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.

  • 1
    $\begingroup$ Have you considered the possibility that it is not possible to detect which-way without destroying the diffraction pattern? $\endgroup$
    – Dan
    Jan 6, 2022 at 0:25
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    $\begingroup$ 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. $\endgroup$
    – H. Walter
    Jan 6, 2022 at 0:37
  • $\begingroup$ See nature.com/articles/36057 $\endgroup$
    – Al Nejati
    Jan 6, 2022 at 1:08
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    $\begingroup$ @H.Walter How do you detect a single photon in a double slit experiment where it contributes to the interference pattern without being absorbed? $\endgroup$ Jan 6, 2022 at 15:57
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    $\begingroup$ 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. $\endgroup$ Jan 6, 2022 at 19:08

3 Answers 3


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.



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.

  • 1
    $\begingroup$ 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. $\endgroup$
    – H. Walter
    Jan 6, 2022 at 0:44
  • $\begingroup$ You can see my answer to a related question here : physics.stackexchange.com/questions/685051/… $\endgroup$ Jan 6, 2022 at 18:50
  • $\begingroup$ Also another related answer here : physics.stackexchange.com/questions/684577/… $\endgroup$ Jan 6, 2022 at 18:52

@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.

  • $\begingroup$ 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. $\endgroup$ May 21 at 20:20
  • $\begingroup$ @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. $\endgroup$ May 21 at 23:52
  • $\begingroup$ 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. $\endgroup$ May 22 at 0:02
  • $\begingroup$ Just read his last sentence: "What I would like to see is the interference pattern disappearing after the addition of the detector." $\endgroup$ May 22 at 2:36
  • $\begingroup$ @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. $\endgroup$ May 22 at 2:49

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