On the geometry of spatial biphoton correlation in spontaneous   parametric down-conversion
  Analytical expressions are derived for the distribution rates of spatial coincidences in the counting of photons produced by spontaneous parametric down conversion (SPDC). Gaussian profiles are assumed for the wave function of the idler and signal light created in type-I SPDC. The distribution rates describe ellipses on the detection planes that are oriented at different angles according to the photon coincidences in either horizontal-horizontal, vertical-vertical, horizontal-vertical or vertical-horizontal position variables. The predictions are in agreement with the experimental data obtained with a type-I BBO crystal that is illuminated by a 100 mW violet pump laser as well as with the results obtained from the geometry defined by the phase-matching conditions. 
Lorenzo M. Procopio, Oscar Rosas-Ortiz and Victor Velazquez
quant-ph