

Part 4 was archived

Part 5

5.0 Otherwise, this seems a useful article defining the mathematics and core nature of [quantum entanglement].

LoneRubberDragon (talk) 09:29, 20 June 2008 (UTC)

One of the difficulties in thinking about this subject is that relativity theory does not really speak of information. Saying that information cannot be transferred at greater than the speed of light is just to say that light cannot travel faster than c and that nothing with a rest mass can even travel that fast. However if a change in one part of a system is reflected in another part of that system and that change does not propagate through space and time, then all bets are off. A trivial and ideal version of such a change would be what happens when one end of a very long and perfectly rigid cylinder is pushed. A pair of atomic clocks on each end would detect movement at exactly the same instant. I said "ideal" because in the real world moving any object is like accelerating a railway train. The engine moves forward and you hear "clink" as the link between the engine and the next car is pulled tight. So the cars actually start moving in sequence and it takes some time before the caboose starts to move. Pulling on a long cylinder works the same way except that it is molecular bonds that are being stretched taut rather than steel links. But in the propagation of a photon from place to place we find an event with no discrete parts, no "cars" to put in individual motion. And what is this "event"? Is it an abstraction? An abstraction from what? Or is it a "thing"? If so, what is it made of? P0M (talk) 08:00, 21 June 2008 (UTC)

"===="

I just wonder about the instantaneous wavefunction probability alterations. QP was formulated for instantaneous local asssumptions of the mathematical operator, but experiments sending objects out object group probabilities on the light cone, is the exact opposite of the assumptions of the local properties of most localized group probability-core problems, and shows itself experimentally, in the behavior of an instantaneous wavefunction alteration transmission operator(!). LoneRubberDragon 2008 06 21 A 0204

I am not sure that I follow what you have written. The phrase "experiments sending objects out object group probabilities on the light cone" is causing me problems. For starts, is the subject of this clause "QP" and "experiments" its verb? Or are you speaking of "experiments that send objects..."?

First for my own correction, I assume the end of this post as a final thoughts section and dialog. I should not have indented my, "I just wonder about ..." section, in implied response to your, "One of the difficulties in thinking about this subject is ..." section, immediately above. It was an unrelated-to-your-comment rumination of my own. LoneRubberDragon (talk) 18:05, 22 June 2008 (UTC)

Now, the idea I was very roughly fleshing out, was that most of QP measurement experiments deal with *spatially localized* probability "groups" (Feynman groups, perhaps?) moving at sum-luminal speeds, producing virtually no signifignat mathematical *spatial group terms* that statistically contribute to the observations outside of that *local-subluminal* group's "sphere of influence wavefunction calculation". With maybe the exception of head to head particle colliders, though they seem to interact often, in a *spatially localized* "pancake" of group probabilities, at "high" but still sub-luminal velocities. LoneRubberDragon (talk) 18:05, 22 June 2008 (UTC)

But somewhat like (1) analyzing mathematical limit equations at infinity, pushes an analytical tool to its extreme properties, or (2) analyzing a photon's Feynman diagram from point a to b through to the infinite limit of all possible probable wavefunction paths, yields a classical "straight line" answer, that (3) sending two photons in opposite directions at the speed of light, and then measuring the polarization from one end of the light cone, seems to affect the polarization probabilities at the other end of the light cone, to be reavealed when the two randomized outcome measurement sets are brought together, is an experiment quite the opposite of a *localized and sub-luminal* measurement of wavefunction collapse with negligible non-localized luminal probability terms. EPR is the quintessentially perfect example of the extreme mathematical limit tests of the single-(dual)-entity wave function's instantaneous collapse by measurement, involving the most non-local single entity example, with probable spatial particle group locations lying perfectly on the edge of an expanding light cone, from the central photon source, being measured. A smart analytical "product" of Einstein selecting the hardest limits of measurement. LoneRubberDragon (talk) 18:05, 22 June 2008 (UTC)

And, unrelated to the immediate paragraph above, to correct the periodic fourier transform comment I made in an earlier exchange; to be accurate, the DC term is not quite in the lowest and highest frequency bin, but rather in the lowest unaliased frequency bin, and highest aliased frequency bin, and the bins correspond to a DC frequency, and a double nyquist limit high frequency that appears identical to DC when periodically sampled. 11:09, 23 June 2008 (UTC)

"===="

I wonder about, or maybe just at, all of this stuff. If the inquisition of one state at a later time can determine the state of something detected at an earlier time is already weird enough without the possibility that we could use this kind of effect to send telegraph messages somehow. But measuring the twist of the tail of an event making the twist of the head of the same event (i.e., the other entangled photon) does not, evidently, send any energy or matter through space. So we seem to be dealing with a kind of "process" without the necessary "pro." It is totally outside normal everyday physics, which is why (I suspect) Einstein thought it defeated QM.

I'm sure Einstein hoped the EPR (influentially termed) peradox would defeat QM, but like Michelson-Morley smartly hypothesiszed, and then smartly disproved their own luminiferous aether hypothesis, in classic science style, that Einstein's EPR paradox was hypothesized, and has currently seemed to confirm the "spooky interaction at a distance" by a "single compound entities" instantaneous wavefunction collapse. It would be nice to telegraph information, but the information appears only when the two sets of measurement data are brought together for later localized analysis well within the entire light cone of the completed experiment's measurements; carefully stated here to outline the greater mathematical context of the no-communication information analysis process involved. LoneRubberDragon (talk) 18:05, 22 June 2008 (UTC)

"===="

One of the double-slit experiments that uses entangled photons to try to get which-path information for an un-meddled-with photon involves some further consequences of the ideas of entanglement that don't seem to have bothered or interested anybody enough to get written about. According to what seems to me to be the conventional way of talking about things, the experiment starts out with a fairly conventional source of photons that can reliably emit one photon at a time. The wave function that leaves the photon source encounters a double slit. According to conventional descriptions a photon either go through the left slit or the right slit. Then that photon either encounters a crystal down-converter in a region near the left slit or a region near a right slit. The next thing that happens is that two photons are emitted from one side or the other of the crystal, they get directed into two kinds of apparatus. Neither of these photons has itself encountered a double slit situation. Regardless of that fact one of them can be given a laboratory inquisition that will force it to reveal its particle nature or its wave nature. The experiment is arranged so that the inquisition and the determination so contrived will occur later in time than the arrival of the free-path entangled photon at its own detector. Nevertheless, the free-path entangled photon will either interfere with itself or fail to interfere with itself depending on what later happens to the other one. It seems remarkable and noteworthy to the people who do these experiments (any everybody else, too)that the interference vs. no-interference determination is done retro-causally. But it seems not to have been worthy of mention by anybody that something apparently went on in the part of the crystal that did not generate a photon. Or, to put it another way, whatever was done to the original wave function by the double slit apparatus appears to have been inherited by both of the photons generated by the down-shift crystals. So here it seems to me that there is what ought to be called a single event that has one starting point, involves first one path, then two paths, and finally eight paths, and that what is manifested at the end of each of these eight paths is a piece with the emission of the photon at the start of that run of the experiment.

In reference to the below Kim Experiment, I am also "in some wonder" about the physical processes involved with the whole class os quantum erasing experiments, in which macroscopic measurment apparatuses, virtually restore wave functions for interference in continued wave function progression. It almost seems a "spiritual" or at least very least an abstract wavefunction process, that setting and eraseing a bit of stored RAM could collapse and restore a wavefunction in the middle of an experiment. But I think the abstraction may be attributable to a perfect symmetry of all probabilistic forces involved in the experiment's mathematics involved. For example, it could be very analogous to the fact that in a sealed unit in zero gravity; that no matter how you move particles, or how complex the arrangement of batteries, masses, electric field exchangers like inductor transformers, and electric coils and magnets (like in motors); that the steady state velocity, translation, angular rotation rate, and angle of rotation, of the unit starting at rest, is zero velocity, zero displacement, zero rotation, zero rotation, and zero spin, respectively, in the group of [Conservation Law], even when quadrature exchange is involved with closed-system electric/magnetic properity (ex)changes.

Except in this QP case, the units that are conserved, appear to be a physical quanitity in units of [measurement|wavefunction-collapse information] for lack of a better unit name like "[gram]". Quite the abstract physical unit, but if the quantum eraser class of experiments show anything about nature, they seemingly show a "new unit" among the physically conserved properties of nature. Wikipedia has an abstract information units article on [Units of Information], and two articles, [Units of Measurement], and [Dimensional Analysis], ragarding mundane units, physically speaking, but I can't remember studying, or seeing articles here on dimenstional analysis units measuring [measurement|wavefunction-collapse information]. LoneRubberDragon (talk) 18:05, 22 June 2008 (UTC)

I would quibble in my own dim-understanding about retrocausality, in general, because there's a sub-class of EPR that uses electrically controlled polarizers to select a polarization by delayed choice of properly time gated photons, so that the opposite photons have virtually no idea of the polarization choice, and still show statistical wavefunction collapse. There is also the consideration that the whole Kim quantum eraser experiment is virtually local to itself, so the switching of the paths, and the erasing of memory cells electromagnetically, could be a conservative property in the units of [measurement|wavefunction-collapse information] affore mentioned, much like mundane conserved physical proerties. LoneRubberDragon (talk) 18:05, 22 June 2008 (UTC)

I will stop writing for a minute at this point to go get the link to the experiment I have in mind. The diagrams provided there will make things easier to visualize. P0M (talk) 00:23, 22 June 2008 (UTC)

Kim experiment

O.K., here is the image I swiped from the Delayed choice quantum eraser article. Note that the conventional description would be that a photon comes out of the laser and either takes the red path or the blue path. Let's say it takes the red path. Then at point "a" in the BBO a pair of entangled photons is emitted, one going to Detector Zero (D-0) and one going to the other part of the experimental apparatus where it ends up in D-1, D-2, D-3, or D-4. But the diagram also shows something going from point "b" in the BBO even though conventional description would say that no photon could have triggered anything there since a photon had to manifest itself at "a" and we only had one photon to begin with.

With regard to the time sequences involved, I think the reasoning the experimenters appear to have used is plausible but perhaps a little too shaky.

The reasoning apparently says that if the BBO were removed and D-0 was at the appropriate place in the path beyond the double slits, then one would get interference and enough photons run through the apparatus would produce interference fringes. (So if things in that part of the experiment were in control of outcomes in the total experiment then one ought always to get diffraction patterns.)

If the BBO were removed and the bottom part of the apparatus were positioned appropriately then it would be possible for a photon to be manifested either in D-3 or in D-4, and, given the way the experiement is set up, photons taking the red path would end up at D-4 but anything going through the blue path would end up in D-3 and play no further part. Similarly, if a photon took the blue path, it could end up in D-3 and anything that took the red path would end up in D-4 and take no further part.

If a photon passes through either Beam Splitter a or Beam Splitter b, then it will interfere with itself because it will arrive at either D-1 or D-2 along with the component of the original wave function that went through on the other side of the double slit apparatus.

So, the argument appears to be, if a photon shows up in D-3 or D-4 then (with the BBO back in place) entanglement forces the photon that arrives at D-0 to not interfere with itself. This then would seem to be a kind of trap-door phenomenon. Even though a photon will manifest at D-3 or D-4 chronologically later than one appears at D-0, the "un-negotiable" nature of what has transpired somehow trumps what occurs earlier at D-0. Basically, it seems, that is because the experiment has interfered with the free propagation of the wave function(s).

And if a photon is detected in either D-1 or D-2, then that result is consistent with a photon interfering with itself, so there is again nothing to prevent the photon appearing in D-0 from interfering with itself.

Another assumption that seems to be clear is that if a photon is transmitted through beam splitter a or beam splitter b, then whatever is complementary to it and is associated with the other path through the double slits will necessarily also be transmitted through the beam splitter on its side of the experient.

It looks to me as though there are several possible single events that start with the emission of a photon in the laser and end with the result that:

• D-0 shows interference and D-1 shows interference
• D-0 shows interference and D-2 shows interference
• D-0 shows no interference and D-3 shows a photon
• D-0 shows no interference and D-4 shows a photon

Each event has its own probability, and only one probability can turn up in each run of the experiment, and that's it.

Maybe that is not such a strange idea. After all, if one sets up a crooked roulette wheel with little lead weights or other gimicks, the outcomes of a thousand turns of the wheel are already predicted as soon as the physical apparatus is there. Still, the way I have figured things out seems to make probability trump causality. Maybe probability comes first in the order of all things.P0M (talk) 01:34, 22 June 2008 (UTC)

Part of intro not suitable for the average well-informed reader.

The current text has:

But, if this is so, then the hidden variables must be in communication no matter how far apart the particles are, the hidden variable describing one particle must be able to change instantly when the other is measured. If the hidden variables stop interacting when they are far apart, the statistics of the measurements obey an inequality, which is violated both in quantum mechanics and in experiments.

I think that this block of text is an example of something that is true but a text that cannot be understood unless you already understand it.

The general reader is going to be terribly confused by this text because nothing has been explained about how hidden variables were supposed to explain the coordination of distant measurements, and nothing has been said about John Bell and his discoveries.

The original idea was that even though there is no discoverable variable that marks one entangled particle for eventual discovery in a certain state and marks its twin with the opposite characteristic, nevertheless, the determination was made from the beginning. So when the first particle is interrogated, its answer (as to spin or whatever) was long ago determined. So it is then no wonder that when the other entangled particle is interrogated it will give the opposite response. The quoted text above implicitly denies the "already determined idea" and says that even if there is some variable in addition to the (spin or whatever) state, the variable would have to be changed at the same time as or after the spin (or other state) is determined. Then it goes on to make implicit reference to Bell's work that was first worked out in thought and only later came to be experimentally verified. P0M (talk) 07:20, 23 June 2008 (UTC)

Add a bit on how entangled particles are produced?

I think it'd be a good idea to include a section that demonstrates how, say, entangled photons are produced via Beta barium borate, and have that link to Spontaneous_parametric_down-conversion as a main article. As it stands, the article doesn't mention much about how particles become entangled, but several of the external links explain it - what do you think? I wanted to post the idea here first, just in case there's a reason that a section on it doesn't already exist. TVK (talk) 19:19, 4 August 2008 (UTC)

Can that actually be done? If so, great! It will hopefully be possible to say something more that that a single photon is emitted by one laser, encounters a beta barium borate crystal, and the result is that the crystal emits two photons in a single event. P0M (talk) 23:56, 4 August 2008 (UTC)

Bell's theorem is incorrectly explained

Bell's theorem is explained in the opposite of the right way. Local hidden variable theory was specifically made to avoid 'talking faster than the speed of light', hence the failure comes not from local variables not updating fast enough (since the local hidden variable theories depend on the hidden variable being set as an entanglement is being created, not 'updated') .... the failure comes from that ANY local hidden variable theory places some restrictions on the maximum correlation between two states (i.e Bell's inequality), quantum mechanics violates this inequality spectacularly(as has been shown by experiment), hence no hidden variable theory is consistent with quantum mechanics. The previous editor states none of this, and actually talks about a completely misleading and incorrect things. I will rewrite the section when I find some time. Any comments or suggestions are welcome. --DFRussia (talk) 23:25, 2 March 2009 (UTC)

Archives

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Though related to the article, I don't think that the posts in this archive are about improving the article. Please correct me if I am wrong and please don't get mad if I moved your post. I'm just trying to make sure that things in the article that need attention get attention. Phancy Physicist (talk) 06:09, 5 June 2010 (UTC)

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The posts in this archive are matters that have been addressed in the past. Phancy Physicist (talk) 07:53, 7 June 2010 (UTC)

Out dated discussions

The posts in this archive are about the validity of concepts that have since been generally accepted. Phancy Physicist (talk) 07:53, 7 June 2010 (UTC)

Quantum Entanglement not violating speed of light

The reasons why Quantum Entanglement can't be used to transmit information faster than the speed of light are really subtle (and really interesting!). This is just another plea for an expert to step in and explain this as well as possible. And no, I'm not even close to being that expert -- I just think it's probably the most important concept in this whole article.

— Preceding unsigned comment added by 207.190.209.8 (talk) 17:59, 16 December 2005 (UTC)

-- Perhaps the best plain-English explanation of why QE cannot be used to transmit information faster than the speed of light, and also why Quantum Teleportation cannot be used to transmit information faster than the speed of light as well:

Photons cannot exceed the speed of light.

Think of it this way: Point A, the origin point where the photon(s) is (are) born, encodes some information on it (them) as spin. The photon(s) then travel (according to our frame of reference) at the speed of light to its (their) respective destination(s), Point B (and Point C), where it (they) is (are) measured.

To our frame of reference, information is transmitted from point A to point B( and perhaps C). Information is never transmitted between point B and point C - they are receiving copies of what is transmitted from A. Point B does not get to choose which way it modulates the photon, which would allow it to transmit information to C - it only gets to choose which way it measures the photon. C gets to choose which way it measures the photon as well. Whether they do so "correctly" for the purposes of decoding what was sent by A is a side issue.

The confusion comes up because B can know what state the photon at C will be by measuring its own photon. This produces a human illusion known as the "speed of bad news". If two people read the same newspaper story, and come to the same conclusion, was information transmitted between the two? No. It can be thought of that way - it doesn't mean it actually is.

Further, to complicate things - in the frame of reference of the photon(s), zero time elapses between creation and destruction (measurement), and according to quantum mechanics, they may be thought of as the same particle because they share the same frame of reference. This does not mean they actually are the same particle.

This brings forth the confusion of thinking of them as actually being the same particle in the minds of many people.

To whit: Quantum mechanics is a descriptive theory, and (as has been stated before) is counter-intuitive. Be careful to separate what is described and what is predicted by the model.

— Preceding unsigned comment added by 69.15.122.87 (talk) 16:29, 26 January 2006 (UTC)

-- The "spooky action at a distance" is a function of the system as well - another "speed of bad news" illusion. To the frame of reference of the photon(s), its (their) state(s) are set at creation, which also happens - to their frame of reference - to be the same as destruction (measurement). Frame of reference of the photon(s) is linked between the 'creation' point and the destruction point(s). To our frame of reference, it still takes time, and "different amounts of time", for the photon(s) to reach their destination(s). One could think of it as taking some time for point C to reach the frame of reference of the photon(s).

As far as the photon's frame of reference is concerned, it involves no distance and 'time' is irrelevant.

It still requires a "conventional" channel for points C and B to discuss their measurements.

— Preceding unsigned comment added by 69.15.122.87 (talk) 18:05, 26 January 2006 (UTC)

Huh? (!!!!)

Intro claims:

Quantum mechanics holds that observables, for example spin, are indeterminate until some physical intervention is made to measure an observable of the object in question.

To me that sounds like: "it is impossible to measure a quantity before its first measurement", which is blatant tautological nonsense, since if one decides to measure a quantity before the first measurements ever, then that new measurement becomes the first. Something different must be what the sentence creator really intended. ... said: Rursus (^mbork³) 13:03, 12 January 2010 (UTC)

The idea is that the particle has no exact value until something happens to it. I changed the offending sentence to:

...quantum mechanics also holds that an observable, for example spin, is indeterminate until a measurement is made of that observable. At that instant, all of the possible values that the observable might have had collapse to the value that is measured.

Is this much more better? Phancy Physicist (talk) 18:36, 8 June 2010 (UTC)

Physicist proves that teleportation of energy is possible

I just found this, [1], which links to this page. It summarizes Masahiro Hotta's discovery, I assume. I don't fully understand what they mean, or what quantum entanglement is, but I thought it would be relevant and someone could add it. Here is a link to his paper, also found on that page. [2] Bsltiger (talk) 01:16, 10 February 2010 (UTC)

Local hidden variables are ruled out by Bell?

I have undone the edit because the original version is essentially correct. Although folk wisdom says that violation of the Bell inequalities is a consequence of nonlocality is locality not the only assumption in derivations of the inequality. Another one is objectivity (in the sense as used in the paper by J.F. Clauser and M.A. Horne, Experimental consequences of objective local theories, Physical Review D10, 526-535 (1974)). The sentence under discussion is referring to the objectivity assumption.WMdeMuynck (talk) 12:04, 2 March 2010 (UTC)

Can you provide an example of a theory which violates Bell inqualities and in which "each particle departs the scene of its "entangled creation" with properties that would unambiguously determine the value of the quality to be subsequently measured"?

The way I read the section of the introduction under discussion here, it appears to be saying that local hidden variable theories can explain quantum entanglement. If this is not the intended meaning, it should at least be rewritten to be clearer.

As an aside, the introduction of the article is already very long, and the paragraph should probably be moved to a separate section in the article. 213.73.197.105 (talk) 16:10, 2 March 2010 (UTC)

Sorry to have made the scope of my remark larger than necessary. Restricting myself to the explanation of strict correlations of measurement results (for instance, of spin observables in a singlet state), there is a historically important choice between two possible explanations of the correlations: on the one hand `von Neumann projection' (the usual explanation given in the context of the Copenhagen interpretation) and `determinism' (assumed by Einstein, Podolsky and Rosen). Even if Einstein's `determinism' is thought to be obsolete, I think the contrast between the two views sufficiently illuminates the difference between quantum and classical mechanics to warrant mentioning here both.WMdeMuynck (talk) 13:41, 3 March 2010 (UTC)

The problem is that in its current form, the paragraph in question appears (to me) to claim that local hidden variables ("each particle departs etc..") can be consistent with known physics (ie. Bell test experiments). I do not understand how you can claim that "[this] version is essentially correct". Again, if I am misunderstanding the article, it should be rewritten. 213.73.197.105 (talk) 16:14, 3 March 2010 (UTC)

What I indeed claim is that no proof exists that all local hidden variables theories satisfy the Bell inequality. However, this talk page is not meant to deal with such discussions. If you are interested in my opinion, please consult my web site, a link to which you can find on my User page.WMdeMuynck (talk) 16:25, 3 March 2010 (UTC)

Then you disagree with the consensus in physics? In that case, the section probably doesn't belong in the article at all, and it certainly doesn't belong in the introduction. 213.73.197.105 (talk) 16:36, 3 March 2010 (UTC)

If Wikipedia were a textbook restricting itself to the consensus of a received view I would agree. However, if it is an encycopedia I think it has to contain alternative views as well, in particular historically and conceptually important ones like Einstein's.WMdeMuynck (talk) 10:10, 4 March 2010 (UTC)

Furthermore, since we will never have seen all possible local hidden variable theories it would probably be impossible to prove that no local hidden variables theory satisfies the Bell inequality. It is important that the article reflect the fact that this is still a live issue. P0M (talk) 00:43, 5 March 2010 (UTC)

I agree the alternative views should be included but they should not be given the same weight as the consensus, when there is a vast majority. The answer here might be an "alternate theories section". This is not to slight alternate theories but to reinforce the idea that wikipedia in general and this article specifically is not the place to debate the validity of the consensus of the scientific community. I will add the section as my last (hopefully) change to this article today.Phancy Physicist (talk) 19:02, 10 June 2010 (UTC)

Muynck is right that contextuality is the key assumption of quantum mechanics that must be violated if any viable hidden variable theory is to be constructed. Where he goes astray from the vast, vast majority of the physics community (as far as I can tell, all of it) is denying nonlocality. The question of realism almost confuses the issue, because the only thing that BT and its experiments say definitively is that the universe is nonlocal (though admittedly definitive is a troublesome word). Ive looked at a couple of his papers, involving rederiving Bell's inequalities without the locality assumption, which is interesting. But there are also infinitely many proofs of Bell's theorem that do not rely on inequalities, as well as a huge body of experimental evidence demonstrating nonlocal effects. Isocliff (talk) 06:43, 28 October 2010 (UTC)

please carefully read article referred to

I have undone the edit because a careful reading of the article referred to shows that the experiment in question relies on classical information transfer. It does not claim to have achieved superluminal teleportation, but just that they have come closer to it because they bridged a larger distance than was done before (which, incidentally, is a dubious claim, see, for instance, W. Tittel, J. Brendel, B. Gisin, T. Herzog, H. Zbinden, and N. Gisin, Experimental demonstration of quantum correlations over more than 10 km, Phys. Rev. A 57, 3229 (1998)).WMdeMuynck (talk) 09:38, 21 May 2010 (UTC)

Layout

I began shaking up the layout for this article. -Phancy Physicist (talk) 14:40, 4 June 2010 (UTC)

Please put your new stuff at the bottom of this discussion page. Otherwise people will ignore it as something that came up a year or so ago. P0M (talk) 17:47, 4 June 2010 (UTC)

I was reading the other posts here and none of them seem to talk about layout which I think is one of the major problems of this article. I am going to call for a clean up of this talk page. Phancy Physicist (talk) 06:09, 5 June 2010 (UTC)

Introduction

I moved around the information in the introduction, some of it to other sections. I don't think the too long tag applies anymore so I removed it. Phancy Physicist (talk) 17:58, 8 June 2010 (UTC)

Background

I think that this section and Concept should be merged in some way. -Phancy Physicist (talk) 14:40, 4 June 2010 (UTC)

Better yet I have changed "Background" to "History" and moved the non-history content to "Concept".Phancy Physicist (talk) 20:49, 10 June 2010 (UTC)

Concept

Mostly material chopped out of the introduction to make it shorter. -Phancy Physicist (talk) 14:40, 4 June 2010 (UTC)

"Speed" of quantum entanglement

I think this section might fit better some where in another section but I haven't figured out where yet -Phancy Physicist (talk) 14:40, 4 June 2010 (UTC)

True. I think as a subsection of concept it'd work better. I'll do that now. Nigholith (talk) 19:12, 8 June 2010 (UTC)

Quantum Mechanical Framework

This technical description is useful but only to those who are looking for it or want to know more. Not necessary for basic understanding. Should be left in the article but at the bottom. -Phancy Physicist (talk) 14:40, 4 June 2010 (UTC)

Recent Changes

I appreciate the energy which which Phancy Physicist has begun to change things around. However I must object to the results. It would appear that s/he has not had much experience with discussion pages.

The ordinary and expected process is to have current matters at the bottom of the page. Recently something from 2004 was moved to this position. The comment from 2004 has been available for about six years now, and has likely received whatever reactions that the participants from then to now are likely to give to it. New contributors are of course free to read the entire body of discussions and to react to old problems by bringing them up again. Typically, to avoid confusion, a new entry will be placed at the end of the discussion page that makes reference to the earlier comment and presents a proposed change to deal with it.

Before doing anything drastic such as reverting this talk page to its original temporal sequence I would like to hear the responses of other contributors. Materials that are candidates for archiving should be moved to the top, not to the bottom. Items of current concern should be at the bottom in temporal sequence. P0M (talk) 20:38, 6 June 2010 (UTC)

I understand what you are saying and I understand the usual protocol of a discussion page. Since this page has not be cleaned in so long I am trying a different approach, in which this current form is not permanent.

I am trying to organize the page into things that are relevant and things that are not. Like it says at the top of the talk page this is not a forum page, but here are a lot of off-topic discussions here. I'm reading though all the posts here and trying to separate them into categories to be eventually archived. It is a page for discussion about improving the article and there are some older posts that are still important and need to be addressed. I don't think just archiving the old posts is the correct manner to do this.

The reason I have not so far followed the strict letter of the law is because the changes I propose are vast and I don't think that they should be done without anyone who wants to comment on the category the posts are in gets a chance to do it. I know that things can be reverted. So rather then have all my changes reverted over an objection to part of it, I would rather allow for piecewise objection.

I just want to make it clear what I was thinking and why I am doing things the way I am. For now, I would ask that rather then revert my changes would you please comment on any of the categorization with which you don't agree. Phancy Physicist (talk) 05:10, 7 June 2010 (UTC)

Is framework section "Too Abstract"?

I understand for those without strong backgrounds in QM and the mathematics that goes with it, that this section probably looks like a different language, but that is the technical nature of the subject. Also all the material above this section is written for a more general audience. I feel that the section could use improvement in some areas but I'm not sure it deserves the "too abstract" tag, if it has a proper warning at the beginning. Any other thoughts on this? Phancy Physicist (talk) 18:22, 8 June 2010 (UTC)

I'd agree, normally the "Framework"et al. section of a physics related subject details its functions in proper and mathematical terms for those with knowledge in the field (As demonstrated to an extreme with the difference between Special relativity and Introduction to special relativity). An introduction section in this case is sufficient in contrast, and thus no need for the abstract tag so far as I can see. Nigholith (talk) 19:03, 8 June 2010 (UTC)

Content without a home.

I cut this from the "Background/History" section because it did not flow well. Now it needs new home.

In addition, experiments are underway to see if entanglement is the result of retrocausality. Quantum mechanics has been highly successful in producing correct experimental predictions, and the strong correlations predicted by the theory of quantum entanglement have now, in fact, been observed.

More orphaned text.

One way to explain these experimental results is an approach known as "local hidden variable theory", in which unknown, shared, local parameters would cause the correlations. Observations pertaining to entangled states appear to conflict with the property of relativity that information cannot be transferred faster than the speed of light. Although two entangled systems appear to interact across large spatial separations, the current state of belief is that no useful information can be transmitted in this way, meaning that causality cannot be violated through entanglement. This is the statement of the no-communication theorem. Even if information cannot be transmitted through entanglement alone, it is believed that it is possible to transmit information using a set of entangled states used in conjunction with a classical information channel. This process is known as quantum teleportation. Despite its name, quantum teleportation may still not permit information to be transmitted faster than light, because a classical information channel is required to complete the process.

Phancy Physicist (talk) 20:57, 10 June 2010 (UTC)

Yet more orphaned text from "Concept" section.

The analysis of entangled particles by means of Bell's theorem can lead to an impression of non-locality, i.e. that there exists a connection between the members of such a pair that defies both classical and relativistic concepts of space and time. This is reasonable if it is assumed that eacThe analysis of entangled particles by means of Bell's theorem can lead to an impression of non-locality, i.e. that there exists a connection between the members of such a pair that defies both classical and relativistic concepts of space and time. This is reasonable if it is assumed that each particle departs the location of the pair's creation in an ambiguous state (thus yet unobserved, as per a possible interpretation of Heisenberg's principle). In such a case, for a given observable quality of the particle, all outcomes remain a possibility and only measurement itself would precipitate a distinct value. As soon as just one of the particles is observed, its entangled pair collapses into the very same state. h particle departs the location of the pair's creation in an ambiguous state (thus yet unobserved, as per a possible interpretation of Heisenberg's principle). In such a case, for a given observable quality of the particle, all outcomes remain a possibility and only measurement itself would precipitate a distinct value. As soon as just one of the particles is observed, its entangled pair collapses into the very same state.

Phancy Physicist (talk) 04:15, 11 June 2010 (UTC)

What Einstein and EPR implied

I believe that the opening sections for "Entanglement" are either reversed or misleading.

Main source: _Age of Entanglement_ by Gilder.

Einstein, Rosen and Podolsky were arguing FOR a Locality Theory that would not contradict Relativity. Bohr appears to be arguing for a Neo-Kantian position that our understanding of Quantum Physics must - by our Categories of Understanding? - be framed in our language that we cannot transcend, mathematical formalism be damned. Over and over it appears that Einstein and Bohr were arguing past each other. They never agreed because they never argued over the same point (at least Bohr didn't...)

IT TURNS OUT that the EPR experiment shows that "Quantum Reality" must be "Non-Local". This is NOT what Einstein sought in his considerations on the subject. Both Kant and the "Arrow Paradox" of Zeno are violated by this "Non-Locality". At the moment particular Predicate(s) for Entangled objects ("Spin", for example) are determined, the Predicate is given to both objects-until-now-entangled, and these ascriptions are given non locally.

Central to this is the "Element of Reality" that EPR claims. If an entangled object has one of its Set Members measured (A statement which has some ambiguous truth value at this point...), a Predicate that can be ascribed to it with certainty, then an "Element of Reality" shall be also ascribed to this Predicate and this Predicate also applies to the "distant" correlated object, now free of its previous entangled state. There is an examination of this given in Gilder's book to this end but the surprise here is that the "Element of Reality" is NOT the considered Predicate that is in the "Element of Reality". It is the Formalism of Entanglement that is "Real". This means that the question of Quantum Physics being "complete" is still not determined. Until the Logic, especially in regards Set Theory, of this EPR experiment is complete, this part of the puzzle is unsolved.

"Quantum Reality" trumps local reality. EPR was not formulated to "Prove" this. Thus, the introduction to this subject is not complete.

Thank you,

Charles

Charles.O.Wilson (talk) 20:31, 20 August 2010 (UTC)

I'm not sure what your issue with the statements about the EPR paradox are but, regardless of the intent of it is formulation, the implications are of EPR are still the same as stated in the introduction. The EPR paradox does not claim the completeness of Quantum Theory only that there are two options. Either non-local interactions exist or the theory is incomplete. It says nothing about completeness if non-local interactions exist. Beyond this I think any interpretation of the EPR should be done on the EPR tak page. Phancy Physicist (talk) 05:12, 22 August 2010 (UTC)

I think the sentence, "The quantum mechanical thought experiment concluded that either nonlocal interaction exists or quantum mechanics is incomplete as a theory", is elliptical at best. The alternative offered after showing that the Copenhagen Interpretation, coupled with Born's "Psi-squared" = A Probability Statement" is incomplete, is that "Yes, Quantum Mechanics is incomplete, but a more complete theory would re-establish a more traditional, causal and LOCAL oriented theory."

The alternative of "Either Non-Local QM or QM is incomplete" as written in the Wiki article came MUCH later. This is not a matter of "Einstein was right" or "Bohr was right". The history appears more straightforward. I merely think that the sentence might be written into 2 sentences showing that the original purpose of EPR was changed at the discovery of Bell's Theorem. Before Bell, the thought was that "When QM is complete, there could be no Non-Local QM" which changed (after Bell) to "Whatever a "Complete QM" means, it must not be a LOCAL QM theory".

CW

Charles.O.Wilson (talk) 17:55, 23 August 2010 (UTC)

I have followed this discussion with some interest. Phancy Physicist is right when thinking that the EPR discussion does not purport to prove that entanglement is due to nonlocal influences in an EPR experiment. In the EPR paper EPR state an alternative explanation: "One could object to this conclusion on the grounds that our criterion of reality is not sufficiently restrictive. Indeed, one would not arrive at our conclusion if one insisted that two or more physical quantities can be regarded as simultaneous elements of reality only when they can be simultaneously measured or predicted. On this point of view, since either one or the other, but not both simultaneously, of the quantities P and Q can be predicted, they are not simultaneously real. This makes the reality of P and Q depend upon the process of measurement carried out on the first system, which does not disturb the second system in any way. No reasonable definition of reality could be expected to permit this."

Unfortunately, EPR associate this alternative explanation once again with nonlocal influences. Today we have a far more convincing argument against the simultaneous reality of P and Q, viz. the Kochen-Specker theorem, basing such non-existence on incompatibility of observables P and Q (which refers to local mutual disturbance of these observables in a simultaneous measurement of these very observables), not basing it on a disturbance of either P or Q by a measurement of an observable of a distant particle (which observables are compatible observables, since belonging to different particles). Note that simultaneous measurement of incompatible observables is possible (and experimentally realized) if inaccuracy of the measurement results is permitted in agreement with the Heisenberg uncertainty relations. Anyway, the notion of entanglement would not be interesting if all observables were compatible (then one single basis would exist for all observables), entanglement being interesting because of non-existence of such a basis. The upshot of these considerations is that already the standard treatment is a simplification because by ignoring the possibility of joint nonideal measurements of incompatible observables the explanation alternative to nonlocality remains out of sight.WMdeMuynck (talk) 19:05, 23 August 2010 (UTC)

So, does anyone else think that the sentence needs to be rewritten? Or is it OK as is?

CW

Charles.O.Wilson (talk) 18:55, 25 August 2010 (UTC)

After reading the comments here, comments over at talk:EPR paradox and the original paper again, I agree that some changes must be made but I'm not sure that what I am thinking would satisfy your objection. I will change the article right now from

"either nonlocal interaction exists or quantum mechanics is incomplete as a theory"

to

"either nonlocal interaction exists or the Copenhagen interpretation of quantum mechanics is incomplete".

This edit will make the article more accurate without dwelling to much on what was intended as a side note in the history of the theory. This more neutral phrasing will also allow this article to be unaffected by the debate over on the EPR talk page.

Phancy Physicist (talk) 11:28, 4 September 2010 (UTC)

I think that your proposal is an excellent one that goes farther in the correct direction than we could have hoped earlier. Good work!

CW Charles.O.Wilson (talk) 22:05, 3 October 2010 (UTC)

I disagree. The Copenhagen interpretation is synonimous with `completeness of quantum mechanics'. Hence, the edit makes it incomprehensible: how can an incomplete interpretation be consistent with a complete theory? What is meant by an incomplete interpretation anyway?WMdeMuynck (talk) 19:28, 4 October 2010 (UTC)

"Spooky action at a distance"

Where in God's name did Einstein ACTUALLY SAY "spooky action at a distance"? It sure as hell isn't in the EPR paper that everyone loves citing.

134.154.212.74 (talk) 06:51, 22 September 2010 (UTC)The Living Alchemist

Please look here for reference 2.WMdeMuynck (talk) 20:28, 22 September 2010 (UTC)

From the source with a link in the above ref 2, Einstein apparently wrote it in German as, "spukhafte Fernwirkungen" which translates as "spooky actions at a distance". Perhaps the quote in its original German form is more believable. --Bob K31416 (talk) 13:33, 24 September 2010 (UTC)

Wolf ticket

There is a big scare notice at the top of the article that has been there for several months: "This article needs attention from an expert on the subject. See the talk page for details."

I don't see any details on this discussion page that indicate what the person up put up that notice was upset about. Am I missing something? On the other hand, the lead of this article now seems to be virtually impenetrable. P0M (talk) 07:06, 5 November 2010 (UTC)

It was I who put this notice; I explained the problem on this talk page, a little time ago. In a nutshell, the summary is confusing and imprecise, and there are conceptual problems in the "concept" section. There is some misunderstanding about the experiment by Gisin et al., and some nonsense about hidden variables (they are a toy model to make a reductio ad absurdum, not an explanation of quantum entanglement!). I will eventually fix it myself if no one steps up. Tercer (talk) 22:17, 5 November 2010 (UTC)

Cause and effect

(122.59.2.149 (talk) 20:36, 6 December 2010 (UTC)) This article starts by saying:

Quantum entanglement, also called the quantum non-local connection, is a property of certain states of a quantum system containing two or more distinct objects, in which the information describing the objects is inextricably linked such that performing a measurement on one immediately alters properties of the other, even when separated at arbitrary distances.

I am confused by the statement "alters properties of the other", as it implies causality. The measurement of one entangled particle reduces the uncertainty about the state of the other particle (to zero). The measurement does need not alter the state of the other particle directly. The states of the two entangled particles are caused by an (unknown) event prior to measurement, not by the measurement itself.

How is this interpretation wrong?

Maybe this (the same here) is a more accurate (but cumbersome) formulation. Boris Tsirelson (talk) 05:51, 7 December 2010 (UTC)

What happened to the openning!?!?

This used to be the opening to this article:

Quantum entanglement, also called the quantum non-local connection, is a property of the quantum mechanical state of a system containing two or more objects, where the objects that make up the system are linked in a way such that one cannot adequately describe the quantum state of a constituent of the system without full mention of its counterparts, even if the individual objects are spatially separated. This interconnection leads to non-classical correlations between observable physical properties of remote systems, often referred to as nonlocal correlations. During the formation of quantum theory, this property of entanglement was recognized as a direct consequence. Quantum entanglement is at the heart of the EPR paradox that was developed by Albert Einstein, Boris Podolsky, and Nathan Rosen in 1935, and was experimentally verified for the first time in 1972 by Stuart Freedman and John Clauser.

Now it is this:

Quantum entanglement, also called the quantum non-local connection, is a property of certain states of a quantum system containing two or more distinct objects, in which the information describing the objects is inextricably linked in such a way that performing a measurement on one immediately alters properties of the other, even when separated at arbitrary distances. Specifically, such a system is said to be in an entangled state if it cannot be written as the tensor product of its constituent subsystems. This discovery posed a serious conceptual challenge to physicists of the day, because faster-than-light influences were assumed to be prohibited by special relativity. In that framework, it was thought superluminal effects would lead to causal contradictions because a change of reference frame can reverse the order of the events. It is now understood that while these nonlocal correlations do occur, they cannot be used to transmit information and thus do not violate causality. Entanglement, which was a recognized consequence of the mathematical formulation of quantum mechanics during its inception, eventually stimulated the discovery of the EPR paradox by Albert Einstein, Boris Podolsky and Nathan Rosen in 1935. This thought experiment, while drawing an incorrect conclusion, helped elucidate the relationship between relativity and quantum theory, and set the stage for John S. Bell to finally resolve the contradiction in 1964 through Bell's theorem. Entanglement has since been verified by a series of experiments beginning in 1972 by Stuart Freedman and John Clauser.

I think that one can describe the idea of Quantum entanglement without talking about tensor products and superluminal effects. I don't propose that the original lead is prefect but I think we took a step backwards here.

Phancy Physicist (talk) 05:57, 8 December 2010 (UTC)

How about this?

Quantum entanglement, also called the quantum non-local connection, is a property of the quantum mechanical state of a system containing two or more objects, where the objects that make up the system are linked in such a way that one cannot adequately describe the quantum state of any member of the system without full mention of the other members of the system, even if the individual objects are spatially separated. Quantum entanglement is at the heart of the EPR paradox that was developed by Albert Einstein, Boris Podolsky, and Nathan Rosen in 1935, and it was experimentally verified for the first time in 1972 by Stuart Freedman and John Clauser.

It is short, to the point and gets a cross the idea of quantum entanglement without going into the details. The rest of the information can be put into the article itself if it is needed.

Phancy Physicist (talk) 06:12, 8 December 2010 (UTC)

I agree that this is better. At least it does not say the misleading "immediately alters properties of the other". Boris Tsirelson (talk) 06:26, 8 December 2010 (UTC)

I agree also. I would only change the sentence "also called the quantum non-local connection", because I've never seen it called that way. Also, it suggests that entanglement is the same thing as nonlocality, which is far from true. A name that is occasionally used and can be illuminating is quantum inseparability. Tercer (talk) 22:07, 8 December 2010 (UTC)

Added in the proposed lead. Let's see how it goes.

Phancy Physicist (talk) 00:38, 10 December 2010 (UTC)

Comments on history section

The last edit by Tercer could be made somewhat more precise by changing its first sentence in the following way: "Entanglement is a consequence of the superposition principle. It is a strikingly non-classical property of quantum mechanics if it is applied to particles rather than waves, that arguably led to an unease with the theory by many of the early scientists."WMdeMuynck (talk) 10:33, 9 December 2010 (UTC)

I wasn't trying to be technically precise, since this is the History section. But are you implying that there exists a phenomenon analogous to entanglement in classical waves? I'd certainly like to know that. Tercer (talk) 13:24, 9 December 2010 (UTC)

In Nielsen and Chuang, p.~96 we read: "We say that a state of a composite system having this property (that it can't be written as a product of states of its component systems) is an entangled state." Here states are represented by state vectors or wave functions. By the superposition principle entangled states are superpositions of product states. Such superpositions can be found in the classical theory of interacting wave fields (like acousto-optics), in which any interaction would yield an entangled state if starting from a non-entangled one. However, I guess that often a stronger notion of entanglement is implied than given by Nielsen and Chuang, viz. in which there are different ways to represent one and the same entangled state (like is the case in the EPR problem). In that case incompatibility of observables is an essential element, which, of course, does not have a classical analogue.WMdeMuynck (talk) 14:26, 11 December 2010 (UTC)

You use a strange language. Nielsen's definition is only sufficient for the particular case of pure states. The general definition, for mixed states, is the one given in this article. I suppose that's what you mean when you say "different ways to represent the same state". It's true that there are infinite convex combinations of pure states that give out the same mixed state; but the density matrix is unique, and that's the representation. There's no essential difference between the pure and mixed cases. In both, what gives rise to entanglement is the tensor product rule for combining quantum systems, in contrast of the direct sum used in classical systems. Tercer (talk) 01:34, 14 December 2010 (UTC)

The language I use is the one used by Schrödinger in his 1935 papers (in Naturwissenschaften), in which he coined the word `Verschränkung' which was later translated as `entanglement'. This language is taken over in the book by Nielsen and Chuang I referred to. I suppose that when you say that "The general definition, for mixed states, is the one given in this article." you refer to a definition of `entanglement' to the effect that `violation of the Bell inequality' is necessary and sufficient for `entanglement'. Unfortunately this definition does not work out as is seen from the existence of (entangled) Werner states for which the Bell inequalities are satisfied.WMdeMuynck (talk) 14:02, 14 December 2010 (UTC)

Allow me to cut and paste:

Extending the definition of separability from the pure case, we say that a mixed state is separable if it can be written as $\rho =\sum _{i}p_{i}\rho _{i}^{A}\otimes \rho _{i}^{B},$ where $\rho _{i}^{A}$'s and $\rho _{i}^{B}$'s are themselves states on the subsystems A and B respectively.

This is the definition I refer to. Tercer (talk) 15:11, 14 December 2010 (UTC)

Thank you. Seems fine with me.WMdeMuynck (talk) 22:13, 15 December 2010 (UTC)

For my opinion, entanglement is (or at least, can and should be) formulated in general terms not mentioning particles, waves, strings, tables and other specific kinds of objects. It is a strikingly non-classical property of quantum phenomena (or predictions, if not quite confirmed yet), without reservations like "if it is applied to particles rather than waves". Boris Tsirelson (talk) 16:15, 9 December 2010 (UTC)

Rewrite of "Concept" section.

I removed the following sentence:

The assumption that measurement in effect "creates" the state of the measured quality goes back to the arguments of Einstein, Podolsky, and Rosen...

because unless I missed something one of Einstein's problems with QM was the every idea that this sentence states. To me it reads like they were arguing for this property.

The bottom three paragraphs still need some attention. Phancy Physicist (talk) 19:51, 7 June 2010 (UTC)

Agreed. But I would be more blunt: the bottom three paragraphs are completely wrong and need to be rewritten. I do have the necessary expertise (grad student in quantum information) and references, but I'm a bit out of time right now; I will do the work in a month or so. For now, I'll just add the expert attention template. Tercer (talk) 17:54, 25 June 2010 (UTC)

The first paragraph of this section — introduced on 19 December — does not address the subject of the section ("concept" rather than measurement). It also has problems with syntax and clarity. I recommend deletion but would defer to others more familiar with the topic. Ontyx (talk) 10:48, 22 December 2010 (UTC)

I agree. P0M (talk) 23:55, 22 December 2010 (UTC)

I deleted it. Tercer (talk) 19:32, 5 January 2011 (UTC)

I rewrote some of the section. I hope that now the concept of entanglement is clear for a laymen. Unfortunately, the end of the section was a mixture of misunderstanding, off topic things and hogwash, so I just deleted it. I think Gisin's experiment does not belong in this article, but I'd like to write something about hidden variables. I just don't know where. Any suggestions? Tercer (talk) 19:32, 5 January 2011 (UTC)

I removed the "attention needed" template. I still think that the article is disorganised and confused, but at least there's nothing wrong in it anymore. Tercer (talk) 19:16, 31 January 2011 (UTC)

I think it is incorrect to write Two particles are entangled when they become fundamentally indistinguishable from each other. - even distinguishable particles may be entangled and indistinguishable particles may nevertheless be separable. Rather, the proper concept is (as written in the introduction of the whole article) that the properties of two particles cannot be described independently of each other (but only a joint description is possible), i.e. I would replace the first sentence of this section by s.th. like "Two particles are entangled if they cannot be described independently (nor by a statistical mixture of independent particles)." --Qcomp (talk) 16:42, 14 March 2011 (UTC)

Sorry, I missed this comment until just now because it's somewhat lost in the middle of this Talk page. I'll copy it to it's own section at the bottom of the page, and will address it there. -- J-Wiki (talk) 02:19, 23 March 2011 (UTC)

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