Abstract

In the Hong-Ou-Mandel[1-3] interferometer, quantum interference is observed when the photons of a spontaneous parametric down conversion (SPDC) pair simultaneously impinge upon the output beamsplitter. We report on an experiment in which interference effects are observed even though the photons of two orthogonal polarization generated in a type-II SPDC do not arrive at the beamsplitter at the same time. In the sense of a quantum eraser, the indistinguishability of the different two-photon Feynmann-amplitudes leading to a coincidence detection is restored by postponing the compensation until after the strongly unbalanced interferometer. We introduce an optical delay line nonsymmetrically only in the one output port of a beamsplitter. The contrast of restored two-photon quantum interference picture has been observed to be 98%. This type of postponed compensation highlights the non-classical nature of the two-photon state produced in SPDC, which can not simply be thought of as two single photons.

© 1996 IEEE