In the book 'Quantum Physics', Alastair Rae uses polarized photons and photon detectors to describe many effects observed in quantum physics. In his typical setup, photons are passed through a polarizer and exit it through two different channels depending on the polarization [itex]\phi[/itex] or [itex]\phi+90^\circ[/itex] they then have. Behind each channel, there is a photon detector to tell through which of the two channels "the" photon exited. Now my question: Where does the wave function representing the photon collapse? Does it happen in the polarizer, when the photon's polarization is decided to be either [itex]\phi[/itex] or [itex]\phi+90^\circ[/itex], or does the wave function collapse only later in the detector? I guess one answer could be that we don't know, but I would hope that there is a canonical, mostly agreed upon answer whether it is (1) or (2).