Talk:Delayed-choice quantum eraser/Archive 1

Rich.lewis 22:14, 10 April 2006 (UTC)

Untitled
I disagree with part in the Discussion section about no usable information traveling backward in time is both unnecessary and wrong. As described on bottomlayer.com in the basic delayed choice(http://www.bottomlayer.com/bottom/basic_delayed_choice.htm) and delayed choice quantum eraser (the experiment performed by Kim, et al. http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm), the "delayed choice" does change the result of the observation on the primary detector. It changes the results of an observation made in the past, that is the whole point.


 * It doesn't change the past observations, just our ability to interpret them. See the section 'Why this is not an ansible". DMPalmer (talk) 19:28, 1 March 2008 (UTC)

I think that the most interesting result of this experiment, also as described on bottomlayer.com, is this shows the underlying flaws in the copenhagen explanation of the double slit experiment. As Wheeler observed, an experimenter may choose to know a property after the event should already have taken place. In other words, as described in the bottomlayer.com explanation, "the observer's choice would determine the outcome of the experiment – regardless of whether the outcome should logically have been determined long ago." Furthermore, Wheeler's thought experiment has been experimentally verified by the results from Kim, et al.

So, I think that comment about causality not being violated takes away from the point of this experiment. Wheeler was disturbed by the fact that causality seemed to be violated by the Copenhagen interpretation, and Kim, et al. have shown experimentally that this is true. It is a fundamental paradox that has never been resolved. Copenhagen said that he himself did not understand it and anyone who said they did was a liar.

Discussion section comments
A few points:


 * The experiment at bottomlayer.com has not, to my knowledge, been carried out. So any expected results from the experiment is speculation and shouldn't be treated as facts in a Wikipedia article.


 * Traveling faster than the speed of light and going back in time are, IMO, essentially the same thing.


 * The delayed choice experiment that was carried out, as is all such experiments of this nature, requires a coincidence detector, that must match (using time coincidence) the detection of the reference particle to it's "twin" slit passing particle. You can, in principle, put the dectection (and choice) of the reference particle millions of light years away in a distance galaxy, but information (or confirmation) about exactly when the particle was detected (to match the corresponding slit particle), and thus which choice was made, can only travel at the speed of light (and will still take millions of years). Thus, DCQE cannot be used for faster then light communications, or to violate causation.


 * Quantum mechanics predicted the strange effects of DCQE, long before the experiment was actually carried out. The Copenhagen interpretation gave an interpretation to exactly what QM was predicting. I don't quite see how the experimental results of DCQE, confirming QM, could contradict the Copenhagen interpretation.

My response
Rich.lewis 22:14, 10 April 2006 (UTC)
 * The experimental setup I was refering to was the delayed choice quantum eraser, which was carried out by Kim et al., included in the references, and the bottom layer has a sort of layman's interpretation which points out some of the paradoxes illustrated by their results. (http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm)


 * The detection of the "twin" particle can be delayed an arbitrary amount of time without putting the secondary detector in a far away galaxy, instead, use a mirror to reflect the twin particle back to a detector near the experimental setup. You could also slow down the twin particle in a number of ways, for example by using electrons instead of photons and slowing the twin by passing it through a decelerating electric field.  This is tricky but in principle can be done, my point being that the detection of the twin particle does not necesrily take place at a distance separated relativisticly from the primary detector.


 * Technically the copenhangen interpretation does predict this bizzare result, which while it is not actually time travel does allow classical information to be transmitted backwards in time. This information could be used to create a paradox.  The fact that the copenhagen interpretation predicts a paradox seems to me to be a contradiction, and by predicting a contradiction this means it is logically false.

If you think about it, the two slit experiment itself boggles the mind. How can something be both a particle and a wave? How can a single "particle" pass through both slits in the experiment to create the interference pattern? Maybe, there is no particle, just a wave. The whole idea of wave particle duality is nonsense. The wave function is real, the particle is not. What we experience as reality is only one possible superposition of states of the wave function, but that is beacuse our experience of reality is limited to only experience one or the other superpositions. In fact, the superpositions always exist, and the wave function does not collapse. The wave function is real, our percepetion of the wave function as a single particle at any one time is an illusion, or more accurately a limitation of our perception. Otherwise, we would have information flowing backwards in time, paradoxes, and logical contradictions.

PS I changed some of my original comments that were not clear. The point I am trying to get at is that the delayed choice qunatum eraser experiment demonstrates a very spooky paradox in QM.

My reply
--Lostart 19:53, 11 April 2006 (UTC)


 * I didn't read very much of the bottom layer article so I can't comment on it directly. I would only say that while there certainly seems to be a lot of paradoxes surrounding QM (and in particular, DCQE), it has nowhere near been settled as to whether the paradoxes as actual, or simply apparent, stemming from our human--thus biased--view of reality. Until the question is settled (if it ever can be), I think it's approprate to present this (in a Wiki article) as active speculation, but not as being an established fact.


 * I do know it's quite posssible (in fact it's been done) to slow a particle, even a photon, even to the point of stopping it for an indefinite period of time. But I'm not at all sure that QM allows you to to use that with entangled particles in such a way that create a causality paradox.

Keep in mind that it is impossible to predict (or determine) the path of each individual particle (one slit or both) using the entangled pair. An individual particle can be detected anywhere on the screen whether you force the particle through one slit or both slits. You need a large collection of particles to determine the pattern. You can (in principle) collect all the particles from the screen, while "holding" all the reference particles until you make the decision (a minute or a year later) about how to read them. But the data you collected on the screen is worthless until you do the second part of the experiment, and use the coincidence detector to corrolate the particles, and let you see the outcome (ie the pattern). You still don't get to see the result until after you make the decision.

I don't have much formal experience in the mathematics of QM, but I'm not aware of anyone doing the mathematical formalism (ie formal proof) of the above experiment, or published a paper establishing that QM allows such causal parodox. If you know of such a published paper please reference it. My guess is doing the formalism would reveal just how QM makes it impossible, perhaps from the Hiesenberg uncertainly principle (ie keeping the particle so long makes it's location so uncertain as to make it impossible to match it with it's twin).


 * The Copenhangen Interpretation (and QM) may predict an apparent paradox/contridiction; not necessarily a real one.

rlewis reply
Rich.lewis 16:42, 14 April 2006 (UTC)

I certainly agree with your comments above, and I'd like to point out that what really shocked me was this experiment in fact has been done, by Kim, et al, who I referenced but probably not very clearly since I buried the link in the text of the article. (http://xxx.lanl.gov/pdf/quant-ph/9903047 Yoon-Ho Kim, R. Yu, S.P. Kulik, Y.H. Shih, and Marlon O. Scully Phys.Rev.Lett. 84 1-5 (2000))  It is true that a single particle could not be used to effectively communicate information over time, you need many observations to construct a pattern which either shows interference (wave like behavior) or no interference (particle like behavior). However, I propose you could construct many identicle experimental aparatuses (sp?) and conduct many observations simulataneously, then actually combine the observations into a single pattern to show particle or wave behavior.

While it is also true that it is fundamentally impossible to predict (or determine) the path of each individual particle (one slit or both), which would amount to knowing both the position and momentum of the particle. However, using exactly this setup to essentially transmit information back in time, on could in essence observe the behavior of a particle after the fact and then send the information back in time to before the experiment was performed, essentially predicting the behavior of each individual particle. This would be so abhorent to the laws of QM that it must in fact be impossible to do this. But, the outcome of the DCQE experiment seems to sugest this may be possible.

Kim et al make some very interesting comments in the introduction to their paper which I will quote here:

"This paper reports a “delayed choice quantum eraser” experiment proposed by Scully and Dr¨uhl in 1982. The experimental results demonstrated the possibility of simultaneously observing both particle-like and wave-like behavior of a quantum via quantum entanglement. The which-path or both-path information of a quantum can be erased or marked by its entangled twin even after the registration of the quantum."

and in the following section...

"Complementarity, perhaps the most basic principle of quantum mechanics, distinguishes the world of quantum phenomena from the realm of classical physics. Quantum mechanically, one can never expect to measure both precise position and momentum of a quantum at the same time. It is prohibited. We say that the quantum observables “position” and “momentum” are “complementary” because the precise knowledge of the position (momentum) implies that all possible outcomes of measuring the momentum (position) are equally probable. In 1927, Niels Bohr illustrated complementarity with “wave-like” and “particle-like” attributes of a quantum mechanical object [1]. Since then, complementarity is often super- ficially identified with “wave-particle duality of matter”. Over the years the two-slit interference experiment has been emphasized as a good example of the enforcement of complementarity. Feynman, discussing the two-slit experiment, noted that this wave-particle dual behavior contains the basic mystery of quantum mechanics [2]. The actual mechanisms that enforce complementarity vary from one experimental situation to another. In the two-slit experiment, the common “wisdom” is that the position-momentum uncertainty relation �x�p ≥ ¯h 2 makes it impossible to determine which slit the photon (or electron) passes through without at the same time disturbing the photon (or electron) enough to destroy the interference pattern. However, it has been proven [3] that under certain circumstances this common interpretation may not be true. In 1982, Scully and Dr¨uhl found a way around this position-momentum uncertainty obstacle and proposed a quantum eraser to obtain which-path or particle-like information without scattering or �Permanent Address: Department of Physics, Moscow State University, Moscow, Russia otherwise introducing large uncontrolled phase factors to disturb the interference. To be sure the interference pattern disappears when which-path information is obtained. But it reappears when we erase (quantum erasure) the which-path information [3,4]. Since 1982, quantum eraser behavior has been reported in several experiments [5]; however, the original scheme has not been fully demonstrated."

But I agree these speculations are not established fact and should not be included in the main article as such. My real objection was to the assertion that it would be impossible to transmit usable, classical information backwards using a DCQE device. I believe this may be possible and remains to be shown by experimental observation if this could in fact be done.

I once saw a program on QM where they were describing how photons can appear to move faster than the speed of light through qunatum tunneling. First, on physicist explained how of course no classical information could actually be TRANSMITED faster than the speed of light, since qunatum uncertainty randomizes the particles and destroys any signal they may carry. The next segment had another scientist who had infact set up a simple experiment to show a microwave signal going through an intervening object, by quantum tunneling, and he had connected the device to a CD player playing classical music. He could then play back the signal (which was the result of photons using quantum tunneling to travel faster than light) and there was distorted but recognizable music coming out the other end. The conclusion was that obviously information CAN be transmitted via quantum tunneling. Scientists often revert to these kinds of "arm waving" arguments to describe what they believe to be an obvious conclusion which may in fact be false.

Article Cleanup (Fact vs. Fiction)
The article as it stands presents wildly speculative information regarding the ramifications of the Delayed Choice Quantum Eraser Experiment to which there is neither empirical nor theoretical evidence.

Whether or not the Delayed “Choice” Quantum Eraser Letter in the Physical Review supports the idea that in this experiment an outcome "in the past" is changed can be debated no more than is possible in Wheeler's original delayed choice experiment. Theoretically the implications are no different for a particle to have entered both slits and then be consigned to one than the probability of a particle's state at a point (in this case a detector) being altered due to delayed observation based on the path of an entangled particle.

The discussion portion of this article should be removed entirely. Its tendency towards science fiction detracts from the overall credibility of the article and falls prey to an unscientific interpretation of QM that has lately become popular. The statement that "quantum mechanics does not seem to have much of a problem with time travel" is incorrect. The Bohm interpretation regarding entanglement was shown recently by Polkinghorne to act more as two singers who sound out of tune unless heard together than two people instantaneously transmitting information. Thus preventing the thin use of the Bohm interpretation to permit time travel. Moreover, QM prevents an outsider from interacting in such a transfer of information, luckily we need not worry as QM prevents the possibility of such a transfer in the first place.

Separating science fact from science fiction is an arduous task. The majority of the media today can rarely tell one from another and supposing too much from antiquated theories and media interpretation can be disastrous. The discussion section of the article page goes far beyond any information presented in the Physical Review Letter and has no proof for the suppositions put forward.

I propose that in order to its scientific reliability this article must present no more information than can be logically derived from the published Letter, unless sufficient proof can be brought forward.


 * Your proposal seems reasonable--go ahead and make the changes you consider necessary. I would recommend not removing the whole discussion section, but just take out the portions that are wildly speculative and/or scientifically inaccurate, and preferably replace them with a better way of saying it.--that's is just my recommendation, of course. --Lostart 16:54, 16 May 2006 (UTC)


 * I'd also like to point out that widely held speculation would not be inappropriate in this (or any Wiki article), as long as it's stated clearly as such, as it adds only adds to information about the subject to the reader. IMO --Lostart 17:03, 16 May 2006 (UTC)
 * I would not describe the discussion section as wildly speculative, or science fiction. I'm not sure what parts you object to, but I made an attempt simply to paraphrase the conclusions from Kim et al. (which I quoted above).  It was not my intent to go far beyond the information presented in that article.  If you object to the statement that QM does not have a problem with time travel, very similar arguments are made by Feynman in his book, QED.  At one point he describes an antiphoton as a photon moving backwards in time, and states that in fact that is exactly what it is.  I agree with this description.  What I really meant is that QM does not produce an "arrow of time."  You can not easily distinguish, from a simple qunatum interaction, the direction in which time is flowing.  Higher level phenomena, for example entropy and the 2nd law of thermodynamics, only emerge when you consider the behavior of large numbers of particles.  If you object to my description of the results of this experiment as "time travel", I meant that changing the outcome of an event, which is the detection of the "first" photon on the "primary" detector, by choosing to observe or not observe the "second" entangd photon (the delayed choice) was analagous to time travel.  This is the central paradox.  I chose to call it time travel and I think that IS an appropriate description of the experimental result. I certainly did NOT mean literally travelling back in time.  I simply meant the outcome of the observation at the primary detector is aparently changed by the action of the "delayed choice" AFTER the photon has already been detected.  That seems like time travel to me.


 * Frankly, I think the results of Scully and Druhl (1982) and Kim et al. (2000) are highly provocative. The paradox which they present, of "apparent" time travel, is analagous to the apparently impossible observation that the speed of light was contant and was NOT relative to the observer's frame of reference.  The puzzling out of that paradox resulted in Eintstein's theory of relativity.  I believe that a similar reconcilition of the delayed choice paradox may result in a revolutionary discovery in QM.  This is why I think it is appropriate to describe this result as time travel.  In a sense that is what is happening.  Of course this is impossible, so some important piece of the puzzle must be missing.  I just think the result of this experiment is important, and describing the paradoxical observations as a kind time travel is appropriate.  Rich.lewis 21:33, 1 June 2006 (UTC)

Looks like Alain Aspect's Orsay team reachs the same conclusions
No need to argue about that : this reference mentions everything there is to know about their own experiment :

Just click here and read

81.64.199.60 13:17, 2 November 2006 (UTC)

Clarification please
To a novice, this section is confusing:

"First, generate a photon and pass it through a double slit apparatus. After the photon goes through slit A or B, a special crystal (one at each slit) uses ..."

I was confused by the significane of the first double slit. And, sure enough, on reading the commentary in http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm I find that:

"(This double slit is a bit of a red herring. It is only a method of randomizing emissions from region A or region B of the crystal, which are themselves the equivalent of the two slits of Young's experiment.)

I was going to try to change the page text to make that "red-herring"-ness clearer (it's what I suspected when I first read the page). However, I really don't know enough to make the change safely. But I think some clarification would help people like me (who are, presumably, the core audience for a non-technical exposition like this). —The preceding unsigned comment was added by 66.69.240.110 (talk) 10:30, 7 February 2007 (UTC).


 * I don't think it is necessary to burden the article with this detail. The original article said that they used a single crystal. If they had just shined a laser on the single crystal they wouldn't have gotten two neat beams through it. P0M (talk) 09:13, 12 December 2007 (UTC)

Revert
Calling legitimate edits vandalism is not appropriate, even if you happen to disagree, nor are wholesale reverts (which includes grammatical corrections) without explanation. My edits were honest attempts to improve the readability and focus of the article. If you disagree with the edits, justify each revert in the talk section, preferably editing the reverts youself, and sign your comments. --Lostart (talk) 04:30, 26 November 2007 (UTC)

Something seems missing in the article
I've found a good image of the original drawing, one that shows D4, and have made an SVG image to go with this article. The article does not as yet explain the function of all the beam-splitters and how they "erase" the path knowledge. P0M (talk) 09:08, 12 December 2007 (UTC)

One function of the BBO, which makes it useful to divide the left-slit path from the right-slit path, is that it polarizes the light differently. The Glen-Thompson prism directs the differently polarized light in diverging directions. Some of the discussions I have found mention the polarization issue, but I haven't found anything very helpful yet. It would be good to know what kind of beam splitters are used. Some beam splitters are polarizing. P0M (talk) 16:32, 12 December 2007 (UTC)

And something is really strange in the experiment
Each photon then interferes only with itself. Interference between two different photons never occurs. —Paul Dirac



The laser pumps out one photon. On the other side of the dual slits this photon disappears and is replaced by two new, entangled, photons, which then go off on different trajectories. (It isn't clear from the published materials why they do so. The diagrams make it appear that the entangled photons are generated moving 90 degrees apart.) It is one new photon that then enters the lens and is directed to a detector screen where it either interferes with itself or doesn't interfere with itself. If BBO were replaced by an analogous photon emitter such as another laser or just an LED then it would not ever produce an interference fringe. For there to be an interference fringe, a "something" has to originate in the alternate spot on the BBO, and it has to be "the same photon," at least according to Dirac.

It looks as though the original photon and the derivative pair of photons have to be entangled themselves. In other words, the derivative photons each have "inherited" the characteristics of the original wave pattern that was simultaneously generated at the exit ports of the two slits. So, in the diagram, the red line going into the lens has to represent something "real," and the blue line going into the lens has to represent something that is equally "real." The successfully interfere despite the fact that their sources are in a photon in one case and the absent photon in the other case. (I just mean that "the photon" that has been so painstakingly contrived to be generated and labeled by polarity near the output port of the double slits would presumably not exist in two instances. And its own progressively expanding wave front ought to proceed from where it is created, not for there and from a second point that is historically related but physically discrete.)

If we follow the method of Huygens, we can trace through the process without reference to the supposed photon with a presumed trajectory. The wavefront moves out of the laser (at the left end of the diagram), encounters the barrier wall, is propagated simultaneously through the two slits, and radiates with diffraction from both slits. Depending on how near the BBO is to the barrier wall, the two wave fronts might or might not be superimposed within the BBO. Regardless, the wave front "collapses" and -- then what? The paper (Kim, et al.) says that a pair of entangled photons are the result.

If we work backward from D0, probability waves have been superimposed and a photon has flashed at a point of high probability. Tracing the probability waves back, how do we account for the second wave? Do the both come from one output port of the BBO? In that case we could presumably block one output port of the BBO and still get interference fringes. Does one come from the second output port? If so, why? Presumably nothing is happening there. But the wave front that came out of the second slit propagated to that point until it collapsed.

If we follow the progress of dual wavefronts through the prism, then there is the logical possibility that they follow mirror courses. In that case if one wavefront propagates to D4 then its mirror version will propagate to D3. This dual wave could collapse in either detector. If the dual wavefronts go through the first two beam splitters, they could end up in D1 or D2. But they could also end up in the same detectors but by symmetrically being reflected or passed by the third beam splitter.

Symmetry seems to be very important in this experiment. If the two wavefronts do not behave symmetrically, one wavefront could end up in D2 and its double could end up in D2 as well. So it would be interesting to check those two detectors for interference fringes.

Was detector D4 so easily just sketched in early on and later omitted entirely because the authors assumed that what was detected by D4 could be symmetrical to what was detected by D3

Are any of these considerations discussed anywhere? P0M (talk) 03:47, 14 December 2007 (UTC)

Problematical assertion
However, it should be noted that an interference pattern can only be observed after the idlers have been detected, and the experimenter plots only the subset of signal photons that are matched with idlers that went to a particular detector such as D1.

I've removed the above assertion for discussion for the following reasons:

I don't think that the first half of this assertion is correct. The article by Kim et al. gives a specific time lag between the firing of D0 and the arrival of the entangled twin photon at one of the detectors from D1 to D4. The coincidence counter lists out matches between the detector for the signal photon and one of the four detectors that pick up the idler photon. Their arrivals are not synonymous.

Moreover, it is not clear from anything that I have read elsewhere that a signal photon might be received but that the corresponding idler photon would yet be lost somehow. Presumably the entire apparatus is shielded from extraneous light sources too. The statement I have removed insinuates that there is a significant subset of measured/observed signal photons that are not matched by measured/observed idler photons.

At the very least there should be an explanation of whether there are too many signal photons or too many idler photons, why this is believed to happen, and a citation provided for each of these assertions if they are to be reinserted into the article. P0M (talk) 08:31, 15 December 2007 (UTC)


 * The assertion you removed here, which I have restored, was not intended to imply that there a significant number of idlers for which the signal photon is not detected at all (I have no idea if this is true or not...I also did not intend to imply that the arrival of the signal and idler were 'synonymous', I don't actually understand what you mean by that). Rather, it's just pointing out that if you look at the total pattern of signal photons at D0, some of them belong to signal-idler pairs where the idler went to D1, some belong to pairs where the idler went to D2, some belong to pairs where the idler went to D3, and some belong to pairs where the idler went to D4. It is only when you look at the subset of signal photons whose corresponding idler went to D1, or the subset of signal photons whose corresponding idler went to D2, that you see an interference pattern. But if you add these two subsets together to get the subset of signal photos whose idlers went to either D1 or D2, then since the peaks of the D0/D1 interference pattern line up with the troughs of the D0/D2 interference pattern (as explained in a sentence shortly after the one you quoted), no interference will be observed in this larger subset. Likewise, the D0/D3 subset and the D0/D4 subset show no interference, so if you look at the total pattern of signal photons whose idlers went to any of the four detectors, this group does not show interference either. I provided a link to a book by Brian Greene where he states that the total pattern of signal photons won't show interference, and this is also illustrated in the description of the experiment here. Hypnosifl (talk) 04:38, 9 March 2008 (UTC)

Polarization


Take a look at this diagram. Where are issues of polarization discussed? It will not do simply to ignore the issue. It would appear that if the lower half of the apparatus were simply taken out then interference fringes could never be observed in the photons that go through the upper paths. The reason is that the BBO crystal gives opposite polarizations to photons that emerge from its two output ports. Other eraser experiments are explicitly built on using polarization to "mark" the paths of photons. (See the Scientific American "do it yourself" article, for instance.) [The article by Jacques et al. (see below) makes the use of opposing polarizations to prevent interference explicit. (Note added P0M (talk) 07:59, 19 December 2007 (UTC))]

The photons that emerge from the BBO are entangled, so changing the polarization of one "twin" will change the polarization in the other one. The apparatus succeeds in producing interference fringes in the upper part of the apparatus because of the tricky way the reflections (which change polarizations) are provided for in the bottom half. If a photon does not get recorded at either D1 or D4, it goes through a triple reflection vs. double reflection process depending on whether it is identified with slit A or with slit B. The net result is that in the end polarizations are not crossed. The changes in polarization work on the entangled photon too, so its polarizations are not crossed and it can interfere with itself.

This part of the experiment is so obvious and so important to the physical process involved that it must have been discussed in published materials by now. Where?

The business about reflections altering polarization is discussed in Sears Optics, pp.171ff. P0M (talk) 06:33, 17 December 2007 (UTC)

Explicit use of polarization is discussed in Quantum eraser experiment.P0M (talk) —Preceding comment was added at 08:45, 17 December 2007 (UTC)

Explicit use of polarization is also mentioned in Experimental realization of Wheeler's delayed-choice GedankenExperiment, by Jaques, et al. P0M (talk) 07:56, 19 December 2007 (UTC)

Ungrounded assertions
The last two paragraphs of the "Discussion" section may be correct, but as written they seems to stand as original research. The passages are not really clear enough to be of greatest benefit to the average well-informed reader. It would be most helpful if whoever wrote them would back them up with references and cite those references in the article. P0M (talk) 09:12, 19 December 2007 (UTC)

Diagrams
The diagrams were definitely needed for this article; I'm glad they got added. A couple of suggestions:

-The small diagrams in the intro; they are hard to read, could that be made bigger (I mean the ones embedding in the text)?

-The main diagram is good, but complicated. I think it would be hard for a reader to conceptualize the experiment from this diagram. I think adding a simplified diagram that shows the experiment on an idealized/conceptual level only would be good.--Lostart (talk) 03:47, 26 December 2007 (UTC)


 * I'm learning how to do SVG and just figured out how to crop images. I just re-uploaded the first diagram. When it works its way through the system it should look better. It's always possible to change the "px" to a higher value.


 * The main diagram has to be that complicated, I think. It's impossible to see how impossible the results are unless you can see what is going on. It might be helpful to add in some lines to show the tricky way the light is made to bounce around in the lower path.


 * If you will check out the article you may agree with me that it leaves many things unsaid. Supplying a simplified diagram could obscure the tricky parts that need to be understood to avoid having readers draw false conclusions. P0M (talk) 04:42, 26 December 2007 (UTC)


 * The main diagram has to show the BBO to establish the presence of the entangled photons. It has to show all of the detectors, and it has to show all of the mirrors and beam splitters because they are what makes it possible to mix the "red path" and the "blue path" stuff going into d-1 and d-2. The only things that could be taken out would be the prism and the coincidence counters, but they don't really make the diagram harder to follow.


 * The thing that is confusing are the two paths shown in the following diagram by the dotted lines.




 * Would this diagram be better for the article? P0M (talk) 05:38, 26 December 2007 (UTC)

Actually I was suggesting ADDING another diagram (with some explanation about it, perhaps in a different section), NOT replacing the existing one. I agree the existing detailed diagram is important to show the actual experiment. Having both a conceptual simple diagram and the detailed one would aid in in the understanding for an uninitiated reader, IMO.--Lostart (talk) 17:02, 26 December 2007 (UTC)
 * How about this diagram then. The experiment is tricky to begin with, and the article about the Kim experiment obfuscates the fact that if they just chopped off the lower half of the apparatus the upper part (to detector 0) would not form an interference pattern because the polarities are opposed. So what I have done is make a schematic by taking out the parts that explain how they actually detect interference and replacing that part with drawings of interference patterns in the detectors, showing the polarities graphically, and using purple arrows to show each time the polarities get switched. The first time I looked at the experiment it made no sense. It sounded like "the magic of human observation." But when I followed it out step by step it became clear that the apparatus is complicated because it takes unequal numbers of reflections to get the polarizations going the same way when they enter detectors one and two. The incomprehensible part, to me, is what that would fix the polarization of whatever goes into detector zero before the corrections can happen in the bottom half.  But that's the quantum mystery factor.

Here is my diagram:

Now I see that I need one more "reflection changes polarity" label, and the text size in the lower left is wrong. And my arrow heads are gone now. Rats. (Inkscape is not perfect.)


 * Fixed that part but now Wikipedia is misinterpreting the SVG image and putting in a big black block in the text -- something that isn't there in the original and isn't even in the SVG if I download it from Commons and run it through Adobe or Inkscape. Maybe I'll just remove the text block and see what happens. P0M (talk) 06:38, 27 December 2007 (UTC)
 * fixed P0M (talk) 07:21, 26 January 2008 (UTC)

The only thing that is not essential to understanding what is going on is the prism, but it doesn't require any thinking on the part of the reader and taking it out would mean redrawing the rest of the diagram to make it appear that the beams diverge magically to go where the apparatus demands that they go. I guess I could remove the first two beam splitters, but that might cause readers to lose their orientation to the more complete diagram, and you don't have to really think about them either. P0M (talk) 05:48, 27 December 2007 (UTC)


 * A diagrammatically much simpler experiment is described in Quantum_eraser_experiment. The tricks with polarity are there, but the time sequence (delayed choice) maneuvers are left out. P0M (talk) 07:48, 27 December 2007 (UTC)

Removed two paragraphs
I have removed the following two paragraphs:

It might initially seem that the "choice" to observe or erase the which-path information of the idler can change the position where the signal photon is recorded on the detector, even after it should have already been recorded. However, as noted above, the total pattern of signal photons never shows interference, and it is only when one looks at a subset of signal photons whose idlers were seen at a particular detector that an interference pattern can be recovered.
 * Now I think I see what this paragraph is trying to convey. If the coincidence counter were taken out of the apparatus and the detection of photons at the four different detectors in the idler path were permitted to continue through a large series of trials, then the experimenters would get a mixture of interfering and non-interfering photons, and there would not be any way to sort them out. But the intention behind provision of the coincidence counter is so that the position of each arriving photon can be mapped to one of four records depending on which detector its twin shows up in. The idea of the experiment is that the apparatus itself randomly delivers idler photons to the several detectors, they arrive wherever they arrive, and when their arrival point is recorded it is found to correspond to the arrival point of the photon in the upper limb of the apparatus -- even though there is a deliberate temporal disparity between the two events.
 * A viewer of a remote "signal photon" detector would randomly receive either "particle" hits or "wave" hits, but s/he wouldn't have any way to learn (except later) whether a hit corresponded to a hit on idler path detector 1, 2, 3, or 4. But the original apparatus was not designed to send a message. It is the equivalent of a geiger counter acting as a telegraph key. To send a message, the apparatus would need to be redesigned so that there would be a real telegraph key that would cause the incoming photons to merge their paths or constrain them from maintaining their paths, therefore arranging for either self-interfering detection of a photon or the delivery of a photon at detectors at the ends of well-separated paths.P0M (talk) 03:45, 14 February 2008 (UTC)


 * I was the one who originally wrote this paragraph, and my meaning was slightly different than what you suggest above. I could have been clearer, but I was referring to the notion that it is possible to control whether the idlers go to a detector that erases their which-path information (D1 or D2) or to a detector that preserves the which-path information (D3 or D4)--to make sure they go to D1 or D2 you can replace two of the beam-splitters with mirrors, and to make sure they go to D3 or D4 you can remove the beam-splitters entirely. Thus, after you have already observed the complete pattern of signal photons at D0, you can either make the choice to erase the which-path information of all the idlers, or preserve the which-path information of all the idlers. One might naively think that this would retroactively determine whether or not the total pattern of signal photons shows interference, allowing you to send information back in time, but it turns out that the total pattern of signal photons never shows interference, even in the case where every single idler has its which-path information erased by being sent to D1 or D2. Interference will be seen in the subset of signal photons whose idler goes to D1, and also in the subset of signal photons whose idler goes to D2, but as noted in the text I re-added to the last paragraph in the section "The experiment", the peaks of the D1/D0 interference pattern line up with the troughs of the D2/D0 interference pattern and vice versa, so that their sum shows no interference.


 * Anyway, if you think that paragraph was unclear I don't object to it being deleted, but I think the article does require some explanation of the fact that the total pattern of signal photons will never show interference and thus you can't use the experiment to send information back in time, since this seems to be a common confusion. Hopefully the section I re-added to the last paragraph in "The experiment" will be acceptable to the other editors of this article. Hypnosifl (talk) 22:46, 8 March 2008 (UTC)

Thus, the experiment would certainly not allow one to send a message back in time, and whether the experiment requires any sort of backwards causality to understand it would depend on one's interpretation of quantum mechanics. The transactional interpretation would interpret the results in terms of genuine backwards causality, but other interpretations such as the Copenhagen interpretation, the Bohm interpretation and the many-worlds interpretation would predict the same experimental results without the need for backwards causality.


 * The first part is a non sequiter as far as I can see. The rest of this paragraph is dogmatic. The experiment is intended to show, and if it has been reliably reproduced, does indeed show that there is a determined relationship between what happens in the two limbs of the experiment. The Copenhagen interpretation generally follows the "rule" that one should not say anything about what one cannot observe. One observes the correlation between "hits" in the two arms of the apparatus at different times. On several criteria it would seems that the two events ought to be separate events, so it is upsetting to our common sense ideas when it turns out that they are not separated after all even though there is no physical connection at the times when photons are detected. Logically, it makes as much sense to me to say that the way the signal photon shows up in the uppermost detector determines which of the four detectors the idler photon shows up in one of the four lower detectors. Our habits of thought suggest to us, however, that the several beam splitters in the idler paths "do something to the photon," and that in turn "does something" to the signal photon.  But that idea is at war with our idea of temporal sequence in causation. Bohm would presumably like some hidden variable to be set the same way for both photons and therefore to determine the relatedness of the outcomes. The "many worlds" interpretation would suggest the creation of one universe for each possible different outcome of each run of the experiment, but it would not account at all for the relatedness of the observed outcomes. Or, to put it another way, there presumably would never be a universe created in which the signal photon arrived in a "particle way" and the idler photon did not, and the same for "wave way" arrivals. P0M (talk) 03:34, 14 February 2008 (UTC)

I don't think thismaterial is relevant, and much of it is very unclear. P0M (talk) 05:44, 26 December 2007 (UTC)


 * Unclear, perhaps, but I would still consider it relevant. I'm just reading about this phenomenon for the first time today. Sending messages back in time was one of the first things that came to mind. Suppose that the idler path was a light year long, similar to what was mentioned at the very top of the talk page. At the end of that light-year-long path, let's say there's an astronaut at an observation post who's waiting to tell us, back on Earth, whether he sees a rogue planet headed on a collision course or something like that. If he sees one on January 1st 2100, he removes the prism for the idler path that let enabled us to detect which course the photons are taking. As a result, as of January 1st 2099, the interference pattern observed at D0 (which is with us on Earth) will change so that it no longer show any hints of a delayed choice being made. Thus we would know that one year in the future, our astronaut friend sees a rogue planet.


 * Dr. John Cramer wrote an article, in Analog I think, in which he suggests a sort of cosmic telegraph operated by (in your terms) moving the prism in and out.


 * At least, that's what this experiment would seem to indicate as a possibility. It doesn't seem like it ought to be possible, but I don't know how to reject that possibility without rejecting the findings of this experiment. That's why I think the material in the 2 paragraphs you mentioned is still important, though a rewrite's definitely needed - I really need the explanation of why sending information to the past is impossible, or my brain's likely to explode. 205.175.225.22 (talk) 23:10, 7 February 2008 (UTC)


 * "Sending information to the past is impossible" is a deduction from theory. To me that stands as a dogmatic statement. The most one should say is that such an event is inconsistent with theories that have shown great utility and have been very well substantiated.


 * There are discussions in Greene's two books on this subject that you might find helpful. Or, put it this way, if it turned out to be possible to send information to the past then lots of complications would ensue. "I'm my own grandpa" might even turn out to work.  People don't like to deal with "impossibilities."


 * The basic idea of what Greene says is that "temporal sequence" is intimately connected with the total probability of a sequence of events. It is possible that one day I throw my eraser at the blackboard at a certain angle, it strikes the board and is deflected into the eraser tray and from there it slides all the way down to the far end of the tray where it grinds to a stop. But so far it has only happened once in my lifetime, so I think it is not very likely. It is very likely that if I open a bottle of pills and sharply strike the bottom of the bottle with a rubber mallet then many of the pills will come flying out of the bottle more-or-less together and go flying all over the room. It is highly unlikely that even one pill thrown back along the original trajectory (even intending to get it into the bottle) would actually arrive there, and getting 100 pills to "unscatter" themselves and pile neatly into the bottle because of the very way they jostled each other on the way toward the bottle would have an extremely low probability. Just as pills have a higher probability of random dispersion, so does energy. If a ball lands in sand its momentum is dispersed into the pile. It is highly unlikely that random movements in the sand would send a surge of energy toward the rock and fling it back the way it came.


 * It is possible, according to George Gamow, that all the air molecules in a room should at the same time all line up pointing due north, and suddenly one part of the room would be in vacuum. (That wouldn't last long, of course.) The probability of that happening would be the product of the probabilities of each molecule to head north. There being a very large number of molecules in an ordinary room under earth normal conditions, the probability of a sudden vacuum developing is low enough that none of us should wear a pressure suit to bed.


 * If we look at very simple systems we may get an idea of what it would be like to "go back in time." What we would need to do would be to arrange a sequence of events that returned upon itself. We have a model for such sequences of events in biological clocks. They can consist of a sequence of chemical reactions that proceed in a circular way. Another model is a kind of circuit used in computers to continually repeat the sending of certain signals to various components on the motherboard. Turning on the computer causes a single chatter-free pulse to be originated. It is delivered to computer chip one, which sends out several signals, one of which is delivered to computer chip two, two does the same thing, delivering a signal to chip three, and (usually after a couple more chips are involved) the last chip in the chain sends its signal back to chip one -- which wouldn't do anything else if it didn't get this signal because the only reason it did anything to begin with was it got the start-up pulse that was triggered by the powering up of the computer. (One good but mysterious way for a computer to get sick is to have a weak chip in this circle that does not reliably send its signal around the merry-go-round. It may pass as being o.k. until the repairman decides on desperate measures and replaces chips in the crucial circuit one by one.)


 * We have to get energy to power the sequence from somewhere, but maybe we could count on "quantum flux" or some other mysterious source. We could "keep time" by watching the little LEDs attached to each chip, and we would see them light up one by one. If we had 12 chips we could have a 12 "hour" day in this little world. But there would only be one day as time is defined in this hypothetical world. To model a world more like our own, we would have to add components.


 * There doesn't seem to be an incipient paradox in this little universe. We would seem to have the kind of universe envisaged in much of early philosophical speculation. In India the idea of kalpas, eternal cycles of time turning back on itself and becoming its own beginning, is particularly prominent.


 * Greene suggests that any sequence of events that led around in a circle, via a wormhole perhaps, would have similarities to this simple circuit. There is no objection that I can see to things turning out differently. Suppose that somebody invents a time machine, hates himself for doing it, and goes back and causes his fetus to be aborted. From there on, the Universe takes a different course. If time is nothing other than the sequence of events (perhaps measured by sequences of events favored by various groups of observers -- I favor the atomic clock), there is nothing inherently paradoxical in this circularity as far as I can see. But it looks like the "trip back" (so to speak) would have to be done in isolation from one's original physical context -- perhaps by going to another inertial frame that does not strongly interact with one's native inertial frame. One's own biological clock would continue to cycle through its changes, cells would age and die, and when one "returned to one's own time-line" it would be as an older individual.  '


 * It seems easy to imagine how circular time would work in the mini-Universe imagined here -- assuming that time is nothing more that the operational definition of time would imply. It is harder to imagine how circular time might apply to our Universe. But if the Big Bang is eventually followed by the Big Crunch that returns everything to the original state "before the beginning of space and time," that sequence would fairly closely follow the model of circular causation outlined above. Would it be then the same "pre Big Bang" whatever you call it, or would it be another "pre Big Bang" -- and does that question have any meaning in the absence of there being any characteristics by which to identify any "thing" at that point?


 * If, in opposition to what the relativity theory seems to be telling us, there is an actual, absolute "flow of time," then circularity would imply different events occupying the same space and time. It looks like there would be a kind of "superposition" in any case of time travel under that interpretation. There might be an Oswald in the book repository and a time traveler with his own weapon in the same room. But the time traveler would co-exist with a volume of air or perhaps co-exist with a volume of hard-bound books. Would a dead Oswald soon co-exist with a living Oswald and then shortly later a living Kennedy co-exist with a dead one? Maybe co-existing with a volume of air would not be too painful. Under those circumstances, meeting oneself on the street would not seem to pose any paradoxes. Killing oneself might be analogous to the Korean War era fighter pilots that riddled their own canopies with machine gun bullets by firing off their machine guns and then going into a power dive whose path intersected the trajectories of those bullets.


 * If causation is deterministic the way Laplace thought it was, then would a span of time be like a span of roadway that can consecutively have different vehicles traveling over it? Would there be a loop in time? The time traveler kills himself, so time goes on without him, so there is nobody to come back in time to kill himself, so... One of the interesting things about quantum mechanics is that it suggests that there is no certainty in causation, so after some number of iterations maybe the time traveler decides not to go back to kill himself.


 * One of the reasons for Kantian philosophy was the argument of what space and time are, an argument that started at least as early as St. Augustine. One view that eventually came to prominence was that space is only a relationship, and that time is only a relationship. But physics seems to be telling us that empty though it may be space has characteristics and existence. Perhaps time may in some way tell us that it too is not merely a relationship. P0M (talk) 05:32, 14 February 2008 (UTC)


 * I don't remember the details, but faster than light communication, were it possible, would enable some sentient entities to send a message from inertial frame one to inertial frame two to inertial frame three, and then back to the first inertial frame where, if the bounces were arranged properly, it could get there before it was sent. (I think it takes three inertial frames to create this paradox.)


 * Some of the stuff that I cut is either flat out irrelevant or else it is relevant in some way that the writer understood but failed to convey (at least to me). Since I've read quite a little about it before, and I've even worked with the numbers enough to start getting familiar with how everything works, I somehow doubt that the average reader coming to this stuff for the first time would be able to make anything of it at all. As far as I can tell, it is all dogmatic assertion.


 * So what should the article say that can be backed up with solid citations?


 * Note that sending a message instantaneously does not in itself violate anything but our sense that we had finally figured out how the universe works and now there is an exception -- the non-locality that Einstein saw and saw as a denial of the possibility that quantum mechanics could be right.


 * Ah, I think I see. There is a problem that Wheeler saw and expressed well in an interview reported in Scientific American. People start out with the idea of there being one photon in one place. They are bound by their lifetime of habits to "see" the photon as a little ball. That being the case, the appearance of a photon at the exit port of side one or side two of some apparatus implies to them that the photon has stayed a discrete entity and has traveled one path to get to the one exit. So if an experimenter can do something a few light years away from the point where the photon either took path A or path B that determines whether the photon took one path or another, then that decisive action must reach back hundreds or thousands of years to the photon that was "deciding" which way to go around the intervening extremely massive body that formed a gravitational lens and give it some kind of shove toward one or the other path. But Wheeler says that it is fundamentally wrong to make such assumptions about the photon. The "photon in flight" responds by appearing as a wave phenomenon or a particle phenomenon depending on how we address it as it reaches where it finally discloses itself. If we permit it to interfere with itself we will get a reading consistent with the formation of an interference fringe. (One photon does not make a fringe, but photons show up in positions along a detector screen where they would not show up if they were not interfering with themselves.) If we segregate the two or more paths it could be traveling along, then it will show up at the end of one of the segregated paths and the most "spread" we will get out of it will be consistent with a diffraction pattern (if even that).


 * You can see what is, IMHO, a very much better depiction of what the photon can be understood to do in the article on Mach-Zehnder_interferometer.


 * So far, nobody has made an "ansible," even just one that can only send morse code. It shouldn't be very difficult to fabricate. If somebody makes one and can send an instantaneous message, e.g., by bouncing a beam off a reflector on the moon that originates in LA and is picked up in NYC, then such a device would excite great interest. As far as sending messages back in time, however, we would need the cooperation of sentient being of some kind traveling in different inertial systems. Without worrying about any special requirements for the velocities involved, until SETI pans out we probably will not have anybody to bounce messages off of unless we send crews out on long star treks.  But being able to communicate instantaneously even with relatively near neighbors such as Mars would have great utility. Imagine being able to drive a Mars rover in real time. P0M (talk) 06:28, 8 February 2008 (UTC)


 * So what you're saying, in a nutshell, is that I'm not necessarily misinterpreting this experiment by believing it opens up a possiblity of backwards causality; that's a reasonable conclusion that, at this point in time, still conflicts with many other things we "know" about quantum physics. There's no resolution yet.


 * Backwards causality is in one sense a common thing, at least on the atomic level. I don't remember who pointed it out before, but some atomic-level interactions are sort of symmetrical with respect to time, i.e., you could look at one interaction as a "going forward in time the ordinary way" event, but there are other events that look just like it except that they go the other way, and you could think of them as being the same thing happening in reverse.


 * Some of the quantum eraser experiments involve entangled photons which are sent through two different experimental set-ups. The two have different optical path lengths, so that the photon that goes through set-up A will hit a detection screen sooner than the entangled photon that goes through set-up B. Nevertheless, something that happens to the entangled photon in B will determine how the one in A behaves. So if the photon in B is coerced into manifesting as a particle, then the "twin" in A will manifest as a particle, but if it is coerced into manifesting as a particle in B then its twin will do the same thing in A. It doesn't sound too bad until you get somebody who wants to make the optical path in B very long so that (in the extreme case) maybe years flow by before anything happens at a detection screen in B, but within minutes something has happened in A.


 * Nevertheless, somebody at B couldn't win bets by communicating with somebody at A to find out where the photon was going to appear at B. The reason is that the guy at B would still have to send out a message at a rate governed by c, which would get there too late to permit cheating.


 * To get to the point that you could cheat you would have to set up the sort of double-bank shot or maybe it is a triple-bank shot. Our everyday habits of thought get in the way of understanding how it works because we have a naive idea that things can occur "simultaneously." P0M (talk) 00:20, 12 February 2008 (UTC)


 * The Wheeler interview you mentioned makes sense. I'd gotten used to thinking of particles as fuzzy little clouds of possiblity that are sort of here, sort of there, sort of everywhere and nowhere, and they only collapse to a sensible particle-like state when the outside world interacts with them. The new part to me is the idea that something's "location" in time is as fuzzy as its location in space.


 * Wheeler was just saying that it is a matter of habit and prejudice to say that "the photon" goes one way or the other way around the high mass body that is doing gravitational lensing. So we imagine that if we can switch how things show up on earth (as particle or as wave) then we have switched how the photon "decided" to go around the high mass body (black hole or whatever). To the extent that the word "goes" still means anything, the photon goes both ways (or Feynman would say that it goes all ways). But the quantum eraser experiment seems to me too to indicate that our idea of location in time as well as in space is not what our macro experience would make us think.P0M (talk) 00:20, 12 February 2008 (UTC)


 * Here's one more idea, more plausible than keeping an instrument perfectly aligned when one part's on the earth and the other part's on Mars. What if we used mirrors to keep the photon bouncing back and forth, giving it a nice long path like a tenth of a light-second, before it reaches the optics in the idler path? Then the device can stay in 1 reasonably sized lab. I'm not suggesting this as a useful device, but as something to investigate causality here. It seems to me that if you were to remove the prism in the idler path (or just block it off by putting something opaque in the path), then the data at D0 would respond to the change 0.1s before the change occurred. I wonder how that would play out in the lab? Would the experimenter attempt to "trick" the device, reaching to block the idler path but pulling his hand away at the last moment, trying to get D0 to exhibit a change but then not taking the action that caused that change? Would his intent to "trick" the device ensure that D0 never exhibited a change at all, except when the experimenter really did block the idler path?


 * There have already been experiments done with entanglement kinds of lab apparatuses with very long paths. They've used fiber optics, typically. Have you checked out John Cramer's article? He's actually working on the kind of experiment you want to see.


 * It seems to me that the useful thing to do would be to have two streams of entangled photons involved. Let's say that you had an interstellar space ship that went to Wolf 359. Halfway there it established a mirror that will automatically orient itself so that it is pointing back toward our sun. It continues on toward Wolf 359. After it gets there and establishes itself, it picks up a laser beam from earth that started out roughly four years earlier. Earth is at the same time picking up the bounced twin that was send to the mirror. (The mirror can't be exactly on-line between Sol and Wolf 359, or the two beams would both get reflected by the same mirror.) So Earth has the twins of photons that are received by the space travelers orbiting Wolf 359.  Getting a spate of photons that show up with particle-like behavior would be a "dash" and getting a spate that show up with wave-like behavior would be a "dot."


 * Before we go to all of that trouble, using one fiber optic cable from NY to SF and another from NY to Omaha and back and then playing with that system should be a good proof of concept. Maybe the easy way to get photons capable of interfering with themselves would be to use two pairs, with a beam splitter at both ends of the telegraph operator's circular cable. The terminal beam splitter would be the telegraph key. The straight pair of cables would just merge their beams and watch to see what happens.  (Actually, they could take turns, and the guy in SF could have an interruptible beam splitter to send with so that the guy in NY could monitor the now-unterminated cable pair to see whether there was any sign of interference.)


 * If that would work, then non-local communication would be possible. But would there be any causal paradoxes? What might work would be to make the Earth-return loop of the interstellar experiment short. Then the Earth-return message could be determined by what would happen about four years in the future. If the same thing were done in reverse, then the Wolf 359-return message with Earth news on it could be determined by what would happen about four years in the future. Or would it? I need to sit down somewhere with paper and pencil and go through this stuff step by step.P0M (talk) 00:20, 12 February 2008 (UTC)


 * Last thing, could you explain how different inertial frames are important? I don't understand how those come into play. Is redshift/blueshift important? I'll do my best to follow along. I studied quantum physics a little bit as an undergrad and spent a summer in a laser optics lab working on a new variety of interferometer (JILA lab at CU-Boulder, pretty cool facility), so I'm no layman but I'm still far from an expert. Thanks a lot for your time. 205.175.225.22 (talk) 17:25, 11 February 2008 (UTC)


 * Our idea of "simultaneity" is naive. Our idea of time is likewise naive.  As long as everybody is moving along together (or in tech-talk, "in the same inertial frame") then "the same time" has a clear meaning.  The faster two observers are moving with respect to each other, the more distortions are introduced into our ordinary-world ideas of time.  If events are not really simultaneous then the alternative is that there is a temporal difference of some sort between them. I'll try to look up the reference in Greene where this stuff is explained. I certainly can't remember it or derive it now (if ever). P0M (talk) 00:20, 12 February 2008 (UTC)


 * See section 4.1 of Time travel. P0M (talk) 17:12, 12 February 2008 (UTC)


 * Thanks for pointing me towards John Cramer. I looked him up after my last post here. His site's like an encyclopedia of mind-blowing ideas. What's even more surprising than the ideas themselves is the fact that he's not a quack, he's a respected published researcher. Generally, scientific ideas this strange come from pseudoscientists selling home time travel kits and ranting about the government conspiracy to silence them.


 * I did have one other thought come to mind. The device in this experiment deals with only 1 photon (well, 2 entangled photons) at a time, correct? Does it have to? If the idler path is 0.1 light seconds long, are we limited to putting out at most 10 photons a second? If having a second photon in the idler path messes things up, then we face a huge restriction in data rate. If that's the case then a 1-light-year device would only handle 1 photon a year, and it takes dozens of photons to determine what kind of pattern we're seeing at D0, so it would take decades to establish 1 bit of the message. The only way I can think of to compensate for that would be to use not just 1 device, but an array of hundreds or thousands so you could get some reasonable amount of statistical certainty. Or am I going off on a pointless tangent, and having a million photons in the idler path at once would work just as well as only one? 205.175.225.22 (talk) 17:52, 12 February 2008 (UTC)
 * No problem with multiple photons. It's easier to talk about what actually seems to happen (and, experimentally, it makes certain thing clearer) when one photon at a time is run through an apparatus. But for practical purposes one would need many photons.
 * I fear that I just have not seen the gotchas yet, but if the apparatus described for this article could be made to have a relatively short path on the upper leg, and an interstellar distance on the bottom leg, then it looks to me as though messages could be received "before their time" and two sets of apparatus could make it possible to conduct conversations over long distances. A kind of "time travel" could result if one leg was very small in comparison to the other, and useful virtually instantaneous conversations could take place if the the pathways were approximately equal.
 * Here's what the schedule for a fictional conversation about an explorer who has suffered a life-threatening injury.


 * P0M (talk) 03:10, 13 February 2008 (UTC)

You may have to click all the way through to the actual svg image on this one. For some reason it is not getting handled right by Wikipedia. The svg is o.k., but when the system tries to produce a png image from it things seem to be going wrong.

Anyway, Earth station generates entangled photons, sends one through a double slit and in oppositely polarized beams on to Wolf 359. When it arrives, the explorers either let it alone or change the polarity so that the two beams can interfere. Back on earth, nearly four years before that happens, experimenters are watching the stream of twin photons to see whether they show up as interfering or not interfering.

The explorers near Wolf 359 have set up the same kind of apparatus, send one stream of entangled photons back toward Sol, and watch their twins to see what the experimenters on the Sol station have done with the received photons. It takes four years for the twins to reach their remote target. If there is a pair of mirrors at the halfway point, then the round-trip photons will come back to Sol at the same time their twins reach Wolf 359, and meddling with them from the Wolf 359 end will influence the behavior of their twins that are just then returning to Earth.

Is that the kind of communication device you were thinking of? I can't remember what Cramer said, but I think he has basically the same idea. Obviously he published it several years ago.

There would be huge collimation problems I believe. But maybe between Earth and Mars it would be useful for running Waldos. P0M (talk) 03:25, 13 February 2008 (UTC)


 * That's not exactly what I was picturing, but the concept is the same. I was imagining one device starting on Earth with its idler arm reaching into the station in deep space, and another device starting on the station in deep space with its idler arm reaching back to Earth. Keeping things perfectly aligned would be practically impossible, but hey, it's a hypothetical, not a patent application. The problem (or advantage) of this arrangement is that instead of being instant, every signal going either direction "arrives" long before it's "sent," which is very handy for some things but makes actual conversation impossible. Your suggestion would work better for real-time communication. My suggestion looks much more paradox-prone, but I'm of the opinion that a paradox could never happen.


 * I see the universe as a stable 4-dimensional entity where every object from the past, present and future has a fixed, stable line of existence. The future conditions of all those lines may be absolutely unknowable from inside the universe, but the lines exist nonetheless. A time-travel paradox would be impossible because only stable loops could be part of a stable 4-d universe, and as a result you can only interact with the past in ways that do not re-write the present. In sci-fi it shows up as the story of a time traveler finding out that the history he knew was all about his own actions in the past. But, like I said, this is just an opinion - and probably a completely untestable opinion at that. 205.175.225.22 (talk) 15:49, 13 February 2008 (UTC)


 * I think the only way you could test it would be to go back in time and change something. ;-) P0M (talk) 01:35, 14 February 2008 (UTC)

Why this is not an ansible
This particular device does not allow communication into the past or faster-than-light by the method of adding or removing a distantly-located quantum eraser and noting a change in the interference patterns as seen in the nearby D0 interference pattern.

You get an interference pattern (at D0) in the D1 case, and also in the D2 case, but not in the D3 case, nor in the D4 case. You could replace the D3 and D4 pick-off half-silvered mirrors with fully-silvered flip mirrors. Flip the mirrors into the beams and you get D3 or D4. Flip them out of the beams and you get D0 or D1. This is a delayed choice quantum eraser where the choice (of whether or not to do that erasure) is under your control. Attach the flip mirrors to a telegraph key and you send morse code.


 * I think your point is that at D0, while getting single photons, there is no pattern. One would need to get enough photons recorded that were all either self-interfering or all not self-interfering to be sure that whatever single photon was recorded was not just an incidental photon that either happened to fall in the center or happened to fall in an area that would be appropriate for the formation of a fringe. Right? P0M (talk) 17:00, 3 March 2008 (UTC)


 * That is not my point. My point is that at D0, if you take a million photons (which are coincident with a quarter million in each of D1,D2,D3,D4) then you will not see an interference pattern.  If you select out the quarter million photons that are coincident with D1, then you will see an interference pattern at D0.  Likewise the quarter-million coincident with D2 will produce an interference pattern on D0, but not the quarter million of D3 nor of D4.  If you take the half-million photons that are in coincidence with either D1 or D2, then you do not get an interference pattern.  (D1:yes, D2: yes, D1+D2:no).  So even if you know that the second photon hit either D1 or D2, if you erase the knowledge of which one it hit, you erase the interference pattern. DMPalmer (talk) 00:23, 6 March 2008 (UTC)


 * I believe that what you say is valid. The apparatus is set up so that a photon may show up at D3 or D4, and in either of those cases there is no pathway by which it can interfere with itself. Or, a photon may show up at D1 or D2, and in both those cases it must interfere with itself because there are pathways open from both slit one and slit two to each of those detectors. I've worked out the reason that photons that show up at D1 and D2 will have opposing polarities. (See diagram above at the top of the section on Polarization.) I haven't had time to work out the phase changes, but it looks like a similar artifact of experimental design must be involved. P0M (talk) 04:48, 6 March 2008 (UTC)

However, flipping the mirrors in or out does not cause a detectable change in the pattern seen on D0. With the mirrors in or out, you still get a broad peak without any interference fringes. (This broad peak looks like what you would get from a single slit.) However, if you take the photons observed at D0 which occurred at the same time as photons observed at D1, you will see an interference pattern. (This can be done simultaneously, as in the Kim et al. experiment, or can be done after the fact, when floppy disks recorded from D1 on Wolf 359 are brought back by slowboat and compared to those recorded from D0.) Likewise, you see an interference pattern in coincidence with D2. But the recorded D0 data does not change at all once you have the D1 and D2 data.

However, the D1 and D2 coincidence interference patterns are 180 degrees out of phase. (See figs 3 & 4 of the paper on the arxiv.) So the D1 pattern looks like  A_C_E_G_ and the D2 pattern looks like _B_D_F_H. If you don't have the D1 and D2 measurements, taking all the D0 data gives you ABCDEFGH, which is exactly the same as for the D3 and D4 cases.

Therefore, you don't know what message was sent until you get the D1/D2 data back via classical channels, and it is too late to play the lottery.DMPalmer (talk) 19:28, 1 March 2008 (UTC)
 * Rather than the complications of the original apparatus, which were designed to permit or prevent interference on a random basis, what would happen if the two paths in the lower part of the apparatus were either caused to diverge or were caused to converge on a single target?  If they converge it would be equivalent to a simple double slit experiment that permits interference and produces fringes. If they were diverged it would be equivalent to a simple double slit experiment with an opaque wall separating the paths from the two slits. 152.17.115.79 (talk) 21:30, 3 March 2008 (UTC)


 * Since this idea that the experiment could be used for FTL or backwards-in-time signalling seems to be a fairly common confusion, I re-added a deleted section discussing the fact that the total pattern of signal photons at D0 never shows interference, and that the interference pattern can only be recovered by looking at the coincidence count between signal photons and idlers which went to one of the which-path-erasing detectors. Hypnosifl (talk) 21:51, 8 March 2008 (UTC)


 * I think it is worthwhile to add this part back in if it can be made entirely clear to the average well-informed reader.


 * Without the bottom (idler) part of the apparatus, interference would be seen at d0 because a part of the light wave emerges from each slit and an entangled wave-function emerges from the two regions (for the red path and the blue path) of the BBO. So it functions just as would a wave-function emerging from the two original slits. But the BBO sends a wave-function along the other two red and blue paths. If there is nothing in that path that "sequesters" a photon by making it show up in a place that only one path can reach (i.e. by collapsing the wave-function there), then it is unclear to me how it could influence what happens in the upper path.  One would have to arrange to have something happen to keep the red and blue paths from merging, and then one would have to have a detector that would, with high efficiency, make a photon "show up" on it, i.e., making the wave-function collapse at that point. (Modified P0M (talk) 07:27, 9 March 2008 (UTC))


 * Once you have that lower (idler) apparatus, however, you have a 50/50 chance of a photon showing up at d3 or at d4, which makes the entwined photon show up on d0 as it would if it had simply been diffracted by a single slit, so there is no interference pattern 50% of the time. The other 50% of the time you should get photons showing up at d0 not as if they had been diffracted but as is they had been parts of an interference fringe. But the sum of two interference fringe patterns with a side-to-side displacement is not a black screen. It should be an evenly lighted screen since the two interference fringes are complementary. So the total result, when the lower (idler) apparatus is in place, should be a central bright patch (the diffraction pattern 50%) on the background of a very wide band of moderate illumination (the joint result of all the photons showing up in their appropriate diffraction fringes).(Modified P0M (talk) 07:27, 9 March 2008 (UTC))


 * So it looks to me as though a "telegraph" could indeed be sent, with "dit" being an ordinary diffraction pattern (when the lower two, idler, paths do not let the wave-function fall on the same detector) and a "dah" being a wide band of moderate background illumination (when the two idler paths are managed so that the wave-function falls, by way of two paths, on the same detector. (Modified P0M (talk) 07:27, 9 March 2008 (UTC))


 * One can also imagine doing things like putting single detectors in place of the two first-surface mirrors in the lower path. That way no matter where the wave-function collapsed and the photon showed up it would be unambiguously at the end of only the red path or only the blue path, and when the apparatus was put in the way of the lower red and blue paths the upper path would necessarily direct the two paths onto the same detector making the wave-functions collapse in such a way as to manifest a diffraction-type pattern. (modified P0M (talk) 07:27, 9 March 2008 (UTC))


 * Your argument is unclear to me. When you say "without the bottom part of the apparatus", what specific parts are you talking about removing? Are you talking about removing all the devices involved in measuring the idler photons--the prism, the beam splitters, the detectors D1-D4, all of it? And why do you think interference would be seen at D0? I don't understand what you mean by "ghost light wave" or by the phrase "nothing in that path that sequesters a photon by making it show up in a place that only one path can reach" (what specific path do you mean by 'that path'? Are you talking about the path of the signal photon or the path of the idler?) I think it would help me understand better if you rephrased your point in the language of signal photons and idlers, and referred specifically to particular detectors and paths.
 * I've tried to make it a little clearer. But to really work it out right I think it will be necessary to look at wave-functions, the collapse of wave-functions, and the results that are consistent for entangled wave-functions. P0M (talk) 07:27, 9 March 2008 (UTC)


 * Right.
 * In the part of the apparatus between the laser and d0 there is a double slit. With the exception of the BBO, the apparatus is the same as is seen in the Double-slit experiment. If the paths from BBO that lead into the Glen-Thompson prism were not treated so as to identify which-path information, what would prevent the light waves in the paths from the BBO to d0 from interfering?


 * As long as the BBO is present to create an entangled pair, the mere fact that the signal photons are entangled with idlers in such a way that there was the potential to measure the idlers in such a way as to determine the which-path information for the signal photons is enough to guarantee that the total pattern of signal photons will not show interference, regardless of what actually happens to the idlers. This is just one of the features of how entanglement works in QM, it's discussed in the physicsforums threads I linked to.


 * In any case, I can assure you that you're incorrect if you think it's possible to get an interference pattern in the total pattern of signal photons at D0, regardless of what you do to the idlers--when the signal photons are entangled with the idlers in such a way that there is even the potential to determine which path they went through by certain measurements on idlers, this alone is enough to destroy the interference pattern at D0, regardless of whether the appropriate measurements are in fact carried out on the idlers. This is just a consequence of the way entanglement works in QM. See the discussion here and here on physicsforums.com, for example (both threads contain a lot of links to papers by professional physicists to back up the points being made). Hypnosifl (talk) 01:46, 9 March 2008 (UTC)


 * If the signal photons' paths are combined, the which-path information is "erased," no? Once that is done, there is no way to get it back. If there were, we could look at a pattern of interference fringes and get the which-path information back from it.


 * Is there some unspoken assumption that the idler photons have some kind of logical priority over the signal photons? I don't see how either idler photons or signal photons can have priority. If that is the case, certain outcomes are possible or impossible for the entire apparatus, no?


 * What do you mean by "combining" the paths of the signal photons? You mean like running the signal photons through an apparatus similar to the one the idlers went through, where idler paths from different slits were combined if they go to detectors D1 or D2? For a pair of entangled particles, we should really just talk about whether the measurements made on both members of the pair preserve or erase their which-path information; in the standard DCQE setup, when the signal photon is measured at D0 and the idler is measured at D1 or D2 this means the which-path information for both is erased, and when the signal photon is measured at D0 and the idler is measured at D3 or D4 this means the which-path information for both is preserved, but in theory you could certainly alter the setup so that even if the idler was measured at D1 or D2, the signal photon could be measured in such a way as to preserve the which-path information for both photons. But I'm not sure if this answers your question--if not, could you elaborate why you think there might be an assumption about the idlers having "logical priority" over signal photons? Hypnosifl (talk) 03:31, 9 March 2008 (UTC)


 * Actually, I have a fair amount of trouble with the language and the logic that some of the writers on these subjects use. I have wanted for some time to work back through everything and try to distinguish "ways of talking about things" from operational definitions. Right now I do not have the time.  Maybe in a few days...


 * Combining paths? It may be part of the problem that I just mentioned. Here is a starter: Consider the basic double-slit apparatus.  Draw a line between the midpoints of each slit and (assuming they are arranged left and right rather than top and bottom) draw a long perpendicular line at the midpoint of that short line. Now construct a thin wall along the long vertical line perpendicular to the wall in which the slits have been made and make it reach all the way to the detector screen, the ceiling and the floor.  Now when a laser is shone on the double slits one beam of light will go out the left slit and hit the detector screen in the left chamber just created,and one beam will come out of the right slit and hit the detector screen in the right chamber. Anything that travels through the left slit will be confined to the left chamber and anything that travels through the right slit will be confined to the right chamber. The "which-path information" is certain. Now remove the wall and the "which-path information" will be lost. Two diffraction patterns will be replaced by one interference pattern, the famous fringes, etc.  The paths have been combined, or, more precisely, whatever travels along each path is not separated from whatever travels along its counterpart path.


 * Now consider:


 * Are not the paths in the upper diagram separated in an analogous way? And does not the introduction of a second beam splitter as shown in the lower diagram combine the paths, or, more precisely, whatever moves along those paths?
 * While we're looking at the simpler apparatus, When a single photon hits the first beam splitter, some people say that a given photon either gets reflected or gets transmitted. If the photon goes one way, what do you prefer to call the what-ever-it-is with which the photon may interfere if circumstances permit it? It is on one level merely a question of terminology, so hopefully you will have a preferred term and I can agree to use it. P0M (talk) 05:31, 9 March 2008 (UTC)


 * One source calls it a "presence," but I don't like that very much.  Another, []which I favor, calls it a "wave-function," and the general discussion there virtually prohibits (as does Wheeler) speaking of the photon as going by one path or the other. The wave-function goes by both paths, and the discovery of a photon in the detector screen is the result of the collapse of the wave-function. Nevertheless, at least in popularizations, people continue to speak of the photon going by one path or the other, seek to determine by which path the photon has really gone, etc. P0M (talk) 06:09, 9 March 2008 (UTC)


 * I'm not sure how familiar you are with the technical details of quantum mechanics, so apologies if I'm telling you things you already know here, but the wave function is a basic part of the mathematical machinery of the Schrödinger picture of quantum mechanics. The wave function is a mathematical function that assigns "amplitudes" (which are complex numbers) to every possible definite outcome that might be obtained from a measurent on the system; for example, for each possible position at which you might find the particle on measuring it, the wave function assigns some amplitude. The complete set of amplitudes at any given moment is the system's quantum state at that moment, and the quantum state changes over time, its evolution determined by the Schrödinger_equation. At the moment you measure some variable like position, the probability of finding any one definite value for that variable is given by the square of the complex amplitude which the wave function assigned to that value immediately before the measurement--this is the Born rule. This is also what is meant by "wave function collapse"--if you want to make further predictions about the behavior of the system after the measurement, you must assume that at the moment of measurement the wave function made a discontinous jump, so that immediately after the measurement all the amplitude for the variable you were measuring became concentrated on the value you obtained, while the amplitude for every other value of that variable dropped to zero. This special sort of state is known as an eigenstate of whatever variable you were measuring. Note that a quantum state which is an eigenstate for one variable may not be an eigenstate for another--for example, in the case of a position eigenstate, where the wave function assigns zero amplitude to all but one possible value of the position variable, this would not be an eigenstate of the momentum variable, this same quantum state would assign nonzero amplitudes to multiple possible values of momentum. Likewise, an eigenstate of the momentum variable cannot be a position eigenstate. This is one way of understanding the position-momentum uncertainty relation, which says that any measurement of position necessarily introduces some uncertainty into the value of the momentum, and vice versa.


 * Anyway, the upshot of all this is that the basic procedure for making predictions in the Schrodinger picture is to construct a wave function for the system (based on information from previous measurements), use the Schrodinger equation to find the evolution of the wave function/quantum state (the two terms are basically interchangeable) over time between measurements, then each time you make a measurement you assume that at the moment of measurement the wave function "collapsed" into the corresponding eigenstate of whatever variable you were measuring, which you can then evolve forward according to the Schrodinger equation until the next measurement.


 * I'd agree that all the talk of "paths" is hard to make sense of in the Schrodinger picture, since the wave function will assign different amplitudes to multiple possible positions at each moment between measurement, and one cannot really talk about the "probability" that the particle is at one position or another (along one path or another) except at the moment of measurement when the wave function behaves differently than it does in between measurements (discontinuously collapsing instead of evolving continuously according to the Schrodinger equation). But the Schrodinger picture is just one of several mathematically equivalent methods of making predictions in QM--there is also the Heisenberg picture, as well as the one that is probably most relevant to the talk of "which-path information", the path integral formulation.


 * In the path integral formulation, if you have some initial measurement of the system and you want to use that to calculate probabilities for the outcome of a later measurement, instead of using the initial measurement to construct a wave function and evolving it forward using the Schrodinger equation, you instead perform a type of quantum-mechanical summation over all possible paths from the initial state (the initial position a particle was emitted from an emitter, say) that end up at the state whose probability you want to calculate (the final position a particle is detected on the screen of a double-slit experiment, say), and this integral will give you the correct probability. In the double-slit experiment, you'd sum over paths which go through the first slit as well as paths which go through the second slit.


 * You can't ordinarily talk about the "probability" the particle took one path or another, since this would imply the total probability of the particle ending up at a given location would just be an ordinary sum of the probabilities of each path that ends up at that location, when in fact the quantum-mechanical sum over paths doesn't work this way, different paths can interfere with one another and cancel each other out. But when physicists say that certain measurements can reveal in retrospect which slit a particle went through, I think this can be understood in terms of the path integral--in a situation where you get the which-path information by measuring an idler, with the idler found to have gone through slit #1 for example, I think that this means you could correctly predict the probability the signal photon is measured at different positions by summing only over paths that went through slit #1, you'd no longer have to include paths that went through slit #2 in your path integral. I'm not absolutely sure this is correct since I haven't seen these calculations done for the type of signal-idler experiments we're discussing, but I know that in the case of the standard double slit experiment without entangled particles, if you measure which slit the particle went through then you only sum over paths that went through that slit in the path integral.


 * Also, although technically you're supposed to sum over all sorts of crazy and wiggly paths, paths which deviate from straight lines tend to mutually cancel each other out, which is the quantum-mechanical answer to why particles tend to move in straight lines, and which also means that for practical purposes you can get pretty accurate probabilities just by summing over the types of straight paths illustrated in the diagrams. In his initial discussion of the double-slit experiment in The Feynman Lectures on Physics, Feynman analyzes the double-slit experiment using just two paths which go in straight lines from the emitter to each slit and from each slit to the position on the second screen where he wants to calculate the probability the particle will be measured, for example.


 * Does this help at all? Hypnosifl (talk) 18:49, 9 March 2008 (UTC)


 * I'm familiar but not extremely familiar with the points you mentioned above. And what you say indicates that you are not the writer who will say thing in a casual way that will misrepresent the complexities of the situation. The way some writers discuss these experiments,  the photon is still a little bullet and it either goes by one path or the other, and the existence of a second path not taken tells us that as it was approaching the split between paths the photon "decided" to take one path or the other. If somebody has no way of figuring out which path it took, then it will interfere with itself (somehow). If somebody has a way of figuring out which path it took, then it will not interfere with itself (for some "reason" of its own). In the extreme case, a photon decides to go around a gravitational lens one way or the other, and it will be seen many years, decades, centuries, later either by a telescope pointed toward one point in the sky or by a telescope pointed toward the other point in the sky. It will not appear in photographic film used by both telescopes. On the other hand, if light from the two telescopes were to be directed onto the same sheet of photographic emulsion, a photon would be recorded at a position in accord with its interfering with itself. (Don't ask me how this thought experiment could be implemented in the real world!) The secondary sources tend to explain this by saying that the choice of which way to observe the photon in the present determines how the photon "decided" to travel.  It sounds so wacky as I try to remember how those writers explain these experiments that I have to doubt that I am getting it right.


 * To me, the idea that some "thing" goes by all possible paths is far easier to accept. For one thing, it doesn't stress my suspension of disbelief so much. A few days ago, I ran across a discussion that handled my problem by speaking of the photon as what is emitted and what is detected. But in between, the author did not speak of the photon. I think he just called it the "light wave."


 * I just wanted to know (1) whether you would say that in the basic double-slit experiment some "thing" goes through both slits, and (2) if you do believe that some "thing" goes through both slits, then what do you favor as a term to use to speak of it to the average well-informed reader?


 * I'm out of time for the present. I hope I have managed to ask the question clearly. P0M (talk) 21:08, 9 March 2008 (UTC)


 * I guess what I'd say is that we can't answer any questions about what is "really" going on between measurements without getting into the issue of the interpretation of quantum mechanics, since different "interpretations", such as the Bohm interpretation or the transactional interpretation or the many-worlds interpretation, could give different answers to this question, and all the different "interpretations" are experimentally indistinguishable so this is not a question that empirical science can settle. But in the context of talking about "which-path information", I think we can just talk about allowable paths in the path interpretation, so that if you know which slit the particle went through, that means all the allowable paths are the ones that went through that slit. You can talk this way while remaining agnostic about whether there was really any "thing" that was following one or all of these allowable paths between measurements. Hypnosifl (talk) 07:01, 11 March 2008 (UTC)


 * Fair enough. (When I mentioned "ghosts" before I was teasing people a little bit. It's very important to not let language and thinking habits get in the way of understanding.) P0M (talk) 13:55, 11 March 2008 (UTC)


 * Not having a lab with the requisite apparatus, I am of course in danger of missing the obvious. Let's take things a step at a time.


 * I put in the missing reflected paths. Thanks for bringing that omission to my attention. P0M (talk) 03:07, 9 March 2008 (UTC)


 * Thanks. If you have the time, could you also modify the angle of the middle beam-splitter so that it looks more like the diagram on this page? As it is the new reflected lines you just added to the diagram don't seem to be obeying the law that "the angle of incidence equals the angle of reflection". Also, could you add the labels "BSA", "BSB" and "BS" to the three beamsplitters as in the diagram on that webpage, so that specific beamsplitters could be referred to in the article if needed? Hypnosifl (talk) 03:39, 9 March 2008 (UTC)
 * I'm not very good with Inkscape. Wikipedia is rather insistent that it be used. The letters will be easy to add. Getting the angles more realistic may be harder than it seems it would be. I'll see. P0M (talk) 05:31, 9 March 2008 (UTC)


 * The new diagram looks good to me--again, thanks for taking the time to draw this up, it definitely makes the article more understandable. Hypnosifl (talk) 18:49, 9 March 2008 (UTC)


 * Actually, I have one last suggestion which should be easy to add--I just remembered that the article refers to "slit A" and "slit B" as these terms are used in the diagram in the paper, and it says the D3 detector gets idlers from slit A while the D4 detector gets idlers from slit B. So, could you add the label A next to the blue slit on the black double-slit screen, and B next to the red one? Hypnosifl (talk) 20:24, 9 March 2008 (UTC)
 * Done. (I should be elsewhere, sigh. But who can I blame but myself?) P0M (talk) 01:44, 10 March 2008 (UTC)

Cramer ansible
The particular device described in this article may have some limitations or special characteristics that obscure the non-local factors. Dr. John Cramer has produced an idea for an experiment that could test the idea of "retrocausal" activity.



I think I read one of his postings that said he was trying to get funds together to do the experiment. Maybe twenty km. of fiberglass cable is expensive. His take on the experiment is that he cannot see any theoretical problems with it, so he gives it at least a slim chance of actually working.

The way of determining whether the conditions are right for interference fringes in the top path is to either put the detector where an interference fringe would "focus," or else pull it far enough back that interference is negligible. I haven't put in the distances between various sets of components.

This experiment is designed so that it could operate with a continuous strong stream of photons being provided by the laser. So there is no need to count coincidences to eliminate the odd photon from outside the lab. P0M (talk) 04:17, 5 March 2008 (UTC)

One possibly more straightforward way of gaining or destroying which-path information would be to modify the Cramer experiment as indicated in the second diagram. With the mirror-shutter closed (in place), there is which-path information available. With it open the two paths are open and the lens converges them onto the detector screen where they will interfere. Once that happens, which-path information is lost. P0M (talk) 03:19, 9 March 2008 (UTC)




 * I recently got into an extended discussion about the Dopfer experiment which Cramer is using as a basis for his own experiment. It started when I read the page here which discusses the Dopfer experiment and speculates how it could be modified to become an ansible, or a device for sending information back in time.(a) Then I came across these posts (b) by someone named "Ben" who seems to have some familiarity with the Dopfer thesis, and claims that there are some misunderstandings in the previous page I linked to which had been arguing the experiment could be turned into an ansible. I posted Ben's comments on this thread (c) from physicsforums.com, and then got into a long discussion with another poster about the thesis and whether Ben's analysis makes sense; if you want to follow the discussion we had (there were a lot of posts on other subjects on that thread too), the relevant posts are #44, #50, #53, #57, #62, #66, #68, #69, #70, #71, #72, #74, #78, #80, #81, #82, and #83 (I was posting as 'JesseM'). I think we managed to come up with some sensible ideas about why, in orthodox QM, Dopfer's experiment could have the results it did and yet this would not open up the possibility of FTL or backwards-in-time communication. Hypnosifl (talk) 20:06, 16 March 2008 (UTC)


 * I just had a look at (b) above. It sounds like Ben believes that any "interference fringes" seen by means of the movable detectors are simply artifacts of the experimental design. I'm not entirely sure what either one of them actually means, but the other guy seems to be clearer in his thinking. Think of it this way: You have normal vision and can see an interference pattern. I have poor vision, so I use the equivalent of a photographic scanner to scroll over the surface of the detection screen (perhaps using a little telescope so my scanner won't cast a shadow on the screen it is trying to scan).  I will get the same information you do, but it will come to me line by line (or maybe musical tone by musical tone if I am totally blind) as the stepper motor moves the scanning head in tiny increments. A significant detail of the experimental set-up is that the scanning head has to pause for some time at each station in order to collect a fair sample.  Since the arrivals of photons are on a probabalistic basis, a short sample might be entirely too high or entirely too low to be representative.  If 32 people flipped a coin 5 times, one of them probably would get all heads or all tails. Somebody who watched only his performance would probably suspect that the coin was loaded. The reason that they do not use a large screen electronic detector is simply because they don't have it. It would have to be not only very sensitive but also very "noise free."  A large screen with a little intrinsic noise (or with lots of stray photons coming in from the general environment) would mean that it would be very hard to be sure what was relevant data and what was distraction.  Even coincidence timing would be problematical if there was enough noise in the system to make multiple hits occur between beats of the timing clock or the recharge time of the photon emitter. P0M (talk) 04:10, 19 March 2008 (UTC)


 * I think Ben just meant that they intentionally made the detectors narrow, because the only way to see an interference pattern at one detector is to do a coincidence count between photon hits there and photon hits at the other detector which lie in a narrow range of positions; if you don't do a coincidence count, the total pattern of photons at a detector will never intereference, just like in the DCQE. Why do you think this is unlikely? After all, to suggest the total pattern of photons at one detector would vary depending on what happened at the other detector would clearly violate Eberhard's theorem. Hypnosifl (talk) 04:41, 19 March 2008 (UTC)


 * I clicked on the wrong link earlier, so I missed what I have now labeled as (a) and went to what I have now labeled as (b). The description at (a) makes it clear that if one eliminates the noise in the experiment (it indicates that some entangled photons do not have equal momenta, etc.) what one does with the idler (i.e., the part of the experiment with the detector that can be placed at two differences from the Heisenberg lens) determines what happens at the detector that is at a fixed distance from its lens.  Using the coincidence counter does not, as far as I can see, do anything to the experiment except removing noise. So it appears that, as far as communication opportunities go, the problem would be how to "purify" the stream of photons coming into the Heisenberg lens. Since some of the photons coming out of the BBO have different energies, a narrow band-pass optical filter could absorb part of them. It is also possible that the BBO crystal could be better constructed or replaced with a more efficient converter. It is possible that the detection screen could be tuned to the frequency of the entangled photons. It is also possible that what looks like an evenly illuminated screen to human vision could still be processed to show that certain areas of the screen are more highly illuminated than random variance could account for.


 * However, I will keep going through the links to discussion. It's always a little difficult to avoid getting sidetracked by wishing that you could get clarifications as you are reading through. P0M (talk) 16:45, 21 March 2008 (UTC)


 * I just checked out (c) above. I do not see what Ben's diagram has to do with the Dopfer experiment. Here is how I see things:


 * In the lower part of the apparatus, entangled photons are sent through a double slit. They then form a or Young's interference pattern on their detector. Presumably a signal is generated by this detector and goes into the coincidence counter.  Meanwhile, lots of photons have shown up on the wall adjacent to the double slits.


 * On the other side of the apparatus, a relatively large number of photons is coming through, since there is nothing corresponding to the double-slit barrier. They will hit the detector screen. Why?  Their entangled mates have had their positions determined by landing on the wall around the double slits.  So the ones on this side will not interfere with themselves since their paths have been determined.  Moreover, they would have landed at random positions even if there were nothing specific set up for their entangled mates.


 * In order to get a high sensitivity reading, an appropriate detector is mounted on a trolley that is moved by a stepper motor. At each position along its course it would pick up a large number of photons that are paired to photons that don't get through the double slits. So the experimental strategy is to filter out the false positives by counting only those photons that match with the reception of a photon that has gone through the double slits.  At some positions along its course there will be virtually no hits, corresponding to the dark fringes in the detection screen associated with the double slits.  At other positions there will be very many hits, corresponding to the bright fringes in the interference pattern.  If the experimenters are being truthful, the results of these readings should be analogous to what you would get by photographing the traditional interference and then scanning it with an ordinary desktop scanner and counting pixels in vertical bands (corresponding to the fringes or to fractional parts of them).  P0M (talk) 22:10, 21 March 2008 (UTC)

(Unindenting) I just finished going through the long set of links provided above. I think I may have a different position than either of the main participants in that discussion.

I've downloaded Dopfer's dissertation. The original article shows, in Abbildung 4.3, an electron source at the bottom of the illustration, a double slit, a screen that would pick up an electron aimed obliquely through the slits, a lens, and two positions for a detector, one at the focal length of the lens, which would bring light waves into registration at a closer distance than would otherwise be the case. In the original Young double-slit experiment, light from the sun was brought into the lab from the outside because the distance between the sun and the photons that are detected at various points on the detection screen are virtually identical. Passing through a single slit, a much nearer point light source would tend to create a wider band because the angle between the source and the two sides of the slit is greater. (It'd the same geometry that makes nearer things look bigger.) Since two slits work the same way, if the light source is not at infinity it is helpful to use a lens to "pull" the divergent light paths together. So the center of the light wave that goes through the left slit would be pulled to the right, and the center of the light wave that goes through the right slit would be pulled to the left, and the center of each light wave will then meet at the center of the detection screen. In that case you will get a good set of interference fringes, and you will not be able to look at a single photon and tell which slit it has come through. If, however, you move the detection screen back and trace the "rays" through the apparatus, you will discover that they no longer meet on the detection screen. Instead, one ray will terminate at one point on the detection screen, and the second ray will terminate at another point on the detection screen. And if the experimenter discovers a photon at one or the other of these two points then it will be clear which path the photon can be associated with. The reason the experimenters do this is that they want to be able to choose either a case where they can see the wave character of light (in the nearer position where the light waves are neatly superimposed) or a case where they can see the particle character of light (in the farther position where they can say that if a photon is detected at position g then it must have gone through slit a).

The next diagram (p. 33, Abbildung 4.4) shows a related experimental set-up in which the electron is replaced by photons generated by passing laser output of 351.1 nm through a crystal that down-shifts them to create two entangled photons of 702.2 nm. The double-split wall interrupts one half of the entangled photons, and many of them terminate on that wall. The ones that find their ways through the double slits then pass through a "helper" lens which is intended to make sure that the light waves are nicely superimposed on the "double-slit detector." In order to avoid having the partner photons that reach the Heisenberg detector, only those that coincide in time with the ones that reach the double-slit detector are counted. If a large-area photon detector screen were used as the Heisenberg Detector, then there would often be more than one photon hitting the screen at the same time, so they have elected to place a narrow detector there. Then they have to make it scan across the width of the expected interference pattern, and recording how many hits per equal length of time are recorded for each increment of its journey. (Essentially, it's acting just like the detector in a desktop scanner. The original Macintosh scanners fit onto the printer head of a Mac printer, and the stepper motor of the printer was made to inch the photosensitive head across the screen, recording the light level for each pixel it could resolve, and then turning the whole thing into a low-resolution scan.)

If the Heisenber Detector is racked to its more remote position, it actually shouldn't have to move very far because it should (I think) just see two bright spots at about the separation of the double slits. But there are still going to be lots of entangled photons that reach the Heisenberg detector even though their partners have come to an early end on the first side of the double slit wall. So the coincidence counting is still going to be important.

In Diagram 4.5, it appears that they are holding what we have been calling the Heisenberg detector in its near position and not having it scan from side to side. Instead, they are having the "double-slit detector" move from side to side. When they do that and plot out the intensities (number of hits per "station" as the detector inches across the space where a traditional detector screen would have been), they get a standard Young interference pattern.

In Diagram 4.6 they appear to be holding what we have been calling the Heisenberg detector in its far position, and scanning from side to side with the double-slit detector. The diagram appears to be trying to express some understanding for why the interference fringes will disappear from the double-slit detector. The crystal appears to act as a reflector, transferring something from the Heisenberg detector to the double-slit detector. The result they diagram is that the double-slit detector sees no interference pattern. It sees a sort of bell curve of intensities.

In Diagram 4.7 they have put the Heisenberg detector back in its near position and they scan with it, recording the hits it gets when the double-slit detector gets a hit The result is an interference pattern.

In Diagram 4.8 they have put the Heisenberg detector into its far position again, and they check its hits that coincide with hits at the double-slit detector as the former racks back and forth. In this case, the detector remotely sees the two peaks that correspond to non-interfering beams of light from the two slits on the other side of the apparatus.

Why aren't 4.6 and 4.8 showing the same results? Apparently because there is no double-slit apparatus at the other end of the experimental setup. I am not sure that I have absorbed all of the implications of this experiment yet. The fact that a detector is racking across an incoming stream of light waves cannot determine whether the light waves are coming in or not, can it? I guess one experiment I'd like to see is to just insert a sheet of white paper in front of the double-slit detector for a while during the running of the experiment. (Or a nice wide digital camera CCD would work too.) Don't have it connected to any coincidence detector.

I think I'll look for some other secondary sources on this experiment. It seems to be worthy of an article of its own. P0M (talk) 04:07, 22 March 2008 (UTC)

I just re-read the article that evidently started this discussion (http://www.paulfriedlander.com/text/timetravel/experiment.htm) and it seems quite good to me. Can anyone identify inaccuracies in that article? P0M (talk) 04:54, 22 March 2008 (UTC)

Future ansible discussions
I think it would be good to find another place to discuss the general issues that gather around delayed choice quantum erasers and the possibilities of instantaneous communications. The apparatus described in this experiment could not be used for such purposes without substantial modification, and the design having been built for automatic production of randomly selected paths it may not be the best example to choose to try to elucidate these questions for the readers of Wikipedia.

Could we find one or two authoritative discussions of whether this apparatus could be used for instantaneous communication? While getting clear on the issues is very important for crafting articles that do not mislead readers, we cannot go farther and put forth our own conclusions. To do so would break the rule against publishing original research in Wikipedia.

Another apparatus (see [[Media:Walborn_EtAl_QuantumEraser.svg]]) is discussed in the article entitled Quantum eraser experiment. The authors of the article that reports the experiment indicate that they have done the experiment in a delayed erasure version, and the presence or absence of the polarizing element before detector P determines whether an interference pattern is detected at detector S. P0M (talk) 07:44, 12 March 2008 (UTC)


 * Well, I found this source which says in section 2.3 that backwards-in-time communication is absolutely ruled out in standard quantum field theory by "Eberhard's theorem", although the paper is mainly about looking for an alternative to quantum field theory which is similar enough that it can't be ruled out by past experimental results and which would allow backwards-in-time communication (but they don't produce a finished theory, only something that can replicate predictions of a simple 'toy' version of QFT). Cramer also mentions Eberhard's theorem in this article (in the paragraph beginning with 'At the AQRTP Workshop ...'), and also says that Eberhard's theorem rules out FTL or backwards-in-time communication in orthodox QM, although he mentions the possible that QM might be incorrect and that the true theory might look like QM with a small nonlinear element introduced into the equations, which would possibly mean FTL (or backwards-in-time) communication might be possible in the new theory.


 * Is there a link to the Walborn article you're talking about? I wonder if the interference pattern at detector S is claimed to be the total pattern of particles at that detector, or just a subset seen through coincidence-counting as in the delayed choice quantum eraser. Hypnosifl (talk) 15:59, 12 March 2008 (UTC)
 * http://grad.physics.sunysb.edu/~amarch/Walborn.pdf P0M (talk) 16:54, 12 March 2008 (UTC)
 * I think those sources should be cited in the article. That would make it "official." I'll have to have a look at Eberhard's theorem. The reason I'm curious is that "backwards-in-time" would seem to imply reversing the flow of time, and probably at minimum involves processes that occur in time, i.e., processes on which one can use the ordinary operational definition of time (watch the second hand go around as you watch the cannon ball fall from the Tower of Pisa), whereas the processes involved in these experiments are non-local and do not involve a process of transmission that one could time. P0M (talk) 16:54, 12 March 2008 (UTC)


 * But the two sources I linked to aren't talking about whether processes can involve backwards or FTL causality, but rather the more narrow question of whether observers could actually communicate information FTL or backwards in time (FTL and backwards-in-time communication would be equivalent in relativity, see Time travel)--the first paper refers to the "backwards-time flows of information" and the Cramer article says that Eberhard's theorem shows QM "cannot be used for FTL observer-to-observer communication". So I'm pretty sure all they're talking about is the results of measurements made by each observer, and whether the results of one observer's measurement can instantly give him any information about what type of measurement was made by another distant observer, not interpretational questions of what is going on "behind the scenes" to explain the process that determined each observer's results.


 * And the Walborn article does seem to be just talking about interference patterns seen in coincidence counts, not in the total pattern of photons at any detector--all their graphs have the vertical axes labelled as coincidence counts. Hypnosifl (talk) 18:48, 12 March 2008 (UTC)


 * By the way, if we're going to cite sources in the article for the impossibility of FTL or backwards-in-time communication in orthodox QM, it might be a good idea to cite the original Eberhard article. The first paper I linked to lists two papers by him:


 * P.H. Eberhard, Bell’s Theorem and the different concepts of locality, Nuovo Cimento, Vol. 46B, No. 2, August 11, 1978, p.392-419. Also see P.H. Eberhard and R.Ross, Quantum theory cannot provide faster-than-light communication, Foundations of Physics Letters, Vol. 2, No. 2, 1989, p.127-149.


 * I assume the second paper would be the more relevant one. Hypnosifl (talk) 20:15, 12 March 2008 (UTC)


 * I don't have access to those articles.


 * One of the social problems seen in recent years seems to have been made worse, if not caused, by mass media sources that make statements about what "science proves" that are later refuted by scientists when new research bring forth new information. Experience in the real world will tell whether people like Cramer can find a way to exploit entanglement to communicate information outside the kind of processes that take time. In the meantime what we can report is that FTL or instantaneous communication is incompatible with theory x.  P0M (talk) 15:15, 13 March 2008 (UTC)
 * Ironically, just after I wrote the above I walked out into the yard where a construction company is putting a new roof on an outbuilding. I mentioned something about global warming to the owner of the construction company. "It's happened before and it'll happen again. Mother Nature will take care of us." He's now got half of the message, but he won't listen to what scientists tell us about how many years it will/would take to get the carbon dioxide content of the atmosphere back down.P0M (talk) 16:54, 13 March 2008 (UTC)


 * Right, what we can say definitively is that FTL communication is incompatible with the current mathematical theory of quantum mechanics. But that doesn't rule out the idea that the current theory is wrong and that it is possible in the real world (personally I doubt it since FTL is the same as time travel in relativity, but you never know), and Cramer actually suggested a way it might be wrong in his article (nonlinear terms added to the equations). Anyway, it would be interesting to know what the current theory predicts about the results of Cramer's experiment--it must give some prediction about it. Perhaps, as with the delayed choice quantum eraser, it would predict the total pattern of photons at each detector would show no interference, but by looking at particular subsets one could recover an interference pattern if the top lens was at the right distance. Hypnosifl (talk) 18:43, 13 March 2008 (UTC)\


 * So what happens if one isolates the two paths that would have merged at either d1 or d2? Presumably d0 would not get an interference pattern from that "side" of the apparatus, no? P0M (talk) 01:34, 14 March 2008 (UTC)


 * You could ensure that all the signal photons end up at D3 or D4 by replacing the beam splitters BSa and BSb with mirrors, for example. In this case the total pattern of signal photons at D0 will just be the sum of the D0/D3 coincidence count and the D0/D4 coincidence count (since every signal photon at D0 is associated with an idler that went to one of those detectors), and neither coincidence count shows interference, so the total pattern of signal photons won't either. On the other hand, you could ensure that all the signal photons end up at D1 or D0 by removing the beam splitters BSa and BSb altogether, in which case the total pattern of signal photons at D0 will be the sum of the D0/D1 coincidence count and the D0/D2 coincidence count. Each of these coincidence counts does individually show interference, but as noted in the last paragraph of the "The experiment" section of the article, the two interference patterns are out-of-phase so the high points of one line up with the low points of the other, meaning that their sum will look like a non-interference pattern. So, no matter what you do to the idlers, the total pattern of signal photons at D0 always looks like a non-interference pattern. Hypnosifl (talk) 05:53, 14 March 2008 (UTC)


 * That's not what I had in mind. My original idea might have been difficult to make happen in the real world. Here is another one: Suppose that we remove BSc and redirect Ma and Mb so that the red and blue beams concide at a single detector.  They would be of crossed polarizations, a factor introduced by the BBO, so a polarizer would have to be introduced into one or the other path to correct for that factor.  P0M (talk) 06:55, 14 March 2008 (UTC)


 * Even if you removed BSc and redirected the mirrors so the blue path and red path would converge on a single detector, wouldn't they be converging on it at different angles so you could in principle tell which path they came from by measuring their momentum, or else wouldn't the path lengths be different so you could tell which path they came from by measuring the delay between the signal photon detection and the idler detection? Basically I don't know if there's any setup that will cause all the idlers to converge on a single which-path erasing detector in such a way that there's absolutely no way to distinguish idlers that came on the blue path from idlers that came on the red path. Hypnosifl (talk) 07:39, 14 March 2008 (UTC)


 * Ideally, one could arrange things so that the path lengths would all be equal, and that would seem (but only seem) to take care of time differentials. But if you do the Young experiment and use a detection screen formed by removing one surface sheet from a piece of corrugated cardboard you will still get an interference pattern. You can also cut a hole in the middle of the detection screen and put a secondary detection screen at some distance behind it without causing any problems with the interference pattern.


 * In the original double-slit experiment, the probability waves converge on the detector screen from different angles. Nevertheless, an interference pattern appears. Even in that simple situation, the times at which photons appear on the detection screen will vary with distances from the slits. But the time and place of positive interference still work out. At pairs of distances along the detector screen the arrival of a high probability part of the wave from the A slit will meet the high probability part of the wave from the B slit and at a random one of those distances a photon will be detected. One might think that the photon would always show up where and when the first high probability parts of the wave hit the screen, but even in this simplest of situations things do not work out in the time sequence that our experience in the macro world would indicate. It seems that one has to say that from the drop in orbital of an electron in the light source to the rise in orbital of an electron in the detection screen is one event. The probability wave that our model(s) say passes between them behaves in such a way that t=d/c elapses during the course of that event, but the photon's "choice" of where to pop up on the detection screen is, paradoxically, time independent. I've seen discussions of momentum measurement too, but I can't remember where. It may be implicit in the original argument where Einstein proposed to remove indeterminancy by physical measurements but Bohr (See:N. Bohr, in Albert Einstein: Philosopher-Scientist, edited by P. A. Schilpp ~Library of Living Philosophers, Evanston, 1949!;
 * reprinted in Quantum Theory and Measurement, edited by J.A. Wheeler and W. H. Zurek ~Princeton University Press, Princeton, NJ, 1983.) proved by Einstein's own relativity theory that it couldn't be done the way Einstein had assumed. (talk) 22:19, 14 March 2008 (UTC)
 * Position and momentum cannot be determined at the same time and information gained about one is information lost about the other. When a photon is manifested on a photographic emulsion, you get a chemical change at molecular size, and you lose momentum information. Gaining momentum information destroys the interference fringes.
 * http://prola.aps.org/abstract/PRA/v57/i3/p1519_1
 * The reason the entangled particles were used in the first place was to try to get around these strictures by measuring momentum of the idler while measuring position of the signal, or vice-versa. Basically all the ways I have seen that attempt to pin down the path of the idler consist of various ways of isolating (marking) the paths. The apparatus in the experiment under discussion was cleverly designed to automatically and randomly swap in a path isolating apparatus (the parts leading to d1 and d2) while swapping out the path merging apparatus (the parts leading to d3 and d4). The only reason that the two interference fringe patterns are complements of each other is that one path reflects from the first-surface side of BSc and the other path reflects from the opposite side of that silvered surface after having passed through a pane of glass. Actually, replacing Ma or Mb with a second-surface mirror might well fix the phase change problem.
 * But let's confront the issue directly. Let's just ask what will happen if part of the time we use the top front-end, and the rest of the time we use the bottom front-end on the idler portion. [[Image: Kim_apparatus+2_new_front_ends.svg|300px|thumb|Two different front ends]]P0M (talk) 02:53, 15 March 2008 (UTC)

(responding to above comments but un-indenting to make more readable) You do raise some good points about it apparently not being possible to determine which slit a particle in the double-slit experiment went through just based on its momentum or on the timing of when it reaches a spot on the screen. It's possible there's something different about the modified DCQE where you rearrange the mirrors to make all the photons go to a single which-path-erasing detector, but I'm not sure.(1) I do have one other speculation, and it involves the fact that idler photons would not really be detected exclusively on the neat paths in the diagram, the paths just show where most of them would be detected; but if you placed a detector slightly to the left or right of one of the detectors in the DCQE, you should get some photons there too, though less (this is definitely true of the detectors in the Dopfer experiment, as you can see by looking at the diagrams on pp. 36-38 of Dopfer's thesis where they always keep one detector at a fixed position while varying the position of the other in order to graph the spatial distribution of hits on the position-varying detector that coincide with hits on the fixed detector; also see the discussion I just added to the "Cramer ansible" section earlier on this Talk page). So, my alternate speculation about what might happen in the modified DCQE where you aim all the photons at one which-path-erasing detector is this: perhaps if you placed an array of detectors side-by-side, or a wider CCD detector that could detect photons at a range of horizontal positions in the plane perpendicular to the path, then you would indeed find photons at a range of horizontal positions. Then if you took a coincidence count between idler photons at some very narrow range of horizontal positions at this detector and signal photons at the D0 detector, you'd see an interference pattern; but the total pattern of signal photons at D0 would still not show interference (as it shouldn't, by Eberhard's theorem), since the total pattern is really the sum of all the coincidence counts for each narrow range of horizontal positions at the idler detector.(2) Hypnosifl (talk) 20:07, 16 March 2008 (UTC)
 * I think one thing that is going on with these experiments is that when they are done with single photons the detector must be a very sensitive device, and technical issues (or maybe just the expense) limits the experimenters to devices with very small input ports. So what they do is to put the detector on a movable platform on tracks that is pulled across the "line of fire" by a stepper motor or something of that sort. They collect hits in this meter for equal lengths of time at each position as it moves from one side to the other, and then they assemble all of the data. That way they can build up an interference pattern on a computer screen or some similar device.
 * I guess they don't do them in total darkness either, and maybe there are some incidental photons that get into the detectors from who knows where, so one of the functions of the Co-incidence detector is to match the arrival of a photon in the detector with the departure of one from the laser. That way if something come into the detector at some other time the assumption is that it wasn't one of their photons and should be disregarded.
 * I had a go at locating the actual experiment reports on the simplest one of these Wheeler-like experiments after I had tried it out with some simple beam splitters and first-surface mirrors and got nowhere. One kind of thing to watch out for is, e.g., that the actual article may mention a very specific type of beam splitter (one that would be out of my financial reach, perhaps) and people who write secondary source materials omit that little detail. So I get the $10 kind and nothing happens.
 * Not having a cheap source of entangled photons at hand, I am very reluctant to depend at all on my own predictions of what ought to happen. If I were to forget to take a single factor into consideration I could get an entirely mistaken picture. For the same reason, I'm not very trustful of the experts who say what will happen without really having tried it. If you can't trust Einstein to get it right then whom could you trust?


 * A single photosensitive charge-coupled device (CCD) can pick up photons from a range of horizontal positions without having to move it back and forth--see here for example. According to the comments by "Ben" that I quoted in my most recent post in the "Cramer ansible" section above, the reason they use photodetectors with a narrow range in experiments like Dopfer's (and in the delayed choice quantum eraser too, presumably) is simply because they want to do a coincidence count rather than register the total pattern of photons--you can only recover an interference pattern by doing a coincidence count where you look at the subset of photons at one detector whose entangled twins were seen at the other detector with its position held fixed. Hypnosifl (talk) 03:46, 17 March 2008 (UTC)


 * (1) above:"It's possible there's something different about the modified DCQE where you rearrange the mirrors to make all the photons go to a single which-path-erasing detector, but I'm not sure." The reason that d3 and d4 pick up interference patterns that are complementary to each other, and that would produce an evenly illuminated screen if they were combined, is that bs3 is constructed so that there is a partially silvered first surface mirror if you look at it from one side, and a partially silvered second surface mirror (like a regular mirror except for not being fully silvered).
 * There is a phase shift when light goes from air to glass and/or glass to air, so the light that bounces off the first surface does not undergo a phase change, but the light that bounces off the other surface is 180 degrees out of phase. When the two beams form interference patterns in the same detector they are flipped, left to right.
 * (2) I need to work through the interferometer versions of the double-slit experiment to get clearer on what is going on. One of the problems with my little physics lab, I just realized, is that the beam that comes out of a laser pointer forms a spot on any detection screen. But what you want is the approximation of a geometrical line of photons, not a geometrical cylinder of photons.
 * Let's follow a single photon, and then keep in mind that in the real world there would be a bunch of other photons being fired along parallel courses. The single photon comes out of the BBO and we could argue about which port it "really" exits from. It is more useful to talk about the probability wave.The probability wave has to follow two paths and emerge from them in such a way that the maxima and minima do not exactly coincide, or otherwise we will not notice any interference.  In the Young experiment this arrangement is guaranteed because of the lateral separation of slit a and slit b. In the interference version you must have to "aim" the two probability waves correctly at the same detector screen.  If you use a single photographic emulsion you can fire single photon after single photon, and their probability waves (assuming the experimenter has gotten things lined up right) will interfere. When they interfere, the probability of a photon's appearing at some points will drop to 0 and the probabillity of a photon's appearing at other points will be enhanced. That means that instead of all of the hits occuring near the center (as would happen when a single slit yields a diffraction pattern in Young's apparatus with one slit blocked), you get very wide dispersion.  If you haven't done the experiment, you ought to try it.  It's very impressive how much the pattern flares out sidewise when the second slit is opened.  (You can see the double-slit apparatus I made with the thinest brads I could buy, some household glue, plastic railway track, etc., here:http://www.wfu.edu/~moran/Physics/) From a distance of about 12 feet the diffraction pattern that appeared on the side of my refrigerator was about the size of my hand. When I opened the second slit it covered all of the side of the refrigerator and probably beyond. Since the bands farther from the center are dimmer, it would need some fancy apparatus to tell where the yield of photons is so low that it is impractical to wait around for one. If you are going to use a high-priced photon detector with a narrow input port, then you have to fire a large number of photons one at a time, and count how many out of some quantity (several hundred perhaps) show up at each place where the little trolley holding the detector has a stop on its route.
 * Back to something I may have said earlier. If the Kim device could be rigged so that temporarily it would permit photons only to show up at d1 or d2, then those would be "particle" finds. (If you measure the output of some apparatus looking for particles you will find particles. If you measure the output of some apparatus looking for waves, you will find waves.) The result on single-path transmissions, the result of something analogous to the double-slit apparatus with one slit blocked, would be a diffraction pattern. As the photograph shows, you are likely to see not just a simple spot in the middle if light is going through a single slit, but a couple of side spots that are not clearly isolated from the central spot. That's because of the diffraction that occurs when light goes through a slit. Probably with an interferometer there would be no diffraction, and then you would get just a spot.
 * If, on the other hand, you rigged the Kim device so that light could not reach d1 or d2, but could reach d3 or d4, then the detector screen would not be black and would not be lit up by a single spot or a diffraction pattern. Instead, it would be evenly illuminated. So the difference between the two set-ups would be center spot vs. evenly illuminated screen. Would d0 reflect the same thing? That seems to be the natural conclusion to draw from the experiment as it has been described.
 * The way Kim et al. did the experiment was governed by the desire to see whether the location of a photon appearing at d0 would match the location of an entangled photon appearing at d1, d2, d3, or d4. There may have been noise in the experiment. (I think I have read something about random quantum effects producing the occasional photon, and maybe the lab the experimenters were operating in was not totally black.) For whatever reason they decided that the practical way of being sure what they were seeing on the various detectors was to detect "coincidences". (If there were path length differences then they would have had to account for a lag between detection times.) I can't remember for sure, but they may not have needed tractors on detectors d1 and d2 because the photons would have shown up at or near the center in all cases. But they probably needed tractors on d0, d3, and d4, and then they would have had to run the experiment for some time to accumulate representative numbers of hits at all the "stations" of the trolleys.P0M (talk) 00:43, 19 March 2008 (UTC)
 * Discussion at http://www.sciencemag.org/cgi/content/full/315/5814/966 and elsewhere seems to me to indicate that at least at the current level of experimental device sophistication there must be problems with interference effects being washed out simply by the presence of unwanted unentangled photons getting through from the laser. I've had a go at reproducing the results of that much simpler experiment just using a continuous laser beam. So far I have not had any success whatsoever. Alignment is not easy to achieve, for one thing. Over long distances it would be extremely difficult to maintain alignment, and probably impossible to screen out all extraneous photons. Probably the best thing for the article is to point out that experts in the field maintain that instantaneous communication would contradict quantum mechanics and/or other highly successful theories. P0M (talk) 07:19, 16 March 2008 (UTC)