Experimental observation of the X-shaped near field spatio-temporal   correlation of ultra-broadband twin beams
  In this work we present the experimental observation of the non factorable near field spatio-temporal correlation of ultra-broadband twin beams generated by parametric down conversion (PDC), in an interferometric-type experiment using sum frequency generation, where both the temporal and spatial degrees of freedom of PDC light are controlled with very high resolution. The revealed X-structure of the correlation is in accordance with the predictions of the theory. 
O. Jedrkiewicz, A. Gatti, E. Brambilla, P. Di Trapani
physics.optics quant-ph