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Some past and some possible future experiments that make use of the photon pairs generated in the process of parametric down-conversion are reviewed and briefly analysed. The experiments demonstrate that the average position in time of an optical photon can be determined with accuracy better than one period, and that two-photon coincidence measurements can carry useful information about the phase of the electro-magnetic field. Several different experiments for exhibiting violations of Einstein locality with down-converted photon pairs are discussed.
Ou et al. (Thu,) studied this question.