In modern experiments,

With the classical setup of photons or electrons passing through double slit, the intensity is kept extremely low, such that one can detect a single particle passing through the double slit at a time.

Why this fact is so important?

Here I am asking for why there is need to keep Intensity to extremely low to detect a single particle ?

  • $\begingroup$ possible duplicate of Slit screen and wave-particle duality $\endgroup$ – John Rennie Apr 18 '15 at 8:56
  • $\begingroup$ Hi Pratik. See the question I've linked for an explanation. having only a single photon at a time present shows that the effect can't be due to two different photons interfering. A single photon must travel through both slits. $\endgroup$ – John Rennie Apr 18 '15 at 8:57
  • $\begingroup$ Hey I have read all the answers of that. BUt I am still not getting any similarities between these, And I am still confused for the above question. $\endgroup$ – pratik Apr 18 '15 at 17:10
  • $\begingroup$ @JohnRennie - I think the question is slightly different, the OP is confused about why you need a single photon. And I think it's useful to have some different answers to the same question at different levels. $\endgroup$ – Martin Beckett Apr 19 '15 at 1:04

It might be clearer if you read it as "detect THE single particle passing through the slits" .

If you use a strong light then there are lots of photons passing through the slits and the interference could be completely classical with different photons going through each slit and interfering at the screen - just like water waves. This was the picture for 100s of years.

With modern detectors it is possible to use a very faint light source so that there is only a single photon in flight at a time - and yet still see interference. So the particle is interfering with itself!

If you are trying to detect which slit the particle goes through and show that knowing this destroys the pattern then you need to have only one particle at a time. Otherwise you could detect which slit a single photon goes through but the billions of other photons would still make a normal patter


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