I am not a physicist, but a fan of physics. I found many videos on YouTube which explain the "double slit experiment" but most of them use animation and that is OK. When I search for any real world experiments; I find only those experiments which show the "interference pattern", but do not find any real world experiment which make the "which-path" measurement so that the "clumped pattern" appears instead of the "interference pattern".

So, is there any real world "double slit experiment" where the which-path measurement made?


4 Answers 4


Real experiments

Quoting "Experiment and the foundations of quantum physics" by Anton Zeilinger :

A recent experiment (Dopfer, 1998) used the so-called process of parametric down conversion to create entangled pairs of photons where a UV beam entering a nonlinear optical crystal spontaneously creates pairs of photons such that the sum of their linear momenta is constant...

In this experiment, photon 2 passes a double slit while the other, photon 1, can be observed by a detector placed at various distances...

(emphasis mine)

You can read more about Dopfer's experiment in this PhysicsSE answer. It also mentions a subsequent and similar experiment done by Dr. John Cramer.

Thought experiments

As for thought experiments, they are quite more common than the real ones. A few examples would be the Wheeler's thought experiments (strictly speaking, these belong to the "delayed choice quantum eraser" group of experiments, however, the underlying motivation for the experiment is the same).

I would also suggest you to read this Wikipedia article for some more directions for finding such experiments.


There are many example’s of which path testing. In a double slit experiment the particles go one path or the other And overtime create an interference pattern. If you block one of the slits you have a single slit experiment with a different type of pattern. Just do the experiment yourself with a laser and double slit. You will see a certain type of interference pattern. Then block one of the slits and you will see a single slit interference pattern which is different.

  • $\begingroup$ There might be several misconceptions in this answer. 1) It's incorrect to assume that the particle goes through one slit or the other, unless you're doing which-path measurements. 2) If one slit is blocked, there won't be any interference, all you'll see is the diffraction pattern. $\endgroup$
    – Miyase
    Commented Jun 15, 2022 at 0:18
  • $\begingroup$ @Miyase I think it’s more incorrect to assume a particle can go through two openings at the same time. A single slit does have an interference pattern. A single edge has a Diffraction Pattern. $\endgroup$ Commented Jun 15, 2022 at 0:29

Dephasing in which path measurements have been a topic of study in mesoscopic physics for quite some time, using nanoscale Aharonov-Bohm interferometers. Here is the first experimental measurement and there are many experimental and theoretical papers that followed (many originating from the same group, but not only).

Another field, where the interference of different paths has been studied for even longer, is the studies of weak localization. These are however more difficult for a newcomer due to the complexity of the underlying concepts and some rather advanced math. Weak localization is arguably a better representation of the two-slit/which-path concepts than the closed mesoscopic interferometers - the latter, despite their seeming resemblance to the hypothetical slits, exhibit a range of complex phenomena, such as phase rigidity, phase lapses due to Coulomb interactions, etc.

Then there are also the interferometry experiments using integer and fractional quantum Hall effect...

Buks, R. Schuster, M. Heiblum, D. Mahalu & V. Umansky, Dephasing in electron interference by a ‘which-path’ detector, Nature volume 391, pages 871–874 (1998). (arXiv version)

enter image description here


It seems to me that the first two experiments measuring the "which-path" of electrons in the double-slit setting appeared in Nature during the same year, 1998:



More recently, the following experiment was probably the first realization of Feynman's thought experiment (from his 1963 lectures):


In that experiment, the physicists could send single electrons to a double-slit and observed the construction of an interference pattern, but they could also mask one of the slit on-demand and witness the destruction of the interferences.

However, note that they didn't do a which-path measurement, which would have to be done after the slits, instead of blocking one slit. In a sense, the "perfect" double-slit experiment (ie where you can send single electrons to a double-slit and choose after they've passed it to measure which-path or not, and observe the resulting pattern on your screen) hasn't been done, up to my knowledge.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.