- #71

PeterDonis

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vanhees71 said:Here just have polarization filters in front of the slits at 90deg relative orientation.

If they are polarization

*filters*, as opposed to quarter-wave plates, then we have a different setup, because each individual photon now passes through only one slit or the other, whereas with QWPs, each photon still passes through both slits, the QWP just shifts its phase.

The two setups are treated differently in terms of the basic math of QM, as follows:

(1) With QWPs, the wave function of the photon after the slits but before the detector screen is a coherent superposition of the two one-slit wave functions, with an appropriate phase applied to each according to how the QWP at the corresponding slit is aligned. The amount of interference seen at the detector screen is then a function of the relative phases at the two slits. This is the kind of setup I took @DrChinese to be describing, since he talked about being able to continuously adjust how much interference there is.

(2) With polarization filters oriented at 90 degrees relative, there is no single wave function after the slits but before the detector screen, because we have to apply the projection postulate: we can only describe the state after the slits as a density matrix showing equal probability of the photon coming from each slit. Of course this means it is impossible to have interference.

We can talk about either setup, but we need to be clear about which one we are talking about.