Spatio-temporal entanglement of twin photons: an intuitive picture
  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. 
Alessandra Gatti, Lucia Caspani, Tommaso Corti, Enrico Brambilla,   Ottavia Jedrkiewicz
quant-ph physics.optics