I have a question that seems simpe: if you have two entangled photons, can you set up two pairs of slits (one pair for the signal photons, and one for the idlers) so that observing which slit the idler goes through tells you which slit the signal photon goes through? It seems that conservation of momentum should let you do this. However, the Walborn et. al. eraser experiment shows that there is interference when you do coincidence counting (in their base setup the signal photons go through a double slit, and the idlers are coincidence counted), so this implies that in a simple setup the idlers cannot reveal which-path information.