Theoretical and Experimental Characterization of Entropic Inequalities Available to Purchase

Nonlinear inequalities based on the quadratic Renyi entropy for mixed two‐qubit states are characterized on the Entropy‐Concurrence plane. This class of inequalities is stronger than Clauser‐Horne‐Shimony‐Holt (CHSH) inequalities and, in particular, are violated “in toto” by the set of Type I Maximally‐Entangled‐Mixture States (MEMS I). Renyi entropy is experimentally obtained by local measurements on two pairs of polarization‐entangled photons. The novel “phase marking” technique allows the selection of uncorrupted outcomes even with nondeterministic sources of entangled photons. Experimental data demonstrate the violation of entropic inequalities which are an example of nonlinear entanglement witnesses.