In the double-slit experiment, we emit a photon that is in a state of superposition (wave form) which travels through both slits to interfere with itself. When we measure which slit it went through, it "collapses" to a particle at that point removing the interference from the other wave.
From that point to the detector wall, does the photon now go back into a wave form to travel from the collapsed point to the detector wall? Is a photon always in a state of superposition while traveling through space?
Best Answer
It's tempting to think of the light as a little ball (the photon), and since little balls have a definite position the little ball has to be in a superposition of a state where it goes through one slit and a state where it goes through the other. However this is not a good description of what actually happens.
The light is not a photon, and it's not a wave - it is an excitation in a quantum field. As a general rule it is a good approximation to model the light as a wave when studying it's propagation and a particle when studying its interactions, but these are only working approximations and should not be taken as literally true.
The light propagating in our double slit experiment does not have a position in the sense that macroscopic objects have a position. To ask the position of the photon is a meaningless question because there is no position. The light travels though both slits because the excitation in the quantum field spans both slits. When the light interacts with something, e.g. it hits the photographic plate or CCD, then the interaction happens at a point (though the position of the interaction is subject to Heisenberg's uncertainty principle). The interaction looks like a localised exchange of energy, so it looks like a photon.