Control of quantum transverse correlations on a four-photon system
  Control of spatial quantum correlations in bi-photons is one of the fundamental principles of Quantum Imaging. Up to now, experiments have been restricted to controlling the state of a single bi-photon, by using linear optical elements. In this work we demonstrate experimental control of quantum correlations in a four-photon state comprised of two pairs of photons. Our scheme is based on a high-efficiency parametric downconversion source coupled to a double slit by a variable linear optical setup, in order to obtain spatially encoded qubits. Both entangled and separable pairs have been obtained, by altering experimental parameters. We show how the correlations influence both the interference and diffraction on the double slit. 
P.-L. de Assis, M. A. D. Carvalho, L. P. Berruezo, J. Ferraz, I. F.   Santos, F. Sciarrino, and S. P\'adua
quant-ph