A linear 50/50 beamsplitter, together with a coincidence measurement, hasbeen widely used in quantum optical experiments, such as teleportation, densecoding, etc., for interferometrically distinguishing, measuring, or projectingonto one of the four two-photon polarization Bell-states $|\psi^{(-)}>$. Inthis paper, we demonstrate that the coincidence measurement at the output of abeamsplitter cannot be used as an absolute identifier of the input state$|\psi^{(-)}>$ nor as an indication that the input photons have projected tothe $|\psi^{(-)}>$ state.
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