Entangled topological features of light

We report the entanglement of topological features, namely, isolated, linked optical vortex loops in the light from spontaneous parametric down-conversion (SPDC). In three dimensions, optical vortices are lines of phase singularity and vortices of energy flow which percolate through all optical fields. This example of entanglement is between features that extend over macroscopic and finite volumes, furthermore, topological features are robust to perturbation . The entanglement of photons in complex three-dimensional(3D) topological states suggests the possibility of entanglement of similar structures in other quantum systems describable by complex scalar functions, such as superconductors, superfluids and Bose-Einstein condensates.