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Multiphoton states constructed from photon pairs generated in the process of spontaneous parametric down-conversion possess frequency and space-time correlations that may carry undesired distinguishing information. It is shown that these correlations may be eliminated if certain conditions in the source configuration are satisfied. For the cases in which these conditions cannot be satisfied because of experimental constraints, it is shown that the correlations may be reduced through proper choices of crystal length and pump bandwidth. The advantage of such source engineering is that it yields much higher count rates, since no photon pairs are lost by predetection spectral filtering.
Grice et al. (Mon,) studied this question.