title: Direct measurement of nonlinear properties of bipartite quantum states.
abstract: Nonlinear properties of quantum states, such as entropy or entanglement, quantify important physical resources and are frequently used in quantum-information science. They are usually calculated from a full description of a quantum state, even though they depend only on a small number of parameters that specify the state. Here we extract a nonlocal and a nonlinear quantity, namely, the Renyi entropy, from local measurements on two pairs of polarization-entangled photons. We also introduce a "phase marking" technique which allows the selection of uncorrupted outcomes even with nondeterministic sources of entangled photons. We use our experimental data to demonstrate the violation of entropic inequalities. They are examples of nonlinear entanglement witnesses and their power exceeds all linear tests for quantum entanglement based on all possible Bell-Clauser-Horne-Shimony-Holt inequalities.
authors.affiliation: Elsag spa, Via Puccini 2, 16154 Genova, Italy. Fabio.Bovino@elsag.it
authors.forename: Fabio Antonio
authors.initials: FA
authors.lastname: Bovino
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Castagnoli
authors.forename: Artur
authors.initials: A
authors.lastname: Ekert
authors.forename: Paweł
authors.initials: P
authors.lastname: Horodecki
authors.forename: Carolina Moura
authors.initials: CM
authors.lastname: Alves
authors.forename: Alexander Vladimir
authors.initials: AV
authors.lastname: Sergienko