  The process of spontaneous parametric down-conversion (SPDC) in nonlinear
crystals makes it fairly easy to generate entangled photon states. It has been
known for some time that the conversion efficiency can be improved by employing
quasi-phase-matching in periodically poled crystals. Using two single-photon
detectors, we have analyzed the photon pairs generated by SPDC in a
periodically poled lithium niobate crystal pumped by a femtosecond laser.
Several parameters could be varied in our setup, allowing us to obtain data in
close agreement with both thermal and Poissonian photon-pair distributions.
