I read in a book the following assertion.
In a double slit experiment photons are passed through the slits and detected at the end plate.
Each of the two slits has a quarter wave plate which alters the polarization of the photons that pass through it in a way different than the other QWP.
Thus a polarizing detecting barrier at the end plate can determine which slit the photon went through.
In such an experiment, there will be no interference pattern at the end plate. i.e. the wave functions collapse.
But if one does either of two things, the interference pattern shows up. Thus if either:
- the quarter wave plates are removed but the polarizing detecting barrier is kept.
- the polarizing detecting barrier is removed but the quarter wave plates are kept.
Then the interference pattern is back.
A. Is this assertion correct?
B. If yes, then where does the wave function collapse when both are in place? At the plates or the end detector?
here's a quote from that book (Biocentrism ch.8 scroll down to "The Most Amazing Experiment"):
We’re back to QT’s complementarity – that you can measure and learn just one of a pair of characteristics, but never both at the same time. If you fully learn about one, you will know nothing about the other. And just in case you’re suspicious of the quarter wave plates, let it be said when used in all other contexts, including double slit experiments but without information-providing polarization-detecting barriers at the end, the mere act of changing a photon’s polarization never has the slightest effect on the creation of an interference pattern.