Nonseparable Werner states in spontaneous parametric down-conversion

Abstract

The multiphoton states generated by high-gain spontaneous parametric down-conversion (SPDC) in the presence of large losses are investigated theoretically and experimentally. The explicit form for the two-photon output state has been found to exhibit a Werner structure very resilient to losses for any value of the nonlinear gain parameter $g$. The theoretical results are found to be in agreement with experimental data obtained by ``entanglement witness'' methods and by the quantum tomography of the state generated by a high-$g$ SPDC.