Hi, My question is the following: As well known, the interference pattern disappears when one determines which slit the photon has gone through. This also holds for atoms and molecules. Now I understand how the actual experiment works for eg. an atom: They send excited atoms that can decay in some cavity coupled to a photomultiplier; this way one can be sure whether the atom has been through slit 1 or slit 2. But how does the experiment work with photons? What does it mean here to determine which slit the photon has gone through? (in particular, without destroying the photon!) Related to this: lasers can be seen by eye if they go through some smoke. What if the double slit experiment is realized in some smokey environment? Does the diffusion on the particles "measure" the photon position? If so, we should see on the screen some interference for photons that have not been measured + two peaks corresponding to photons that have been measured. Is that correct? If so, does anyone have references to provide? Many thanks!