X-entanglement of PDC photon pairs
  We investigate the spatio-temporal structure of the bi-photon entanglement in parametric down-conversion (PDC) and we demonstrate its non-factorable X-shaped geometry. Such a structure gives access to the ultra-broad bandwidth of PDC, and can be exploited to achieve a bi-photon temporal localization in the femtosecond range. This extreme localization is connected to our ability to resolve the photon positions in the source near-field. The non factorability opens the possibility of tailoring the temporal entanglement by acting on the spatial degrees of freedom of twin photons. 
E. Brambilla, L. Caspani, O. Jedrkiewicz, L. A. Lugiato and A. Gatti