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With respect to Thomas Young's double-slit experiment, I have heard that it can be done with a single photon. But how does the single photon go through both slits? And how far apart are the slits?

Qmechanic
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Kurt Hikes
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  • It makes no sense to do the experiment with just one photon. Assuming your detector is sensitive enough to reveal a single photon, all you get is one dot on the screen. What you probably heard about was, doing the experiment with an extremely low intensity light source---so low that it is unlikely that there is ever more than one photon in the apparatus at any given time. But as lots of photons go through, one after the other, the dots that they mark on the screen gradually add up to reveal an interference pattern. – Solomon Slow Jan 23 '18 at 01:15
  • Is it really possible for one photon to make a dot on the screen? – Lambda Jan 23 '18 at 02:29

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The electromagnetic field is interacting with the slits and detector placed around it (say the screen) everywhere. You can make the field so weak that by Fermi's golden rule the probability of the field interacting with any point on the screen is so small that you see only one "hit" at a time. This does not mean however that we dealt with a photon travelling through any of the slits arriving at the screen. A photon is not a localized particle. It is a quanta that a can be annihilated from field when interacting with the fields in the screen with certain probability.

Jan Bos
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