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ar ho tro pairs generated by downconversion in bulk crystals can be a which dr ton pairs waveguid which en with a s This fea neering t photons input face of a 1-mm-long quasi-phase-matched KTP cteristics ave been injected objective, onverted well over have the pled out possible novel ways for controlling the spatiotemporal characteristics of correlated photon pairs. The use of nonlinear waveguides thus provides a route to the efficient, if random, generation of “heralded ” single photons, as an alternative to solid-state sources.10 Observation of parametric downconversion in nonlinear waveguides has been the subject of three recent experiments. Two of them11,12 consisted of detecting Hanbury-Brown–Twiss-type coincidences with a laser-diode collimating lens and transmitted through a RG665 red filter and a half-wave plate that rotates the polarization by 90±. The separation of the twin photons is performed with a zero dispersion line employing two Brewster-angle SF10 prisms and two f 10 cm lenses. In the Fourier plane after the f irst prism, a multimode fiber tip mounted on a translation stage is used to collect the trigger photons and send them to a fiber-coupled photon-counting module. Thechieved only by significant spatial filtering, amatically reduces the useful fraction of pho-. In contrast, downconversion in a nonlinear e can provide output in a single spatial mode, sures good interference without f iltering ignificantly larger sample of photon pairs. ture, combined with the potential of engi-he temporal properties of the downconverted in quasi-phase-matched structures,9 makes waveguide. The production and the chara of the sample used in our experiment h described elsewhere.14,15 The light power into the waveguide, measured before the is 22 mW. The bandwidth of the downc light for the parameters of our experiment is 100 nm, and the downconverted photons same polarization as the pump field. The output from the waveguide is couGeneration of correlated photo by downconversion i