SpectraCat and I have been discussing what happens to entangled photons after encountering a polarizing beamsplitter (PBS). A beer rides on the outcome (in addition to one I already won), so you know I can't let this go. SpectraCats says: collapse of the wave function occurs at the PBS. I say: the photon must be detected after the PBS for collapse to occur. Now, the relevant issue is whether in a delayed choice experiment, the collapse occurred before or after some event (it doesn't matter for the discussion what event). The key is that in the experiment, which is referenced below, collapse should be measured relative to the PBS or the detectors. http://arxiv.org/abs/quant-ph/0201134 Experimental Nonlocality Proof of Quantum Teleportation and Entanglement Swapping Authors: Thomas Jennewein, Gregor Weihs, Jian-Wei Pan, Anton Zeilinger My argument is simple: it is the final context of the entire experiment that matters. After encountering a PBS, the outputs can - in principle - be reassembled and thus the "measurement" would be erased. SpectraCat disagrees with this, at least in the situation where no erasure occurs. I am starting a new thread so we can discuss the issue of collapse - and how to probe it - assuming that collapse is a physical event. Your thoughts are welcome on the above question.