EINSTEIN-PODOLSKY-ROSEN STATE FOR SPACE-TIME VARIABLES IN A 2-PHOTON INTERFERENCE EXPERIMENT - COMMENT

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 mecha nics predicts perfect visibility interference. In explanation of this discrepancy, the authors conclude that the reduced visibility is due t o ''nonperfect phase matching of the down-conversion,'' the parent pro cess of the correlated photon pairs. They imply that the nonperfect ph ase matching stems from the finite size of their down-conversion cryst al. We argue that the crystal length was not a limiting factor in thei r measurement, citing three similar experiments which achieved higher visibilities [Kwiat, Steinberg, and Chiao, Phys. Rev. A 47, R2472 (199 3); Brendel, Mohler, and Martienssen, Phys. Rev. Lett. 66, 1142 (1991) ; Europhys. Lett. 20, 575 (1992)].