  We explore a promising method of generating pure heralded single photons. Our
approach is based on parametric downconversion in a periodically-poled
waveguide. However, unlike conventional downconversion sources, the photon
pairs are counter-propagating: one travels with the pump beam in the forward
direction while the other is backpropagating towards the laser source. Our
calculations reveal that these downconverted two-photon states carry minimal
spectral correlations within each photon-pair. This approach offers the
possibility to employ a new range of downconversion processes and materials
like PPLN (previously considered unsuitable due to their unfavorable
phasematching properties) to herald pure single photons over a broad frequency
range.
