Entanglement and non-classical correlations

Alexander Streltsov

Bogaert, Pieter, Davide Girolami

Non-classical correlations denote the ways in which quantum systems can be correlated beyond what is possible in classical physics. They include but are not limited to entanglement: non-entangled states can also exhibit forms of non-classical correlations. We here review the literature on metrological measures of non-classical correlations and show how they relate to increased probe sensitivity in quantum metrology protocols such as interferometric parameter estimation and state discrimination.

Andrea Chiuri, Giuseppe Vallone, Mauro Paternostro +2 more

The study of the connections between entanglement and non-classicality has seen an increasing interest in the last few years. Several forms of entanglement witnesses have been proposed to reveal and quantify the entangle ment of a particular state or a particular class of states. Non-classicality indicators have been proposed so as to quantify quantum correlations in two-qubit states. It has been demonstrated that there are interesting situations in which non-entangled states (i.e. separable) can behave in a non-classical way.

Alexander Streltsov

Anil Kumar Tiwari

Elias Amselem

The rapidly developing interdisciplinary field of quantum information, which merges quantum and information science, studies non-classical aspects of quantum systems. These studies are motivated by the promise that the non-classicality can be used to solve tasks more efficiently than classical methods would allow. In many quantum informational studies, non-classical behaviour is attributed to the notion of entanglement. In this thesis we use photons to experimentally investigate fundamental questions such as: What happens to the entanglement in a system when it is affected by noise? In our study of noisy entanglement we pursue the challenging task of creating bound entanglement. Bound entangled states are created through an irreversible process that requires entanglement. Once in the bound regime, entanglement cannot be distilled out through local operations assisted by classical communication. We show that it is possible to experimentally produce four-photon bound entangled states and that a violation of a Bell inequality can be achieved. Moreover, we demonstrate an entanglement-unlocking protocol by relaxing the condition of local operations. We also explore the non-classical nature of quantum mechanics in several single-photon experiments. In these experiments, we show the violation of various inequalities that were derived under the assumption of non-contextuality. Using qutrits we construct and demonstrate the simplest possible test that offers a discrepancy between classical and quantum theory. Furthermore, we perform an experiment in the spirit of the Kochen-Specker theorem to illustrate the state-independence of this theorem. Here, we investigate whether or not measurement outcomes exhibit fully contextual correlations. That is, no part of the correlations can be attributed to the non-contextual theory. Our results show that only a small part of the experimental generated correlations are amenable to a non-contextual interpretation.