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In the double-slit experiment, the "observer" is the one actually firing one photon. How does the observer fire one photon? Does not one have to "observe and "measure out" a single photon? How does one isolate one photon without collapsing it to begin with?

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  • $\begingroup$ This business with the "observer" is what is confusing you. It is not the observer firing the photon, but a macroscopic experimental setup that allows the intensity of light to fall so low that only one photon at a time registers at the detecting screen. The whole experiment could be done by a robotic setup. see this double slit single photon experiment : sps.ch/en/articles/progresses/… $\endgroup$ – anna v Jan 22 '17 at 16:30
  • $\begingroup$ see the answer of mine here physics.stackexchange.com/questions/306571/… $\endgroup$ – anna v Jan 22 '17 at 16:54
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You reduce the intensity of the source of photons until it is statistically sufficiently likely that there is only one photon in the experiment at a time.

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