# Sequential Double Double Slit Experiment?

Let's say you arrange the double slit experiment so that there are two sets of two slits one after the other. Like so:

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If you measure which slit the photon went through in the first set, will the wave function remain collapsed as it passes through the second set so that you get two points on the detector screen, and not an interference pattern?

• Possible duplicate of Multiple Double slit experiment – By Symmetry Aug 19 at 15:08
• That question doesn’t address having a detector located at one of the slits along the way. – John Aug 19 at 15:09

You can however also imagine a way to know which slit the photon went through without absorbing it. For example, you can make it so that when the photon passes through the upper slit its polarization is $$H$$, while when it goes through the lower slit it's $$V$$ (assuming the polarization doesn't change in the rest of the evolution). Then by measuring the polarization at the screen at the end you are effectively measuring which slit the photon went through in the first stage. This is just an example of how you could do this. An ever simpler scenario is that you just block one of the initial slits, which then gives you the same identical evolution as if you measured the photon to have passed through the other slit in some other way.