It is said that in the double slit experiment one will prevent the interference of the photon with itself by simply observing which path the photon.

Here is a very first Google result for "double split experiment":

To find out, you might place a detector by the slits, to see which slit an electron passes through.


A second Google result:

But if they try to find out by setting up detectors in front of each slit to determine which slit the photon really goes through



What are these detectors they talk about? How can one observe which slit the photon went through without absorbing the photon in the detector? In which case it is obviously not forming interference pattern on the screen?

  • $\begingroup$ Just be careful whether they are taking about photons or electrons or molecules. For photons the observation is not possible because photon gets absorbed if a detector was used. For electrons (it's a thought experiment), if you observe it AFTER the slits it gets a new wave function due to the interaction ....the prior interaction with the slits is nullified. For photons, electrons, molecules the EM field is always present and mediating the interactions ... the EM field is wavelike. $\endgroup$ Jan 2 at 14:54

1 Answer 1


It means any way of interaction with photon, which forces it to collapse it's wavefunction when going through slits. In layman terms it means that photon is forced to choose specific slit instead from going through both slits at once and hence producing interference pattern at the screen.

For example, imagine constructing a single-photon source which emits each even photon vertically polarized and each odd photon - horizontally polarized. Before or after (I believe it doesn't matter) one slit you put vertically polarized filter and in second slit - horizontally polarized filter. So now you will surely know that photon with relevant polarization will pass only through required slit.

So added polarization filters is enough to destroy (polarized) photon superposition of states and hence produce just the ordinary pair of high intensity bands on the screen, instead of interference pattern.

It doesn't even matter if photon polarization is measured at the screen itself or not. For "photon path observation" or more technically, for photon wavefunction collapse is just enough to encode "which slit" information into photon itself.

  • $\begingroup$ This is very interesting! Could it be possible to coat the final srceen with such a polar filter surface? Then what happens is that one measure both where the photon is coming on the screen and which slit it has passed through. Right? Then the interference should disappear, I think; but the which way measurement only happens at the very end, not on the way. @Agnius Vasiliauskas $\endgroup$ Jan 30 at 10:06
  • $\begingroup$ "Polarized" screen will not change anything, IMHO, because interference will be already destroyed due to polarizing filters in the slits. Wavefunction is destroyed not because of measurement, but because photon is forced to choose particular path, due to filters in slits. Hence the overall effect for a photon is like there would be no pair of slits, but just 1 slit. Photon path collapse can happen due to many reasons, - measurement, environment noise, filters (artificially directing photon to some slit) , etc. It's not about measurement in narrow sense. $\endgroup$ Jan 30 at 10:34
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    $\begingroup$ You're right, I messed up my previous question. Let me ask another question related to this topic. Is it possible in practice to make a wich way measurement directly at the location of the detecting screen? So the detector is not behind the slits and not in the way. But can the screen itself measure in some way which slit the photon or particle has passed through? Is there some kind of tricky surface? @Agnius Vasiliauskas $\endgroup$ Jan 30 at 13:48
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    $\begingroup$ Interesting question, but as it is a completely different question with a wide and probably non-trivial answer,- I would suggest for you to register new post for this type of question. My initial intuition tells me that IF it's possible then probably such screen would destroy interference pattern, as it will become "a huge detector". I'm afraid "cheating" is not an option for photon :-) But maybe I'm wrong. $\endgroup$ Jan 30 at 14:38

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