  We show that the wave packet of a biphoton generated via spontaneous
parametric down conversion is strongly anisotropic. Its anisotropic features
manifest themselves very clearly in comparison of measurements performed in two
different schemes: when the detector scanning plane is perpendicular or
parallel to the plane containing the crystal optical axis and the laser axis.
The first of these two schemes is traditional whereas the second one gives rise
to such unexpected new results as anomalously strong narrowing of the biphoton
wave packet measured in the coincidence scheme and very high degree of
entanglement. The results are predicted theoretically and confirmed
experimentally.
