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Basically I am not satisfied with the answers to this question.

The question is asking for records (images, videos) of the double slit experiment with a which-way detector. And although the answers give some interesting information, they don't point to any such records.

Any help is appreciated since all videos and articles suspiciously skipping the detector or simplifying it as a 3d cat or fictional cartoon eye.

It is possible to observe a photon and other particles without absorbing it.

So why can't I find an experiment which shows the double slit experiment with a which-way detector on the internet?

What I would like to see is the interference pattern disappearing after the addition of the detector.

H. Walter
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    Have you considered the possibility that it is not possible to detect which-way without destroying the diffraction pattern? – Dan Jan 06 '22 at 00:25
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    Yes. I mean I know it is, but that’s the point. I am trying to find an experiment with the diffraction pattern destroyed. Sorry if I wasn’t clear in my question. – H. Walter Jan 06 '22 at 00:37
  • See https://www.nature.com/articles/36057 – Al Nejati Jan 06 '22 at 01:08
  • And also https://ui.adsabs.harvard.edu/abs/1991PhRvL..67..318Z/abstract – Al Nejati Jan 06 '22 at 01:11
  • It is not possible for a photon to be observed without it being absorbed by something. – Bill Alsept Jan 06 '22 at 04:03
  • @BillAlsept Yes it is possible to detect a photon without absorbing it: http://pages.erau.edu/~reynodb2/colloquia/nature400-239.pdf And I am pretty sure it is possible to observe molecules without absorbing them. Molecules that are also capable of producing the interference pattern. (I don't know about electrons) From the article: "Here we report a cavity quantum electrodynamics experiment in which we detect a single photon non-destructively." – H. Walter Jan 06 '22 at 07:19
  • But the double slit experiment can be done with other particles, like electrons or some molecules. And molecules are observable twice for sure. Where do people who make statements such as "electron behavior appears to change when being observed" get the confidence and the knowledge to say that? – H. Walter Jan 06 '22 at 07:26
  • Thanks @AlNejati, I think Figure 2 from X. Y. Zou, L. J. Wang, and L. Mandel is what I was looking for, more or less. – H. Walter Jan 06 '22 at 08:42
  • @H.Walter ok I will make it an answer then, that way it's easier to refer back to in the future. – Al Nejati Jan 06 '22 at 09:12
  • The question remains: how were people convinced of that before 1991? – H. Walter Jan 06 '22 at 09:25
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    @H.Walter How do you detect a single photon in a double slit experiment where it contributes to the interference pattern without being absorbed? – Bill Alsept Jan 06 '22 at 15:57
  • @BillAlsept I don’t know. I just know the article I posted in my previous comment seems legit and was posted by someone else on this forum as a reply to my question. I am not even a physics major, I am just trying to understand where people get their certainty when they talk about this stuff. – H. Walter Jan 06 '22 at 16:06
  • @H.Walter see my edit – Al Nejati Jan 06 '22 at 19:00
  • Here is more history, Feynman got the ball rolling: https://www.eurekalert.org/news-releases/831858. He had the best grasp of QM at the time and popularizered the DSE with particles in famous lectures. – PhysicsDave Jan 06 '22 at 19:05
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    Yes photons detected are absorbed/destroyed. In the experiment you reference above the photon "presence" is detected but not position or momentum .... So no information is gained. – PhysicsDave Jan 06 '22 at 19:08
  • I doubt that the amount of information is strictly zero @PhysicsDave It would mean the experiment is as good as nothing – H. Walter Jan 07 '22 at 20:04
  • @H.Walter Photons are individual particles That can only go through one slit at a time. A single slit forms a unique interference pattern as many photons diffract and scatter left and right across the detection screen. A double slit creates two of these patterns that overlap on the detection screen, creating a new and different interference pattern. A photon only goes through one of the slits and the only way to find out which one is to block one of the slits (absorbing photons) which illuminates the double slit pattern, leaving you with a single slit pattern. – Bill Alsept Jan 08 '22 at 00:42
  • @PhysicsDave I used to tell people to read Feynman on quantum mechanics. I don't do that anymore. Feynman had a different, but also very poor understanding of quantum mechanical ontology IMHO. A modern reader should avoid all of these authors from the early days. Curiously the young Heisenberg had a few years during which he seemed to have had enlightenment, but that got lost again as he aged. By the time he wrote "Physik und Philosophie", I think, he was just as wrong about how QM actually works than most of his fellow "founders". I could be wrong. I haven't re-read that book in a long time. – FlatterMann May 20 '23 at 20:29
  • @BillAlsept Photons aren't moving, at all. Energy flows in the free field, but it is not made up of photons in an atomistic way. That's the great ontological misunderstanding about quantum mechanics. It is NOT "Newtonian corpuscular theory 2.0". – FlatterMann May 20 '23 at 20:31
  • @FlatterMann that's silly, I can't think of anything more ontological sounding than saying "FREE FIELD" without even explaining what it is. I mean "Energy flowing in the free field" is not much different than saying Spiritually connected or some statement based on faith or just because... One thing we know for certain is that particles exist. Photon particles can explain every phenomenon to do with light. You cannot physically describe the propagation of light or a light waves without incorporating individual photons. Just saying "a field" is kicking the can down the road. – Bill Alsept May 21 '23 at 03:43
  • @BillAlsept I have never seen a particle in my entire life, even though machines that were partly designed by me have detected many trillions of quanta of energy. The word "particle" belongs into the dustbin of history, together with its friends phlogiston and aether. They are all based on the same trivial objectification fallacy. – FlatterMann May 21 '23 at 05:33
  • @FlatterMann now you are really being silly. There are particles everywhere from dust particles protons to atoms, dust grains of sand. Even a planet can be considered a particle. If you don't even believe in particles there's literally nothing for you to discuss here. – Bill Alsept May 21 '23 at 06:22
  • @BillAlsept So you are saying that dust particles don't rotate? That's not what Landau and Lifshitz are saying. They are defining the word "particle" as the name of an approximation in which we neglect the rotation to simplify our lives. In other words, a "particle" in physics is a man-made abstract and not an object or category of objects. I am not a philosopher, but it abstracts are generally not considered real. They exist only in our hearts and minds and on paper. – FlatterMann May 21 '23 at 13:32
  • @FlatterMann of course they can rotate, any particle can. That's my point, Its easy to prove particles exists, They can have size, momentum, speed, direction, rotation, energy and most of all they can be counted. It is this field you mentioned that is philosophical and used more for approximation. When it comes to light, words like field or wave are for the hand waving conversations. You still have not attempted to physically describe either one. It can't be done. – Bill Alsept May 21 '23 at 15:01
  • @BillAlsept So you are basically saying that Landau and Lifshitz have written a bad textbook. – FlatterMann May 21 '23 at 15:02
  • @FlatterMann, your responses diverge from the original question, which is about detecting the path of particles through slits and requesting visual evidence. Instead of addressing the question, you veer into philosophical discussions and assert that particles do not exist, implying (in your opinion) that the question has a false premise. You could have provided a relevant video, such as the one found at this link: https://youtu.be/dJywiz37lNQ, which demonstrates both single and double slit interference. Which can be explained with individual photon particles. – Bill Alsept May 21 '23 at 16:18
  • @BillAlsept One can't detect the path of quanta because quanta do not have a path. Energy never had a path, not even in classical mechanics. Energy flows between systems. It's a system property. – FlatterMann May 24 '23 at 16:52
  • @FlatterMann you just need to reason through what your saying, which is really nothing. Your idea can't even be wrong because it makes no claims at all. Lets say a device here on Earth emits a single photon (there are thousands of scientific experiments that claim they can do this. Do you disagree with them?) The devise emits a single photon every minute and eventually a devise on the moon detects one. Can you physically describe how that quanta of energy (photon) went from A to B? I'm sure what ever you dream up will be far, far more complicated than a single photon going from here to there. – Bill Alsept May 24 '23 at 23:30
  • @BillAlsept Energy in single photons flows exactly as Maxwell's equations predict. Single photon physics is, except for emission and absorption processes that require matter interaction, identical to the mean field theory. – FlatterMann May 24 '23 at 23:41
  • @FlatterMann, why can't you provide a physical description of how energy moves from point A to point B? A particle/photon theory can easily explain all phenomena of light both physically and mathematically. Why resort to complex and intricate concepts that can't adequately be explained (for example, what comprises the field itself). Your just deferring the problem to a later time. Simply stating that energy flows through the field without delving into the nature of the field is not much different than someone claiming a spiritual connection based on faith alone. What's a field without photon? – Bill Alsept May 25 '23 at 03:54
  • @BillAlsept Quantum mechanics is not an atomistic theory. It can, however, take energy from one part of nothing and move it to another. At the scale of the entire universe it can even make very large amounts of energy out of nothing, it seems. I wouldn't complain too much about that... without this capacity neither of us would be here. – FlatterMann May 25 '23 at 06:16
  • @FlatterMann I'm definitely not complaining, I am absolutely interested in all of it. I am interested in what you say but could be more specific when you say things like: take, move and make. What is is happening when you say that? – Bill Alsept May 25 '23 at 07:31
  • @BillAlsept The same thing is "happening" as in classical mechanics, just at smaller scale. Energy was always a systems property and it is still a systems property. Expecting a property to behave like an object is simply a category error that the human mind is prone to. – FlatterMann May 25 '23 at 13:23
  • @FlatterMann Of course its the same thing at a much smaller scale. There is nothing smaller or proportionately more energetic than individual photons. You can't describe anything smaller than that. You say the same thing is happening, then please describe it without handwaving. – Bill Alsept May 25 '23 at 15:38

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Most real-world versions of the experiment that I've seen don't actually use two slits, but rather an optical circuit with separate paths using beam-splitters. In "Induced coherence and indistinguishability in optical interference" by Zou et. al., they discuss an experiment with two coherently pumped optical down-converters. The down-converters generate both signal and idler photons. The path of the signal photons is measured using a pair of photon counters configured to detect the idler photons. Non-classical interference patterns are observed that depend on the configuration of a beam stop placed in the idler path. As they say in the paper, this is strange from a classical point of view and only makes sense in the context of quantum effects occurring.

(This kind of setup is a Mach-Zehnder interferometer and has a long history).

As to why people were so 'confident' that a which-way detector destroys the interference pattern before the experiment was conducted in the literal sense, it's because this is a straightforward prediction of quantum mechanics, and quantum mechanics itself has a lot of experimental verification. As far as I know, the actual experiment with a which-way detector wasn't carried out even in Feynman's time and remained purely a thought experiment. He says so himself (The Feynman Lectures on Physics, 1965, vol. 3, 1.4):

We should say right away that you should not try to set up this experiment (as you could have done with the two we have already described). This experiment has never been done in just this way. The trouble is that the apparatus would have to be made on an impossibly small scale to show the effects we are interested in. We are doing a “thought experiment,” which we have chosen because it is easy to think about. We know the results that would be obtained because there are many experiments that have been done, in which the scale and the proportions have been chosen to show the effects we shall describe.

He doesn't make specific the 'many experiments' he's referring to, however it's not unlikely that he's referring to Compton scattering. The core of the argument that interference disappears when you use a which-way detector is based on wavefunction collapse; the earliest experiment that I'm aware of that demonstrates wavefunction collapse (of a single photon) is Compton's 1925 experiment. In the years 1925-1927 there were a lot of further experiments which culminated in the 1927 Solvay conference and subsequent debates on collapse and various interpretations. A lot of the details of how this understanding evolved have been lost in the re-telling.

More modern perspectives on this experiment have been given, with some more history and discussion here.

Urb
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Al Nejati
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https://phys.org/news/2011-01-which-way-detector-mystery-double-slit.html

In this link they describe a which way experiment with electrons done by the Italians.

After this there were many thought experiments to emphasize the point.

PhysicsDave
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    Thanks for the answer but it doesn’t really answer my question. “A team of scientists has found a clue as to why electron behavior appears to change when being observed”. Clearly people seem to know for sure (take for granted?) the fact that electron behavior appears to change when observed. What I am interested in is proof of that change. Show me the two slot experiment without and with a detector and the resulting patterns on the wall. – H. Walter Jan 06 '22 at 00:44
  • You can see my answer to a related question here : https://physics.stackexchange.com/questions/685051/is-my-understanding-of-the-double-slit-experiment-correct/685146#685146 – PhysicsDave Jan 06 '22 at 18:50
  • Also another related answer here : https://physics.stackexchange.com/questions/684577/can-molecules-used-in-double-slit-experiments-interact-with-light/684677#684677 – PhysicsDave Jan 06 '22 at 18:52
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@H.Walter your final question was: "What I would like to see is the interference pattern disappearing after the addition of the detector." There are images and videos like this: youtu.be/dJywiz37lNQ that show the difference. I am sure there are much better ones. But keep in mind that both Single and Double slit experiments have interference. It takes hundreds, thousand or millions of photon detections at the screen to form a good pattern and detecting (absorbing) one of them back at the slits before it gets to the screen would not change the pattern enough to be noticed. When you completely block one of the slits (in other words detecting/blocking EVERY photon at that slit) you will see the difference between Double Slit interference and Single Slit interference.

Bill Alsept
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  • That video just shows the difference between two slits and one slit. It doesn't show what happens when you measure which slit the photon went through. – Peter Shor May 21 '23 at 20:20
  • @Peter Shor its not the best video but it shows more than just the difference between the double and single slit experiments. It shows it actually happening as you block (Same as measure) one of the slits. When you measure a slit to see if a photon is there, then you intercept the photon and it does not make it to the screen. Which is the same as blocking. – Bill Alsept May 21 '23 at 23:52
  • The loss of interference when you block the photon in a two-slit can be explained purely classically. It will also happen for water waves. I think the OP wants something quantum mechanical. They remark "It is possible to observe a photon and other particles without absorbing it." So I don't believe this answers the question. – Peter Shor May 22 '23 at 00:02
  • Just read his last sentence: "What I would like to see is the interference pattern disappearing after the addition of the detector." – Bill Alsept May 22 '23 at 02:36
  • @Peter Shor, it's important to note that the behavior of water waves passing through a slit differs from that of photons passing through a slit. Specifically, when photons pass through a single slit, they produce an interference pattern, whereas water waves do not exhibit this phenomenon. The comment made by the original poster regarding photon absorption holds significant relevance in relation to my answer. – Bill Alsept May 22 '23 at 02:49
  • I thought single-slit interference patterns were explained by Huygens' Principle. And I thought Huygens' Principle was applicable in both the classical and quantum cases. Which of these assumptions did I get wrong? – Peter Shor May 31 '23 at 13:07
  • @Peter Shor water waves can easily be explained as molecules pushing against each other and when you block one slit you can obviously see why the interference goes away and it obviously has nothing to do with knowing the which way information. A light wave is made of individual photons moving in straight lines until they're diffracted or scattered left or right and this creates what looks like a wave. Huygens just said wave front or wavelet and did not say what they were or what they were made of. Water waves through a single slit do not create interference but lots of photons can. – Bill Alsept May 31 '23 at 15:10
  • Just because water waves can be explained as molecules pushing against each other, this does not mean that they do not exhibit interference. They do, even when going through a single slit, because of Huygens' Principle. Huygens lived in the 17th century, so he did not know anything about quantum mechanics; his principle applies to water waves as well as light waves. – Peter Shor Jun 01 '23 at 16:05
  • @ Peter Shor No, if water waves going through a single slit created an interference pattern like photons do when they go through a single slit, then I think you would be able to show me an example. Remember what a single slit interference pattern looks like with light? You don't see that with water waves. – Bill Alsept Jun 01 '23 at 18:05
  • I can show you an example — Video. From the caption of the video: "With a narrow single slit opening we see a plane wave emerge as a spherical wave on the other side of the barrier. This can be explained by the Huygens-Fresnel principle which states that each point in the slit forms its own spherical wavefront source. With a wider slit, more point sources arise, causing interference patterns due to the larger angles involved with a broader spatial distribution of these sources." – Peter Shor Jun 01 '23 at 19:15
  • @Peter Shor your video show diffraction but where is the familiar single slit interference pattern that photons create? – Bill Alsept Jun 01 '23 at 22:04
  • All right -- if you don't believe a video, can you explain to me what is different about the equations for light going through a single slit and water waves going through a single slit that makes them have different interference patterns? If this has anything to do with quantum mechanics, this would be a macroscale demonstration of quantum mechanics, which would be amazing. – Peter Shor Jun 01 '23 at 23:47
  • And why do you think 19th-century physicists didn't worry about the difference between the interference patterns of water waves and light waves, if they don't behave exactly the same? – Peter Shor Jun 01 '23 at 23:48
  • @Peter Shor water waves through a SINGLE slit demonstrates waves and diffraction but NO interference. If you see an interference pattern, please highlight it or something. With coherent photons going through a single slit you DO get an interference pattern. That's the difference. – Bill Alsept Jun 02 '23 at 05:40
  • @Peter Shor as for 19th century physicists, I'm sure they too wondered why photons through a single slit create interference but water waves did not. – Bill Alsept Jun 02 '23 at 06:15
  • At this point, it's clear that you're set in your opinions, and I'm not going to convince you. But wouldn't you think that light waves behaving differently from water waves would have been one of the great mysteries at the turn of the 19th century that led to the discovery of quantum mechanics, and that it would be mentioned in all the histories of quantum mechanics, like the photoelectric effect and the ultraviolet catastrophe? But the histories don't mention it at all. The reason? Water waves and light waves actually follow the same equations, and both show interference from a single slit. – Peter Shor Jun 02 '23 at 10:41
  • @Peter Shor I agree but it's not my opinion, it's just an observation. Where is the interference patterns you keep talking about? It should be easy to point out. I would truly be interested in seeing it. Thanks for the time Peter – Bill Alsept Jun 02 '23 at 15:16
  • I think the reason you can't see exactly the same interference pattern in the video is that they don't have a screen or photodiode to project the waves onto. I don't know whether there's an experiment that remedies this. – Peter Shor Jun 02 '23 at 15:28
  • @Peter Shor If you find one, I would love to take it to chat, Thanks – Bill Alsept Jun 02 '23 at 15:35