Generation of spatially pure photon pairs in a multimode nonlinear   waveguide using intermodal dispersion
  We present experimental realization of type-II spontaneous parametric down-conversion in a periodically poled potassium titanyl phosphate (KTiOPO4) nonlinear waveguide. We demonstrate that by careful exploitation of intermodal dispersion in the waveguide it is feasible to produce photon pairs in well defined transverse modes without any additional spatial filtering at the output. Spatial characteristics is verified by measurements of the M2 beam quality factors. We also prepared a postselected polarization-entangled two-photon state shown to violate Bell's inequality. Similar techniques based on intermodal dispersion can be used to generate spatial entanglement and hyperentanglement. 
Michal Karpinski, Czeslaw Radzewicz, Konrad Banaszek
quant-ph physics.optics