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With the advent of quantum information, the violation of a Bell inequality is used as evidence of the absence of an eavesdropper in cryptographic scenarios such as key distribution and randomness expansion. One of the key assumptions of…

Bell's theorem shows that local realistic theories place strong restrictions on observable correlations between different systems, giving rise to Bell's inequality which can be violated in experiments using entangled quantum states. Bell's…

Bell's theorem is typically understood as the proof that quantum theory is incompatible with local-hidden-variable models. More generally, we can see the violation of a Bell inequality as witnessing the impossibility of explaining quantum…

The assumption of free will - the ability of an experimentalist to make random choices - is central to proving the indeterminism of quantum resources, the primary tool in quantum cryptography. Relaxing the assumption in a Bell test allows…

Quantum Physics · Physics 2013-12-30 James E. Pope , Alastair Kay

Bell's theorem is a no-go theorem stating that quantum mechanics cannot be reproduced by a physical theory based on realism, freedom to choose experimental settings and two locality conditions: setting (SI) and outcome (OI) independence. We…

Quantum Physics · Physics 2015-05-13 M. Pawlowski , J. Kofler , T. Paterek , M. Seevinck , C. Brukner

A classical system violating the Bell inequality is discussed. The system is local, deterministic, observers have free will, and detectors are ideal so that no data are lost. The trick is based on two elements. First, a state of one…

Quantum Physics · Physics 2009-09-25 Marek Czachor

We analyze a cryptographic protocol for generating a distributed secret key from correlations that violate a Bell inequality by a sufficient amount, and prove its security against eavesdroppers, constrained only by the assumption that any…

Quantum Physics · Physics 2014-12-02 Ll. Masanes , R. Renner , M. Christandl , A. Winter , J. Barrett

Bell inequalities are intended to show that local realist theories cannot describe the world. A local realist theory is one where physical properties are defined prior to and independent of measurement, and no physical influence can…

Quantum Physics · Physics 2014-12-01 Jan-Åke Larsson

The use of Bell's theorem in any application or experiment relies on the assumption of free choice or, more precisely, measurement independence, meaning that the measurements can be chosen freely. Here, we prove that even in the simplest…

Quantum Physics · Physics 2016-11-25 Gilles Pütz , Denis Rosset , Tomer Jack Barnea , Yeong-Cherng Liang , Nicolas Gisin

The violation of a Bell inequality is an experimental observation that forces one to abandon a local realistic worldview, namely, one in which physical properties are (probabilistically) defined prior to and independent of measurement and…

Experiments motivated by Bell's theorem have led some physicists to conclude that quantum theory is nonlocal. However, the theoretical basis for such claims is usually taken to be Bell's Theorem, which shows only that if certain predictions…

Quantum Physics · Physics 2007-05-23 Henry P. Stapp

Bell inequalities rest on three fundamental assumptions: realism, locality, and free choice, which lead to nontrivial constraints on correlations in very simple experiments. If we retain realism, then violation of the inequalities implies…

Quantum Physics · Physics 2021-05-20 Pawel Blasiak , Emmanuel M. Pothos , James M. Yearsley , Christoph Gallus , Ewa Borsuk

This article contains a review of Nelson's analysis of Bell's theorem. It shows that Bell's inequalities can be violated with a theory of local random variables if one accepts that the outcomes of these variables are not predetermined prior…

Quantum Physics · Physics 2014-09-30 Benjamin Schulz

Bell's theorem supposedly demonstrates an irreconcilable conflict between quantum mechanics and local, realistic hidden variable theories. In this paper we show that all experiments that aim to prove Bell's theorem do not actually achieve…

Quantum Physics · Physics 2025-01-10 Andrea Aiello

Bell gave the now standard definition of a local hidden variable theory and showed that such theories cannot reproduce the predictions of quantum mechanics without violating his ``free will'' criterion: experimenters' measurement choices…

Quantum Physics · Physics 2007-05-23 Adrian Kent

Device-independent randomness generation and quantum key distribution protocols rely on a fundamental relation between the non-locality of quantum theory and its random character. This relation is usually expressed in terms of a trade-off…

Quantum Physics · Physics 2018-03-20 Olmo Nieto-Silleras , Cédric Bamps , Jonathan Silman , Stefano Pironio

The paper considers the claim that quantum theories with a deterministic dynamics of objects in ordinary space-time, such as Bohmian mechanics, contradict the assumption that the measurement settings can be freely chosen in the EPR…

Quantum Physics · Physics 2015-06-24 Michael Esfeld

In a Bell test, the set of observed probability distributions complying with the principle of local realism is fully characterized by Bell inequalities. Quantum theory allows for a violation of these inequalities, which is famously regarded…

Quantum Physics · Physics 2022-06-01 Mariami Gachechiladze , Bartłomiej Bąk , Marcin Pawłowski , Nikolai Miklin

The interpretation of the meaning of Quantum Mechanics has faced controversy since its inception. Bell's inequalities are a touchstone in this controversy. Their observed violation demonstrates that at least one of the hypotheses involved…

Quantum Physics · Physics 2026-03-27 Mónica Agüero , Juliana Bordieu , Alejandro Hnilo , Marcelo Kovalsky , Myriam Nonaka

Solid experimental evidence has now been obtained that confirms the violation of Bell's inequality in tests of maximally entangled qubit pairs. This violation is widely interpreted as definitive proof of the impossibility of describing…

Quantum Physics · Physics 2025-10-20 David H. Oaknin
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