Observations of unusual diffraction and interference by two-photon correlation measurements are reported. The signal and idler beams produced by spontaneous parametric down-conversion are sent in different directions, and detected by two distant pointlike photon counting detectors. A double slit or a single slit is inserted into the signal beam. Interference-diffraction patterns are observed in coincidences by scanning the detector in the idler beam. PACS numbers: 42.50.Dv, 03.65.Bz Spontaneous parametric down-conversion (SPDC) [1]is the most effective source of two-photon light, consisting of pairs of correlated photons. The essentially quantum nature of the corresponding two-photon state [2,3] has been confirmed in a number of two-photon correlation experiments [4]. This quantum feature allows us to demonstrate an unusual two-photon effect [5], which looks very strange from the classical point of view. The SPDC light beam, which consists of two orthogonal polarization components (usually called signal and idler), is split by a polarization beam splitter into two beams, and detected by two distant pointlike photon counting detectors for coincidences (see Fig. 1). A Young's double-slit

No abstract

We report on a "postponed compensation" experiment in which the observed two-photon entangled state interference cannot be pictured in terms of the overlap of the two individual photon wave packets of a parametric down-conversion pair on a beam splitter. In the sense of a quantum eraser, the distinguishability of the different two-photon Feynman amplitudes leading to a coincidence detection is removed by delaying the compensation until after the output of an unbalanced two-photon interferometer.

No abstract