  We draw an intuitive picture of the spatio-temporal properties of the
entangled state of twin photons, where they are described as classical
wave-packets. This picture predicts a precise relation between their temporal
and transverse spatial separations at the crystal output. The space-time
coupling described by classical arguments turns out to determine in a precise
way the spatio-temporal structure of the quantum entanglement, analysed by
means of the biphotonic correlation and of the Schmidt dimensionality of the
entanglement.
