Quantum entanglement -> retro/FTL communication?

[Sidhe]

I've been assured by somebody who used to teach QM that there exists a proof that entanglement cannot be used to for FTL information. Yet Ayatollah's variation of the experiment seems to contradict that. There has to be an answer one way or another. Either you can transmit information FTL, or something will prevent such an experiment from working as I expect.

To Mise's question I am more apt to accept a no answer is known response.

I see yes well there is an answer to that, Bell's inequality touches on that as do various Alice and Bob type thought experiments, google quantum entanglement. essentially there is still a classic means of information transfer so that FTL is not possible in quantum entanglement.

There is also an answer to the quantum eraser which shows how it tallies with QM and CI: see below.

The original experiment is obviously bunk and had it of proved retro-causality we would know about it, it's a bit like P.T.Barnum putting on a show. That's why that is unanswerable, because it's ludicrous science

Here's something that explains it

Spoiler :

Referring to the first set of photons as "signal photons" and the second set as "idler photons", the way it works is that the total pattern of signal photons actually never shows interference--even if you measure all the idlers in such a way that the which-path information is erased, you will see interference if you do a "coincidence count" between signal photons and idlers which went to a certain detector, but if you add all the subsets, they add up to a non-interference pattern. So, it's impossible to find an interference pattern until after the idlers have already been measured, ruling out the possibility of any backwards-in-time hijinx.

Even in the case of the normal delayed choice quantum eraser set up where the which-path information is erased, the total pattern of photons on the screen does not show any interference, it's only when you look at the subset of signal photons matched with idler photons that ended up in a particular detector that you see an interference pattern. For reference, look at the diagram of the set up in Fig:2 at the bottom of this analysis:

http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm

In this figure, pairs of entangled photons are emitted by one of two atoms at different positions, A and B. The signal photons move to the right on the diagram, and are detected at D0--you can think of the two atoms as corresponding to the two slits in the double-slit experiment, while D0 corresponds to the screen. Meanwhile, the idler photons move to the left on the diagram. If the idler is detected at D3, then you know that it came from atom A, and thus that the signal photon came from there also; so when you look at the subset of trials where the idler was detected at D3, you will not see any interference in the distribution of positions where the signal photon was detected at D0, just as you see no interference on the screen in the double-slit experiment when you measure which slit the particle went through. Likewise, if the idler is detected at D4, then you know both it and the signal photon came from atom B, and you won't see any interference in the signal photon's distribution. But if the idler is detected at either D1 or D2, then this is equally consistent with a path where it came from atom A and was reflected by the beam-splitter BSA or a path where it came from atom B and was reflected from beam-splitter BSB, thus you have no information about which atom the signal photon came from and will get interference in the signal photon's distribution, just like in the double-slit experiment when you don't measure which slit the particle came through. Note that if you removed the beam-splitters BSA and BSB you could guarantee that the idler would be detected at D3 or D4 and thus that the path of the signal photon would be known; likewise, if you replaced the beam-splitters BSA and BSB with mirrors, then you could guarantee that the idler would be detected at D1 or D2 and thus that the path of the signal photon would be unknown. By making the distances large enough you could even choose whether to make sure the idlers go to D3&D4 or to go to D1&D2 after you have already observed the position that the signal photon was detected, so in this sense you have the choice whether or not to retroactively "erase" your opportunity to know which atom the signal photon came from, after the signal photon's position has already been detected.

This confused me for a while since it seemed like this would imply your later choice determines whether or not you observe interference in the signal photons earlier, until I got into a discussion about it online and someone showed me the "trick". In the same paper, look at the graphs in Fig. 3 and Fig. 4, Fig. 3 showing the interference pattern in the signal photons in the subset of cases where the idler was detected at D1, and Fig. 4 showing the interference pattern in the signal photons in the subset of cases where the idler was detected at D2 (the two cases where the idler's 'which-path' information is lost). They do both show interference, but if you line the graphs up you see that the peaks of one interference pattern line up with the troughs of the other--so the "trick" here is that if you add the two patterns together, you get a non-interference pattern just like if the idlers had ended up at D3 or D4. This means that even if you did replace the beam-splitters BSA and BSB with mirrors, guaranteeing that the idlers would always be detected at D1 or D2 and that their which-path information would always be erased, you still wouldn't see any interference in the total pattern of the signal photons; only after the idlers have been detected at D1 or D2, and you look at the subset of signal photons whose corresponding idlers were detected at one or the other, do you see any kind of interference.

Still doesn't answer the fundamentally odd nature of photons, but it does explain the quantum erasor experiment is consistent with QM and CI.

 

[Ayatollah So]

There is also an answer to the quantum eraser which shows how it tallies with QM and CI: see below.

Aha, that really is a trick. So, the use of the coincidence detector is (horrible pun coming up) no coincidence. It's essential. No FTL communication, case closed.

 

[warpus]

The wave function colapses when we have information, when we erase that information then the wave function reappears, just as CI maintains. So the act of erasing the info or measurement is what influences decoherence, you take away the effect of that measurement by erasing the information you get a wavelike pattern again, ala CI, also postulated by the man who first thought up the delayed two slit thought experiment. And confirmed by experiment 30 years later.

Can you elaborate on that? So you perform your measurement and the wave collapses.. I get that part.. How do you 'erase' this measurement to get the particple to convert back into a probability wave?

Remember that all the quantum physics I know is self-taught.. but I am a math major, and all the stuff so far in this thread makes sense.. but that one detail I am not too sure about.

 

[Sidhe]

Can you elaborate on that? So you perform your measurement and the wave collapses.. I get that part.. How do you 'erase' this measurement to get the particple to convert back into a probability wave?

Remember that all the quantum physics I know is self-taught.. but I am a math major, and all the stuff so far in this thread makes sense.. but that one detail I am not too sure about.

Slightly misleading that passage, probably because I was a little hazy on the details myself, until I read the above spoiler, sorry that's a bit of a fudge, I'd ignore that passage and just take the above stuff as what is happening k. Have you read the spoiler above? It clears it all up and explains how in fact that they all collapse precisely when there meant to, according to sub FTL laws of QM?

What I should have made clear is that all the results in the experiment are the result of decoherence, so there is no mysterious event happening in retrocausality as each result shows the only difficulty is looking at the last figure which when the two patterns are combined show a decoherence event, the results indicate that any collapse happens precisely when it is meant to and strictly according to sub FTL principals.

 

[Mise]

<cool shizzle>

It STILL doesn't solve our original problem: How does the 1st photon know that the 2nd photon will, at some point, be detected?

1) The yellow (and green) line(s) show that the which-path information is known by the 1st photon at the point of detection, since, if we did not reconcile our results with detectors D1 and D2, we would see bullet-like curves (the green/yellow lines).
2) The blue and red lines show that which path information is not known by photon 1, hence the interference patterns.

Common sense tells us that (1) and (2) are contradictory and cannot both be true at the same time; QM tells us that they can (and are) both true at the same time, since the photon goes through (A or B) and (A and B) at the same time. Reality agrees with QM.

It STILL, however, says nothing about the CI

EDIT: Actually, I suppose the green/yellow line is indeed a "superposition of the two possible states" (red and blue). Okay, so it is explained by the CI

EDIT2: Actually..... it should be the other way around, surely? A superposition of two bullet-like curves yields an interference pattern? I.e. the which-path information is present in the no-path interference pattern, and by reconciling which-path information with no-path information, we reconstruct the bullet-like pattern; this experiment shows it to be the other way around -- we use the no-path information to reconstruct the which-path pattern. In other words, you shouldn't get a bullet-like pattern at D0; that we do see a bullet-like pattern is purely coincidental, and simply falls out from the maths (which I can't understand... I'll have a look at it).

EDIT3: Unless...... The which-path information still contains the no-path information (i.e. not the other way round), and the eraser does in fact erase the which path information, leaving only the no-path information. Okay, geddit now. The which-path pattern actually contains more information than the no-path pattern... Makes sense.

 

[Sidhe]

It STILL doesn't solve our original problem: How does the 1st photon know that the 2nd photon will, at some point, be detected?

1) The yellow (and green) line(s) show that the which-path information is known by the 1st photon at the point of detection, since, if we did not reconcile our results with detectors D1 and D2, we would see bullet-like curves (the green/yellow lines).
2) The blue and red lines show that which path information is not known by photon 1, hence the interference patterns.

Common sense tells us that (1) and (2) are contradictory and cannot both be true at the same time; QM tells us that they can (and are) both true at the same time, since the photon goes through (A or B) and (A and B) at the same time. Reality agrees with QM.

It STILL, however, says nothing about CI's assertion of superposition and when it says by x we decohere again that is another principal of CI

Being as the two slit as used as basic proof of CI or the superposition principals, and this is basically a complicated 2 slit experiment, I think it says something don't you? When it uses words as if it was passing through both slits etc, it's talking about CI's superposition of single photons.

It doesn't? Well since the graphs when superimposed show decoherence I think you'll find it does show precisely that.

They cancel each other out, notice the peaks and troughs, if you compare them they show a decoherence.

This confused me for a while since it seemed like this would imply your later choice determines whether or not you observe interference in the signal photons earlier, until I got into a discussion about it online and someone showed me the "trick". In the same paper, look at the graphs in Fig. 3 and Fig. 4, Fig. 3 showing the interference pattern in the signal photons in the subset of cases where the idler was detected at D1, and Fig. 4 showing the interference pattern in the signal photons in the subset of cases where the idler was detected at D2 (the two cases where the idler's 'which-path' information is lost). They do both show interference, but if you line the graphs up you see that the peaks of one interference pattern line up with the troughs of the other--so the "trick" here is that if you add the two patterns together, you get a non-interference pattern just like if the idlers had ended up at D3 or D4. This means that even if you did replace the beam-splitters BSA and BSB with mirrors, guaranteeing that the idlers would always be detected at D1 or D2 and that their which-path information would always be erased, you still wouldn't see any interference in the total pattern of the signal photons; only after the idlers have been detected at D1 or D2, and you look at the subset of signal photons whose corresponding idlers were detected at one or the other, do you see any kind of interference.

 

[Mise]

Being as the two slit as used as basic proof of CI or the superposition principals, and this is basically a complicated 2 slit experiment, I think it says something don't you?

It does, because the CI was made before the consequences of the CI were investigated (i.e. this experiment). That says to me that a proof of QM is NOT a proof of the CI. Just because the CI explains the contemporary QM'al results does not mean that the CI and QM are identical... So please stop using proofs of QM as proofs of the CI. You actually have to go into the details of the CI and see how it applies in EACH QM'al experiment/result to prove that the CI explains the QM for that specific case. Thus far, you haven't done this. My edit a few posts up attempted to do this, but I don't know how true it is...

It doesn't? Well since the graphs when superimposed show decoherence I think you'll find it does show precisely that.

No they don't, they show the green line in the graph I posted above.

They cancel each other out, notice the peaks and troughs, if you compare them they show a decoherence.

They don't "cancel out". It's got nothing to do with "cancelling out". The detection rate from R01 + R02 + R03 + R04 should add up to the detection rate from D0. That's what I've done in the graph that I posted (omitting the contribution from R03 and R04, as their form is identical to the form of the superposition of R01 and R02, so it would not change the form of the superposition).

Oh, and it STILL doesn't address the problem of future knowledge of detection

 

[Sidhe]

The first slit experiment was done by young back in the 19th century I think about 1860. And again another experiment was done back in Einsteins day and lead to a postulate that a single photon would perhaps interfere with itself, thus CI came about based on the duality of light and the photo electric effect. The later someone actually managed to show in 1960 I believe that a single photon does in fact interefere with itself in a two slit experiment, thus proving CI, to be at least inferably true, then someone complicated the two slit by suggesting a delay, then someone complicated the two slit further by introducing an eraser aspect, and hey presto again it showed the same as the original two slit in 1916 or so. Thus confirming that the current theories are true. And also simultaneously agreeing with the superposition/particle wave duality idea postulated by Bohr the mac daddy of CI. I'm not saying it's the big I am of CI, but it certainly verifies the idea that we cannot view what is happening to a photon because by doing so we decohere it, which is fundamentally what CI is all about.

Anyway I think basically we're in agreement apart from some technical issues and a little bit of semantics. I had a great deal of difficulty with this myself until I read that particular discussion.


http://en.wikipedia.org/wiki/Copenhagen_interpretation

All versions of the Copenhagen interpretation include at least a formal or methodological version of wave function collapse.[4], in which unobserved eigenvalues are removed from further consideration. (In other words, Copenhagenists have never rejected collapse, even in the early days of quantum physics, in the way that many worlds adherents do).

The existence of collapse as an objective process, with obvious implications about the reality of the wave function, is more contentious.

It is maintained by some[5] that the concept of collapse of a "real" wave function was introduced by John Von Neumann and was not part of the original formulation of the Copenhagen Interpretation[citation needed]).

Double Slit Diffraction - Light passes through double slits and onto a screen resulting in a diffraction pattern. Is light a particle or a wave?

The Copenhagen Interpretation: Light is neither. A particular experiment can demonstrate particle (photon) or wave properties, but not both at the same time (Bohr's Complementary Principle).

The same experiment can in theory be performed with electrons, protons, atoms, molecules, viruses, bacteria, cats, humans, elephants and planets. In practice it has been performed for light, electrons, buckminsterfullerene, and some atoms. Matter in general exhibits both particle and wave behaviors.

Oh, and it STILL doesn't address the problem of future knowledge of detection

Actually that's precisely what it does or am I missing something? I think it shows clearly that there is nothing spooky or FTL going on. And also that there is no future knowledge.

 

[Mise]

Right.... so when does the first particle's wavefunction collapse?

 

[Sidhe]

Right.... so when does the first particle's wavefunction collapse?

You read this? But did you understand what it means about no FTL? Or backwards in time hi-jinx?

Referring to the first set of photons as "signal photons" and the second set as "idler photons", the way it works is that the total pattern of signal photons actually never shows interference--even if you measure all the idlers in such a way that the which-path information is erased, you will see interference if you do a "coincidence count" between signal photons and idlers which went to a certain detector, but if you add all the subsets, they add up to a non-interference pattern. So, it's impossible to find an interference pattern until after the idlers have already been measured, ruling out the possibility of any backwards-in-time hijinx.

Even in the case of the normal delayed choice quantum eraser set up where the which-path information is erased, the total pattern of photons on the screen does not show any interference, it's only when you look at the subset of signal photons matched with idler photons that ended up in a particular detector that you see an interference pattern. For reference, look at the diagram of the set up in Fig:2 at the bottom of this analysis:

You did read this bit right and also the bit at the bottom.

This confused me for a while since it seemed like this would imply your later choice determines whether or not you observe interference in the signal photons earlier, until I got into a discussion about it online and someone showed me the "trick". In the same paper, look at the graphs in Fig. 3 and Fig. 4, Fig. 3 showing the interference pattern in the signal photons in the subset of cases where the idler was detected at D1, and Fig. 4 showing the interference pattern in the signal photons in the subset of cases where the idler was detected at D2 (the two cases where the idler's 'which-path' information is lost). They do both show interference, but if you line the graphs up you see that the peaks of one interference pattern line up with the troughs of the other--so the "trick" here is that if you add the two patterns together, you get a non-interference pattern just like if the idlers had ended up at D3 or D4. This means that even if you did replace the beam-splitters BSA and BSB with mirrors, guaranteeing that the idlers would always be detected at D1 or D2 and that their which-path information would always be erased, you still wouldn't see any interference in the total pattern of the signal photons; only after the idlers have been detected at D1 or D2, and you look at the subset of signal photons whose corresponding idlers were detected at one or the other, do you see any kind of interference.

Clearly this shows that only when you look at the non idlers you don't see interference as the graphs cancel. And you get a non interference pattern to in the corresponding idlers, in other words your getting accordance with the idler and other photon each time. No faster than light shizzle, just each time seeing what eventually you end up seeing. Nothing to see here, just plain crazy old QM.

In other words, the results precisely tally each time with idler and non-idler.

 

[Mise]

1) In what way does that explain when the wavefunction collapses?

2) As I've said before, it's a superposition. That means they add up. Nothing "cancels". The interference information is still there. And even if it wasn't, it would PROVE that the photon knows that it will be detected. Read through it all again, pay close attention to the graphs and what they are actually showing.

 

[Sidhe]

1) In what way does that explain when the wavefunction collapses?

2) As I've said before, it's a superposition. That means they add up. Nothing "cancels".

wavefunction collapses when you detect it, and this is confirmed by both entangled photons matching each others results. If it decoheres, you get a pattern of decoherence result, if it doesn't you get interference pattern, precisely as QM predicts, that's essentially what the whole thing says, the idlers results always exactly match the non idlers results. There is nothing funny going on. I'm not sure how I can make it clearer than that post does.

Look essentially at the point of decoherence you get something that appears to be decoherence, then at the end you get an interference pattern, in actual fact, your just seeing what you should see.

When you see the information from both, you see precisely what you should see in both cases and at that time, do you see? The time when it becomes apparent is not retrocausal it's merely when all the results are in.

This is bloody hard to explain.

Besides that post says it better than I ever could.

 

[Mise]

In other words, the results precisely tally each time with idler and non-idler.

Yes they do, but they show which-path information, not no-path information, which goes against everything you're saying...

 

[Mise]

wavefunction collapses when you detect it, and this is confirmed by both entangled photons matching each others results. If it decoheres, you get a pattern of decoherence result, if it doesn't you get interference pattern, precisely as QM predicts, that's essentially what the whole thing says, the idlers results always exactly match the non idlers results. There is nothing funny going on. I'm not sure how I can make it clearer than that post does.

Okay, so the first photon's wavefunction collapses when it hits D0. But that results in which-path information showing up on the detector (green line on the graph I posted). That means that it knows that its path will be observed, before it is observed. Hence the future problem...

Read edit 3 in the post with the graph in it.

 

[Sidhe]

Read this below again your not understanding when we know decoherence or non decoherence we know this at time 4 and only if we know it's position does it decohere, and this agrees at time 4, and only if we have no position information does it not, and we observe interference at time 4 just as we do if we don't know it's position. The information is consistent at any point in the experiment. It decoheres exactly when it is meant to and appears to agree at time 4 and vice a versa. At time 6 all results are consistent with all information.

Yes they do, but they show which-path information, not no-path information, which goes against everything you're saying...

No they show which path at time 4, ie if we know position then it shows decoherence then,then if it shows the converse no path then at time 4 we see interference. Accordingly time 6 shows that no path = interference, path = decoherence at the time we obtain this information at time 4. No FTL or retrocausality is observed only what we expect.

Comment: To the physicist, the results "are all consistent with prediction." To the layperson, the results should be shocking. Let us review the course of the experiment as it unfolds, beginning when the incoming photon from the laser generates an entangled pair at the crystal.

Time 1. The entangled pair leaves either region A or region B of the crystal. The signal photon heads off to detector D0, and the idler photon heads off to the interferometer.

Time 2. The signal photon is registered and scanned at detector D0 according to its position. This information (the position of the signal photon upon "impact" at D0) is sent on its way to the Coincidence Circuit.

Time 3. The idler photon reaches the first pair of beamsplitters, BSA, BSB. There, QM makes a choice which direction the idler photon will go &#8211; either to detectors D3, D4; or to the quantum eraser BS and on to detectors D1, D2.

Time 4a. If the idler photon is shunted to detectors D3, D4, it is detected with which-path information intact. Then and only then do we know which-path information for its twin signal photon that already has been detected, scanned, registered and recorded at D0.

Time 4b. If the idler photon passes through to detectors D1, D2, it is detected with no which-path information (the which-path information having been "erased" at BS).

Time 5. The Coincidence Circuit correlates the arrival of a signal photon at detector D0 with the arrival of its twin at D1, D2, D3, or D4. If the correlation is with an idler arriving at D3 or D4, then we know (after-the-fact) the which-path information of the signal photon that arrived earlier at D0. If the correlation is with an idler arriving at D1 or D2, then we have no which-path information for the signal photon that arrived earlier at D0.

Time 6. Upon accessing the information gathered by the Coincidence Circuit, we the observer are shocked to learn that the pattern shown by the positions registered at D0 at Time 2 depends entirely on the information gathered later at Time 4 and available to us at the conclusion of the experiment.

The position of a photon at detector D0 has been registered and scanned. Yet the actual position of the photon arriving at D0 will be at one place if we later learn more information; and the actual position will be at another place if we do not.

Ho-hum. Another experimental proof of QM. This is the way it works, folks.

 

[Mise]

Sidhe, what is on the detector D0 at time 2? ...

That's the crux of the matter. If which-path information has not been observed, it would look like the yellow line in my graph. If which path information has been observed, it would look like the yellow line again, only shifted up somewhat (the peak in the middle would be twice as high, the form roughly the same). The text that you posted previously (referring to an online discussion) implies that it would look like the green line. The green curve implies which-path information, the yellow curve implies no-path information. D0 should contain both no-path information and which-path information (yellow curve shifted up somewhat), and the coincidence detector extracts the which-path information from the D0 curve by correlating results from D1-D4. The text that you quoted most recently says nothing about what would be seen on D0 at time 2 - it simply repeats the experiment. Which one do you agree with? Perhaps it would help if you sketched the curve that you think would be seen on D0?

Also, you keep mentioning decoherence, but the CI doesn't refer to decoherence at all, only general wavefunction collapse (which is related but not the same) -- to be clear, are you using decoherence to mean wavefunction collapse?

 

[Sidhe]

Sidhe, what is on the detector D0 at time 2? ...

That's the crux of the matter. If which-path information has not been observed, it would look like the yellow line in my graph. If which path information has been observed, it would look like the yellow line again, only shifted up somewhat (the peak in the middle would be twice as high, the form roughly the same). The text that you posted previously (referring to an online discussion) implies that it would look like the green line. The green curve implies which-path information, the yellow curve implies no-path information. D0 should contain both no-path information and which-path information (yellow curve shifted up somewhat), and the coincidence detector extracts the which-path information from the D0 curve by correlating results from D1-D4. The text that you quoted most recently says nothing about what would be seen on D0 at time 2 - it simply repeats the experiment. Which one do you agree with? Perhaps it would help if you sketched the curve that you think would be seen on D0?

Also, you keep mentioning decoherence, but the CI doesn't refer to decoherence at all, only general wavefunction collapse (which is related but not the same) -- to be clear, are you using decoherence to mean wavefunction collapse?

Obviously by decoherence I mean wave function collapse, usually when talking about CI the terms are the same, in fact I don't know of any case we're decoherence means something else but I'd be intrigued to find out what?

In this case obviously and I question your assertions based on your graph, if indeed it is true fine but we should either see what path information, that agrees at time 4 or no path information that agrees at time 4, if that is not the case then the whole physics world is mistaken and you may have a point, but I'm going to err on the side of the scientists that assert that any given time at t0 t2 t4 and t6 what you see is what you get. If not then you have found an immense flaw in QM. I am not qualified to discuss your assertions, only what is accepted science. If your correct you should be taking this to the science community, not me, don't forget I'm not even a first year physics student yet, just a semi laymen, I suggest you take this question to a physics forum, where I suspect you will have more luck.

To be honest all that green line looks like to me is the bullet peak, or a peak that you get from decoherence.

An r1 and r2 simply are path information patterns that equate to decohesion IIRC.

That's the point in the discussion and that is stolen directly from a physics forum, the guy in question is pretty well qualified I can tell you that much. So take it up with him, I can give you the address of the forum by pm if you'd like?

EDIT: and that's a nifty graph package what is it?

 

[Mise]

Obviously by decoherence I mean wave function collapse, usually when talking about CI the terms are the same, in fact I don't know of any case we're decoherence means something else but I'd be intrigued to find out what?

Well, decoherence is a mathematical framework that defines a method via which wavefunction can happen. In the CI, the wavefunction is explicitly not real (i.e. its a mathematical tool). Decoherence doesn't require the wavefunction to be abstract, nor real (i.e. it can apply to both the CI or MWI). The terms aren't strictly the same, as decoherence in CI is a (possible) method for wavefunction collapse, whereas the CI doesn't specify any method. I just wanted to make sure that we were on the same page when I referred to wavefunction collapse and you referred to decoherence.

In this case obviously and I question your graph, if indeed it is true fine but we should either see what path information, that agrees at time 4 or no path information that agrees at time 4, if that is not the case then the whole physics world is mistaken and you may have a point, but I'm going to err on the side of the scientists that assert that any given time at t0 t2 t4 and t6 what you see is what you get. If not then you have found an emmense flaw in QM. I am not qualified to discuss your graph, only what is accepted science. If your correct you should be taking this to the science community, not me, don't forget I'm not even a first year physics student just a semi laymen, I suggest you take this question to a physics forum, where I suspect you will have more luck.

To be honest all that green line looks like to me is the bullet peak, or a peak that you get from decoherence.

An r1 and r2 simply are path information interference patterns that equate to decohesion IIRC.

EDIT: and that's a nifty graph package what is it?

Excel

It would probably help at this stage to explain what the graph I posted is, and how I constructed it.

I wanted to know what the output on detector D0 would look like. To do this, I supposed that the sum of the outputs at detectors D1,2,3 and 4 would be the same as the output at D0 (all idler particles have a corresponding signal particle and are all counted).

The blue and red lines are the joint detection rates from R01 and R02 respectively. Basically, I looked at the graphs from the report and read off the datapoints, stuck them into excel, and plotted them.

The yellow line is the sum of the blue and red lines, i.e. the joint detection rate from R01 + the joint detection rate from R02. This is what the text you quoted previously (referring to an online discussion) was talking about. The text said that if you add the two curves up, you get a bullet-like pattern. The yellow line is clearly not a bullet-like pattern -- and how can it be a bullet-like pattern! -- It's simply a sum of two sinc functions, which yields another sinc function... NOT a gaussian, as per a bullet-like distribution. The author clearly expected a gaussian (the green trendline, which he incorrectly assumed was "more correct" than the yellow line), as he goes on to say that this proves that which-path information is detected at D0, and the interference patterns only come out when you correlate the D0 data with D1 and D2 data.

There are two things wrong with this:
1) The sum is clearly not a gaussian, and cannot be, since it is the sum of two sinc functions.
2) Even if the sum WAS a gaussian, this proves which-path information was detected at D0, regardless of whether which-path information was observed at D1 or D2, which means that the signal particle knew that it was going to be detected.

Now, the full pattern at D0 should be the sum of the joint detection rates for each of the other detectors, i.e. R01 + R02 + R03 + R04. This will yield a sinc function (from R01 + R02) + a gaussian (R03 + R04). This has an unusual form (i.e. it is neither an interference pattern, nor a bullet-like pattern, but a superposition of the two), and, naturally, contains both which-path information and no-path information.

The question remains: Why do we see this combined pattern, and not either/or? How do the bullet-like pattern's particles know that they will be detected? How do the interference pattern's particles know that they won't be detected? Why don't we see this pattern when we observe the interference pattern on a screen?

 

[Mise]

Wait.... scrap all that.... the sum of two sinc functions can in fact equal a gaussian.... The green line probably IS "more correct".

Well that's crazy. I need to think.

EDIT: Okay, back to EDIT3.... this leaves us with the question:

Why don't we see a bullet-like gaussian on a screen in the fourier plane?

Okay... another shot at this... can anyone tell me what a sinc + a gaussian would look like? Is it a sinc function? Because if it is, then that explains EVERYTHING! Woo!

I'm pretty sure that a sinc + a gaussian is a sinc... which is what you would see on D0. This is also what you would see on a screen in the fourier plane. So what you see on a screen in the fourier plane contains which-path information! You just have no way of extracting it. What the delayed eraser thingo does is enable you to extract it, using entanglement. Cool.

 

[Sidhe]

Well, decoherence is a mathematical framework that defines a method via which wavefunction can happen. In the CI, the wavefunction is explicitly not real (i.e. its a mathematical tool). Decoherence doesn't require the wavefunction to be abstract, nor real (i.e. it can apply to both the CI or MWI). The terms aren't strictly the same, as decoherence in CI is a (possible) method for wavefunction collapse, whereas the CI doesn't specify any method. I just wanted to make sure that we were on the same page when I referred to wavefunction collapse and you referred to decoherence.

Ok that's clear, I guess I should of specified it was context specific.

Excel

Ah I don't have Excel. Explains it.

I'm guessing you've answered your own question? Talking about a Gaussian as in a Gaussian function? Mathematically you are looking at a wave function collapse yes? Or are you trying to establish that a wave function mathematically is a real picture of what is happening "visibly", which we know it is not?

The thin green line

To me though if you look at the yellow line it's equivalent numerically to the green line as an integral, but I maybe wrong here?

It certainly doesn't look like an interference pattern.