Preservation of spatial entanglement in surface plasmon polaritons

Two-photon states emitted from spontaneous parametric downconversion (SPDC) can exhibit quantum correlations in various degrees of freedom. On the other hand, plasmonic nanostructures show enhanced transmission thanks to the conversion of light into surface plasmon polaritons. It has been shown previously that polarization, time-energy and orbital angular momentum entanglement are preserved in such structures. The fact that surface plasmons are collective charge density waves, consisting of billions of electrons makes this observation remarkable. Here we show that spatial entanglement is also preserved by characterizing it after the photons interacted with gold nanopillar arrays. The detection is realized with novel single photon avalanche diode (SPAD) arrays with high spatial and temporal resolution. By measuring the spatial correlations of the photon in the near- and far-fields, we can characterize the state with respect to spatial entanglement with and without the nano-structure.