Comment on ''Einstein-Podolsky-Rosen state for space-time variables in a two-photon interference experiment''

A recent article by Shih, Sergienko, and Rubin [Phys. Rev. A 47, 1288 (1993)] reports a two-photon interference effect in which fringes with a visibility of 59% were observed; for an ideal system, quantum mechanics predicts perfect visibility interference. In explanation of this discrepancy, the authors conclude that the reduced visibility is due to ''nonperfect phase matching of the down-conversion,'' the parent process of the correlated photon pairs. They imply that the nonperfect phase matching stems from the finite size of their down-conversion crystal. We argue that the crystal length was not a limiting factor in their measurement, citing three similar experiments which achieved higher visibilities [Kwiat, Steinberg, and Chiao, Phys. Rev. A 47, R2472 (1993); Brendel, Mohler, and Martienssen, Phys. Rev. Lett. 66, 1142 (1991); Europhys. Lett. 20, 575 (1992)].