Experimental observation of a skewed X-type spatio-temporal correlation of ultra-broadband twin beams

This work presents 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 high resolution. The revealed correlation is skewed in space-time in accordance with the X-structure predicted by the theory.