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For criticisms of Popper's experiment, see for example, D. Bedford and F. Selleri, Lett. Nuovo Cimento
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52, 470 (1985). A. Sudbery, in Microphysical Reality and Quantum Formalism, A. van der Merwe et al., eds. (Kluwer Academic, Dordrecht, 1988). Many of the criticisms concern the validity of a point source for entangled two-particles. However, a “point source” is not a necessary requirement for Popper's experiment. What is essential is to learn the precise knowledge of a particle's position through quantum entanglement. This is achieved in our experiment by means of a “ghost image”.
For discussions of the effect of the size of the source on one-particle and two-particle diffraction, see for example, M. Horne, Experimental Metaphysics, R. S. Cohen, M. Horne, and J. Stachel, eds. (Kluwer Academic, Dordrecht, 1997).
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In type-I SPDC, signal and idler are both ordinary (or extraordinary) rays of the crystal; however, in type-II SPDC they are orthogonally polarized, i.e., one is ordinary and the other is extraordinary.
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