  We propose experimental measurements of the logarithmic negativity, which
quantifies quantum correlations using Gouy phase measurements in an asymmetric
double-slit interference experiment for twin photons. This is possible because
both quantities have analogous dependence with the spatial confinement by the
slits and enables one to manipulate the portion of entanglement by the Gouy
phase. In order to obtain those measurements, we need to work in a regime where
the position correlations between particles are strong, therefore we
investigate such correlations for biphotons. Since we would like to handle
entanglement quantifiers through the Gouy phase, we analyze the Gouy phase
difference for two entangled photons in an asymmetric double-slit interference
experiment.
