Generation of correlated photons in controlled spatial modes by downconversion in nonlinear waveguides

We report the observation of correlated photon pairs generated by spontaneo us parametric downconversion of a 400-nm pump pulse in a quasi-phase-matche d KTiOPO4 nonlinear waveguide. The highest ratio of coincidence to single-p hoton count rates observed near 800 mn exceeds 18%. This suggests that nonl inear waveguides will be a promising source of correlated photons for metro logy and quantum information processing applications. We also discuss possi bilities of controlling the spatial characteristics of the downconverted ph otons produced in multimode waveguide structures. (C) 2001 Optical Society of America.