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Counterfactual measurement in quantum mechanics

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Voting period is over. Please don't add any new votes. Voting period ends on 4 Apr 2013 at 09:56:48 (UTC)

Original – Counterfactual measurement in quantum mechanics. How could the photon possibly have acquired information about M without an exchange of energy?
Edit 1 – Counterfactual measurement in quantum mechanics. How could the photon possibly have acquired information about M without an exchange of energy?
Reason
This figure illustrates a mind-blowing result of quantum mechanics, the phenomenon of counterfactual measurement. A Mach-Zehnder interferometer is adjusted so that 100% of the output impinges upon detector B. Having been adjusted in such a manner, 100% of the light will continue to reach B even if the the light intensity is reduced so that only one photon at a time travels through the apparatus. It appears as if the wave function of each individual photon travels both paths and engages in interference at the last beam splitter, so that only the wave to B is constructive. In figure (a), although the photon is illustrated as having taken the "northern" branch of the interferometer, it interferes with itself so that only detector B detects the photon. The same holds for figure (b), although the photon is considered to have taken the "southern" branch of the interferometer. Figure (c) illustrates the situation where an obstacle has been introduced on the "southern" branch of the interferometer. 50% of the photons are deflected by mirror M, and the remaining photons are split 25% to detector A, and 25% to detector B. Figure (d) illustrates the fundamental paradox raised by this demonstration. An individual photon arriving at detector A must have traversed the "northern" path and could not have interacted with mirror M. The arrival of a photon at detector A constitutes proof that an obstacle exists on the "southern" path, but no exchange of energy has taken place between the photon and obstacle M. How could the photon possibly have acquired information about M without an exchange of energy?
Articles in which this image appears
Original appears in Mach–Zehnder interferometer.
Edit 1 appears in the article Counterfactual definiteness but is not seven-day stable.
FP category for this image
Wikipedia:Featured pictures/Sciences/Others
Creator
Tatoute (original author), Quibik (edits), and Stigmatella aurantiaca (edits and English-language labels)
  • Support as nominator -- Support both versions, with preference given to Edit 1. Stigmatella aurantiaca (talk) 09:56, 26 March 2013 (UTC)[reply]
  • Comment I don't see why the final beam splitter is there. As far as I can see, it would mean that a photon could take the "southern" path and arrive at A (after being diverted by the beam splitter) even in a classical interpretation, which rather defeats the point of what the experiment is claiming to show (i.e. that to get to A, the photon cannot have passed through M classically). EDIT: Wait, no, ignore me. M is a mirror, not a half silvered mirror. MChesterMC (talk) 16:41, 26 March 2013 (UTC)[reply]
    • If we were to remove the final beam splitter, then in Figures (a) and (b), the photons would be split 50/50 between the two detectors A and B. There would be absolutely no mystery about how photons could reach detector A in Figure (c); indeed, no photons should reach detector B with M in the way. On the other hand, the experiment as illustrated works even if we attenuate the light source so that only a few photons per minute pass through the apparatus. The path of the photon is affected even though no energy is exchanged between the photon and the blocking object M. — Stigmatella aurantiaca (talk) 17:00, 26 March 2013 (UTC)[reply]
    • Wait. It looks as if you interpreted blocking object M as a beam splitter rather than a mirror. Perhaps this means that the illustration needs some revision to eliminate any possible confusion between the two? — Stigmatella aurantiaca (talk) 17:05, 26 March 2013 (UTC)[reply]
      • Yeh, I had a bit of a brain fart and forgot how mirrors worked... Although, as dllu says below, a bit more differentiation would be nice. I'd also question what the difference would be between adding the mirror M, and removing the bottom left mirror, which would make for a cleaner diagram. MChesterMC (talk) 12:03, 27 March 2013 (UTC)[reply]
        • I've been exchanging ideas with dllu. You can look at his talk page to see what I have so far in terms of an "Edit 1" version, which I might be posting here in a few hours if dllu gives his blessing. So far as removing the bottom left mirror goes, I'm sure that after spending several hours getting the mirrors, beam splitters, light sources and detectors set up, any experimenter would be rather reluctant to remove any of the major optical components from the optical bench unless he or she had to. Adding a blocking mirror is much less disturbing to the experimental setup. Stigmatella aurantiaca (talk) 12:24, 27 March 2013 (UTC)[reply]
  • Comment:
  • The ellipsoid shape of the mirrors is confusing (they are not curved mirrors, and they are not lenses) considering the diagram is a top-down view. Consider changing it to flat mirrors (like in File:E-V bomb-testing.svg).
  • The appearance of the light source is quite ugly and looks like an incandescent light bulb (which is misleading, since incandescent light bulbs are most definitely not suitable for generating single monochromatic photons). It is probably better to change it into something more abstract-looking.
  • The appearances of the actual wave packet photon vs the "ghost" photon should be made more clear to indicate which is which, especially when viewed at thumbnail scale.
  • Despite the title of this nomination, the image does not appear in the counterfactual definiteness article.
Other than that, I am inclined to support because I believe that a good diagram very significantly contributes to understanding of the concept in the article. As a side note, maybe the File:Elitzur-Vaidman bomb tester.svg picture should be placed into the Elitzur-Vaidman bomb tester article. dllu (t,c) 20:02, 26 March 2013 (UTC)[reply]
I had wished to make a minimal number of changes from Tatoute's original illustration (which, after searching through Commons, appeared to be the best there is for illustrating the phenomenon), but yes, I agree with you that there is a fair amount to be desired in the figure. Let me make some more dramatic edits and offer it as an alternative. Give me a day. Stigmatella aurantiaca (talk) 20:55, 26 March 2013 (UTC)[reply]
That's fine, take your time :) Let me know if you need any help. dllu (t,c) 02:39, 27 March 2013 (UTC)[reply]
  • Edit 1 is now available. Although the appearance of this version may be greatly modified from Tatoute's original figure, it preserves the basic pedagogical elements of his presentation that made his illustration of counterfactual measurement the best on Commons. Stigmatella aurantiaca (talk) 04:56, 28 March 2013 (UTC)[reply]
  • Support Edit 1: it is a straightforward, clear, and effective illustration of the concept. dllu (t,c) 06:03, 28 March 2013 (UTC)[reply]
  • Comment I believe the picture would be more inviting if there was a clearer delineation and/or more space between the four sections. At the moment, at first sight it looks like one big complicated forest. Also, on my screen, at the sort of size shown above and used in the article, the beams are virtually invisible (Edit 1 even worse than original in this respect). (compare this with the beams in, say, File:Mach-Zender interferometer fringe localization.svg, which are very clear even at smaller sizes). Because of these concerns I feel unenthusiastic about the image as it currently appears. 86.160.220.22 (talk) 18:29, 28 March 2013 (UTC)[reply]
  • I understand your concern regarding the visibility of the 'beams', especially at thumbnail scale, but in this experiment only single photons are sent instead of continuous beams so the red dotted lines serve only to indicate the optical path. Feel free to suggest any way to represent the optical path without making them look like solid laser beams. (perhaps Stigmatella aurantiaca can just make the width of these dotted lines bigger) By the way, when voting, you should log into an actual account since votes from an IP address do not count. dllu (t,c) 19:19, 28 March 2013 (UTC)[reply]
It was a comment, not a vote. However, I'm not sure that excluding people without accounts from a "voting" process conforms with wider Wikipedia policy. If it does, please point me towards that policy. 86.160.220.22 (talk)
I know it was a comment. At the top of WP:Featured Picture Candidates there is the following sentence: Consensus is generally regarded to be a two-third majority in support, including the nominator and/or creator of the image; however, anonymous votes are generally disregarded, as are opinions of sockpuppets. See also the last item in guidelines for what an IP user can or cannot do. dllu (t,c) 20:55, 28 March 2013 (UTC)[reply]
OK, thanks. I have on occasions voted in other parts of Wikipedia without being logged on, and no one has ever commented that it was improper. It only seems to be here that people have an issue with it. However, I note the policy* 86.160.220.22 (talk) 21:17, 28 March 2013 (UTC) *Though having said that, I notice that the page you link to explictly says at the top that it is not a policy or even guideline, and should be treated "with discretion".[reply]
Although the guidelines state that anonymous votes are "generally" not counted, I personally have no problem with your being able to vote on this image. Your concerns are legitimate, I will do my best to address them, and the image will be improved as a result of your input, no matter which way you ultimately throw your vote. Stigmatella aurantiaca (talk) 21:45, 28 March 2013 (UTC)[reply]

Tatoute (talk) Hello, i am Tatoute, the original author of this picture. first i Apologize for my terrible english. I am very honored to be cited here. I have no problem at all about the fact that edits be done. Please let me make some explanation relative of the original picture: the original picture was intentionally made as most international as possible, impling only numbers and letters (i did not found idea to remove roman letters). Introducing english indication reduce the availability of your work. Indication of semi-reflective nature of 2 mirror was done by letting light beam seen through (for beam splitter) & not seen through for mirrors. I agree about the remark about elliptical shape: it may be confusing, parallelogram would be better, but reducing mirrors as simple line remove the immediate perception due to the small color gradient in them. I did use incandescent light bulb as light source for simplicity: Wikipedia is not for specialist, so basic illustration may be clearer. Please note that Mach Zender_interferometer has to work even with light bulb... Regards, Tatoute —Preceding undated comment added 23:22, 30 March 2013 (UTC)

Not Promoted --Armbrust The Homunculus 10:05, 4 April 2013 (UTC)[reply]

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