16 August 2013 Local Orthogonality for Quantum Correlations

Examples from graph theory used in applying the principle

First intrinsically multipartite principle for quantum correlations introduced in Nature Communications The correlations between the results of measurements performed on entangled quantum particles may lead to correlations that are impossible to explain classically. This form of correlations with no classical analogue is known as non-locality and represents one of the most intrinsic quantum properties. Moreover, quantum non-locality is an information resource behind novel protocols for secret-key distribution and randomness generation.

Quantum physics allows for non-local correlations, but is also compatible with the no-signalling principle, that is, Einstein’s causality: the presence of these correlations cannot be used for any form of faster-than-light communication. However, the no-signalling principle per se does not imply quantum correlations, as there exist supra-quantum correlations that are compatible with this principle, i.e. quantum physics is not the most non-local theory. The fundamental question then emerges: why is quantum physics not maximally non-local? What are the fundamental principles behind the limited non-locality of quantum physics? Why quantum correlations? In recent years, an intensive theoretical effort has been devoted to answering this question, and new principles have been proposed to explain the non-locality of quantum physics.

Despite partial progress, a limitation of this approach is that, until now, (i) many of the proposed principles have been formulated in a scenario consisting of two quantum particles, but (ii) intrinsically multipartite principle are necessary to characterize quantum correlations. In a paper recently published in Nature Communications by the group led by ICREA Professor at ICFO, Antonio Acín, they have introduced the concept of Local Orthogonality, the first intrinsically multipartite principle for quantum correlations. This work studies the consequences of the principle and proves how its multipartite character allows revealing the non-quantumness of correlations for which any bipartite principle fails. The authors believe that Local Orthogonality is a crucial ingredient for understanding no-signaling and quantum correlations.