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A unified graph-theoretic approach to monogamy relations between nonlocality and contextuality

Debashis Saha, Ravishankar Ramanathan

Published Jun 13, 2016• 2 citations
OpenAlex

Abstract

A unified view on the phenomenon of monogamy exhibited by Bell inequalities and non-contextuality inequalities arising from the no-signaling and no-disturbance principles is developed using the graph-theoretic method introduced in \textit{Phys. Rev. Lett. 109, 050404 (2012)}. The sufficient condition for the existence of a monogamy between any two Bell or non-contextuality inequalities in terms of a decomposition of the commutation graph representing all the observables in the setup into induced chordal graphs, is studied. The condition is used to show that any two cycle inequalities, whether studied as non-contextuality or Bell inequalities and for any length of the cycle, are monogamous under certain conditions with each other. As a main application, a novel type of trade-off is then derived, namely Bell inequalities that do not exhibit monogamy features of their own can be activated to be monogamous by the addition of a local contextuality term. This is illustrated by means of the well-known $I_{3322}$ inequality. The non-necessity of the condition for the existence of a monogamy relation is shown by means of an explicit counter-example. In the derivation of novel no-signaling monogamies, we uncover a new feature, namely that two-party Bell expressions that are trivially classically saturated can become non-trivial upon the addition of an expression involving a third party with a single measurement input. This implies that the study of Bell inequalities (and the corresponding classical polytopes) with single inputs for some of the parties becomes interesting.

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A unified view on the phenomenon of monogamy exhibited by Bell inequalities and non-contextuality inequalities arising from the no-signaling and no-disturbance principles is developed using the graph-theoretic method introduced in \textit{Phys. Rev. Lett. 109, 050404 (2012)}. The sufficient condition for the existence of a monogamy between any two Bell or non-contextuality inequalities in terms of a decomposition of the commutation graph representing all the observables in the setup into induced chordal graphs, is studied. The condition is used to show that any two cycle inequalities, whether studied as non-contextuality or Bell inequalities and for any length of the cycle, are monogamous under certain conditions with each other. As a main application, a novel type of trade-off is then derived, namely Bell inequalities that do not exhibit monogamy features of their own can be activated to be monogamous by the addition of a local contextuality term. This is illustrated by means of the well-known I3322I_{3322}I3322​ inequality. The non-necessity of the condition for the existence of a monogamy relation is shown by means of an explicit counter-example. In the derivation of novel no-signaling monogamies, we uncover a new feature, namely that two-party Bell expressions that are trivially classically saturated can become non-trivial upon the addition of an expression involving a third party with a single measurement input. This implies that the study of Bell inequalities (and the corresponding classical polytopes) with single inputs for some of the parties becomes interesting.
A unified view of the phenomenon of monogamy exhibited by Bell inequalities and noncontextuality inequalities arising from the no-signaling and no-disturbance principles is presented using the graph-theoretic method introduced by Ramanathan et al. [Phys. Rev. Lett. 109, 050404 (2012)]. We propose a hitherto unexplored tradeoff relation, namely, Bell inequalities that do not exhibit monogamy features of their own can be activated to be monogamous by the addition of a local contextuality term. This is illustrated by means of the well-known I3322{\mathcal{I}}_{3322}I3322​ inequality and reveals a resource trade-off between bipartite correlations and the local purity of a single system. In the derivation of no-signaling monogamies, we uncover a unique feature, namely, that two-party Bell expressions that are trivially classically saturated can become nontrivial upon the addition of an expression involving a third party with a single measurement input.
In this Letter, we demonstrate that the property of monogamy of Bell violations seen for no-signaling correlations in composite systems can be generalized to the monogamy of contextuality in single systems obeying the Gleason property of no disturbance. We show how one can construct monogamies for contextual inequalities by using the graph-theoretic technique of vertex decomposition of a graph representing a set of measurements into subgraphs of suitable independence numbers that themselves admit a joint probability distribution. After establishing that all the subgraphs that are chordal graphs admit a joint probability distribution, we formulate a precise graph-theoretic condition that gives rise to the monogamy of contextuality. We also show how such monogamies arise within quantum theory for a single four-dimensional system and interpret violation of these relations in terms of a violation of causality. These monogamies can be tested with current experimental techniques.
Monogamy is a fundamental property of Bell nonlocality and contextuality. In this article, we study the nnn-cycle noncontextual inequalities and generalized Clauser-Horne-Shimony-Holt (CHSH) inequalities in detail and find sufficient conditions for those inequalities to hold. According to those conditions, we provide several kinds of tradeoff relations: monogamy of generalized Bell inequalities in a nonsignaling framework, monogamy of cycle-type noncontextual inequalities, and monogamy between Bell inequalities and noncontextual inequalities in a general no-disturbance framework. Finally, some generic tradeoff relations of generalized CHSH inequalities for nnn-party physical systems, which are beyond the one-to-many scenario, are discussed.
Abstract We analyze monogamous relationships of abritrary entropic non-contextuality (ENC) inequalities via a graph theoretic approach. While ENC inequalities are important in quantum information theory, a theoretical and experimental understanding of their monogamous nature is still elusive. We analyze conditions for ENC inequalities to exhibit a monogamous relationship and derive the same explicitly for an entropic version of the Bell-CHSH scenario. Next, we consider two different sets of three qubit states and experimentally show that they exhibit monogamy of entropic Bell-CHSH inequality. The monogamous relationship of these inequalities is theoretically exhibited via an insightful proof and is experimentally demonstrated on an NMR quantum information processor. We also develop a novel and easy-to-implement experimental method to evaluate entropies on an NMR quantum information processor using only the information about expectation values of the observables.
Quantum mechanics imposes limits on the statistics of certain observables. Perhaps the most famous example is the uncertainty principle. Similar trade-offs also exist for the simultaneous violation of multiple Bell inequalities. In the simplest case of three observers, it has been shown that if two observers violate a Bell inequality, then none of them can violate any Bell inequality with the third observer, a property called monogamy of Bell violations. Forms of Bell monogamy have been linked to the no-signaling principle, and the inability of simultaneous violations of all inequalities is regarded as their fundamental property. Here, we show that the Bell monogamy does not hold universally and that in fact the only monogamous situation exists for only three observers. Consequently, the nature of quantum nonlocality is truly polygamous. We present a systematic methodology for identifying quantum states, measurements, and tight Bell inequalities that do not obey the monogamy principle for any number of more than three observers. The identified polygamous inequalities enable any subset of [Formula: see text] observers to reveal nonlocality, which is also shown experimentally by measuring Bell-type correlations of six-photon Dicke states. Our findings may be exploited for multiparty quantum key distribution as well as simultaneous self-testing of multiple nodes in quantum networks.