We report the observation of an optical vortex in the correlations of photons produced from spontaneous parametric down-conversion. The singularity appears in a non-local coordinate plane consisting of one degree of freedom of each photon.
@article{arxiv.0902.1659,
title = {Observation of a Non-local Optical Vortex},
author = {R. M. Gomes and A. Salles and F. Toscano and P. H. Souto Ribeiro and S. P. Walborn},
journal= {arXiv preprint arXiv:0902.1659},
year = {2009}
}