… We report the observation of an optical vortex in the correlations of photons produced
from spontaneous parametric down-conversion. The singularity appears in a … Here we
report the experimental observation of a nonlocal optical vortex associated with a phase
singularity which manifests in the spatial correlations of two entangled photons. The vortex
appears in a two-dimensional plane of the two-photon phase space that is composed of one
spatial degree of freedom of each photon. The …