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Journal of Mathematical Physics

Bell’s inequalities and quantum field theory. II. Bell’s inequalities are maximally violated in the vacuum

Stephen J. Summers, Reinhard F. Werner

Published Oct 1, 1987• 144 citations
Publisher

Abstract

In the context of the study of Bell’s inequalities carried out in Paper I [J. Math. Phys. 28, 2440 (1987)], it is proven that Bell’s inequalities are maximally violated in the vacuum state by suitable spacelike separated observables for both Bose and Fermi free quantum field theories.
DOI
10.1063/1.527734

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Bell’s inequalities are briefly presented in the context of order-unit spaces and then studied in some detail in the framework of C*-algebras. The discussion is then specialized to quantum field theory. Maximal Bell correlations β(φ,𝒜(𝒪1), 𝒜(𝒪2)) for two subsystems localized in regions 𝒪1 and 𝒪2 and constituting a system in the state φ are defined, along with the concept of maximal Bell violations. After a study of these ideas in general, properties of these correlations in vacuum states of arbitrary quantum field models are studied. For example, it is shown that in the vacuum state the maximal Bell correlations decay exponentially with the product of the lowest mass and the spacelike separation of 𝒪1 and 𝒪2. This paper is also preparation for the proof in Paper II [S. J. Summers and R. Werner, J. Math. Phys. 28, 2448 (1987)] that Bell’s inequalities are maximally violated in the vacuum state.
We continue our study of Bell's inequalities and quantum field theory. It is shown in considerably broader generality than in our previous work that algebras of local observables corresponding to complementary wedge regions maximally violate Bell's inequality in all normal states. Pairs of commuting von Neumann algebras that maximally violate Bell's inequalities in all normal states are characterized. Algebras of local observables corresponding to tangent double cones are shown to maximally violate Bell's inequalities in all normal states in dilatation-invariant theories, in free quantum field models, and in a class of interacting models. Further, it is proven that such algebras are not split in any theory with an ultra-violet scaling limit On montre que les algebres d'observables locales correspondant a des domaines en coins violent au maximum les inegalites de Bell dans tous les etats normaux. On caracterise les paires d'algebres de Von Neumann qui commutent et qui violent maximalement les inegalites de Bell. On montre que les algebres d'observables correspondant a des cones doubles tangents violent aussi maximalement les inegalites de Bell dans tous les etats normaux des theories invariantes par dilatation, dans les modeles de champs libres et dans des modeles en interaction. On montre que ces algebres ne sont «split» dans aucune theorie avec une limite d'echelle UV
We study the violation of Bell-Mermin-Klyshko (BMK) inequalities in initial quantum states of scalar fields in inflation. We show that the Bell inequality is maximally violated by the Bunch-Davies vacuum which is a two-mode squeezed state of a scalar field. However, we find that the violation of the BMK inequalities does not increase with the number of modes to measure. We then consider a non-Bunch-Davies vacuum expressed by a four-mode squeezed state of two scalar fields. Remarkably, we find that the violation of the BMK inequalities increases exponentially with the number of modes to measure. This shows that the BMK inequalities are useful to classify the initial quantum state of the Universe.
Bell inequalities are certain probabilistic inequalities that should hold in the context of quantum measurement under assumption of a local hidden variable model. These inequalities can be violated according to the theory of quantum mechanics and have also been violated experimentally. Bell inequalities were therefore historically used to disprove local hidden variable models as an interpretation of quantum mechanics. An interesting question is to what degree Bell inequalities can be violated according to the theory of quantum mechanics. The degree to which a particular Bell inequality can be violated is quantified by the largest violation of the Bell inequality. A central question we address in this thesis is how large the largest violation can become when considering all possible Bell inequalities. In particular the CHSH-inequality, a well-known Bell inequality, has a largest violation equal to the square root of 2 and we want to find a Bell inequality with largest violation exceeding the square root of 2. This thesis consists of two main parts. In the first part we formally define Bell inequalities and the largest violation and prove several theorems about Bell inequalities. In the second part we search for Bell inequalities with largest violation exceeding the square root of 2. We do this by first approximating the largest violation of a given Bell inequality using numerical optimization. Next, using this approximation, we use numerical optimization to maximize the largest violation. Due to run-time restrictions we were only able to consider Bell inequalities with a relatively small number of terms and were unable to find any among them with largest violation exceeding the square root of 2.
Bell's inequalities in algebraic quantum field theory have been studied by Landau, Summers and Werner, and Halvorson and Clifton. Halvorson and Clifton showed that many normal states on local algebras associated with space-like separated bounded open regions violate Bell's inequalities under the the axioms of algebraic quantum field theory. In the present paper, I point out that many faithful normal states on local algebras associated with space-like separated bounded open regions violate Bell's inequalities under the axioms of algebraic quantum field theory.