My understanding of the famous double-slit experiment is that, when both slits are open, a wave-pattern interference arising from the two slits is observed. This is a different pattern than summing the pattern from each slit individually. This is not unusual as the same thing happens with water and sound waves, for example.
The odd/novel things (with regards to classical physics) are:
- the interference pattern is observed even if you only send one photon or electron etc through at a time (i.e. it appears the particle 'interferes with itself'). And
- If you detect which slit the photon or electron etc goes through, there is no more interference pattern - you observe the sum of the individual slits. i.e. the observation destroys the interference pattern because the position became known, so the photon could not 'interfere with itself' any more. And this holds even if you only have a detector on one slit but not the other.
My question is - has an experiment actually ever been performed to validate #2?
As per What is the *detector* in the double slit experiment and how does it work? , the top answer refers to "a long series of debates between Bohr and Einstein" -- which is not an experiment but rather a conversation.
Likewise Wikipedia states that Feynman "proposed (as a thought experiment) that if detectors were placed before each slit, the interference pattern would disappear." -- that is, a thought experiment, not an actual experiment.
None of the answers at this question Young's double-slit experiment with detectors cite an actual experiment that was performed.
I found a link to an article titled "Which-way detector unlocks some mystery of the double-slit experiment", but it doesn't seem to answer the question. It appears they simply change the setup of the experiment into one where there is no interference.
I'm presuming the answer is 'yes' but I haven't been able to find anything yet.
UPDATE: the answer here links to one experiment that uses a 'blocker' rather than a detector. The second link points to a YouTube video showing measurement of photon path by using polarization filters. But this doesn't demonstrate that it is knowledge of the photon's path that resulted in the wavefunction collapse, but rather that horizontally and vertically polarized photons do not cause an interference pattern.
Further the 'eraser' of a 45-degree polarized filter, doesn't demonstrate that erasing the knowledge of which slit the photon passed through re-creates the interference pattern, but rather simply that photons that have the same polarization do cause an interference pattern. i.e. the photons, after being horizontally and vertically polarized, then recombine to have the same polarization, which then results in an interference pattern again.
UPDATE 2: This part of the video describes exactly this experiment (placing an electron detector at the slit): https://youtu.be/U7Z_TIw9InA?t=243 , but I've been unable to find a paper about this or reference to an actual experiment being done.