Take the famous double slit (thought) experiment. The wave function for the photon is a superposition of two orthogonal states, one for each slit passage. But it is claimed to collapse into one of these two states when the photon hits the screen beyond, and then continue in this one state. But when the photon hits the screen, it is totally absorbed and ceases to exist. How can one then claim that its wave function collapses into a state? It does not exist anymore, in any state at all, or...?