Detection of the ultranarrow temporal correlation of twin beams via   sum-frequency generation
  We demonstrate the ultranarrow temporal correlation (6 fs full width half maximum) of twin beams generated by parametric down-conversion, by using the inverse process of sum-frequency generation. The result relies on an achromatic imaging of a huge bandwith of twin beams and on a careful control of their spatial degrees of freedom. The detrimental effects of spatial filtering and of imperfect imaging are shown toghether with the theoretical model used to describe the results. 
O. Jedrkiewicz, J.-L. Blanchet, E. Brambilla, P. Di Trapani and A.   Gatti
quant-ph physics.optics