This is a better form of a previous question I asked: Double slit experimental procedure
Basically, I want to know what constitutes a measurement of which slit a particle goes through in the double slit experiment. For the following three setups:
- Two slits.
- Two slits with devices that assign the particles perpendicular spins.
- Two slits with devices that assign the particles perpendicular spins, and where the screen can tell the spin of the electron.
In the first, we have interference. In the third, we have no interference because we have measured which slit the electron passed through. In the second, is there interference or no interference?