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

We report the observation of correlated photon pairs generated by spontaneous parametric downconversion of a 400-nm pump pulse in a quasi-phase-matched KTiOPO(4) nonlinear waveguide. The highest ratio of coincidence to single-photon count rates observed near 800 nm exceeds 18%. This suggests that nonlinear waveguides will be a promising source of correlated photons for metrology and quantum information processing applications. We also discuss possibilities of controlling the spatial characteristics of the downconverted photons produced in multimode waveguide structures.