We prove that for any two commuting von Neumann algebras of infinite type, the open set of Bell correlated states for the two algebras is norm dense. We then apply this result to algebraic quantum field theory—where all local algebras are of infinite type—in order to show that for any two spacelike separated regions, there is an open dense set of field states that dictate Bell correlations between the regions. We also show that any vector state cyclic for one of a pair of commuting non-Abelian von Neumann algebras is entangled (i.e., nonseparable) across the algebras—from which it follows that every field state with bounded energy is entangled across any two spacelike separated regions.
We prove that for any two commuting von Neumann algebras of infinite type, the open set of Bell correlated states for the two algebras is norm dense. We then apply this result to algebraic quantum field theory—where all local algebras are of infinite type—in order to show that for any two spacelike separated regions, there is an open dense set of field states that dictate Bell correlations between the regions. We also show that any vector state cyclic for one of a pair of commuting non-Abelian von Neumann algebras is entangled (i.e., nonseparable) across the algebras—from which it follows that every field state with bounded energy is entangled across any two spacelike separated regions.
We prove that for any two commuting von Neumann algebras of infinite type, the open set of Bell correlated states for the two algebras is norm dense. We then apply this result to algebraic quantum field theory—where all local algebras are of infinite type—in order to show that for any two spacelike separated regions, there is an open dense set of field states that dictate Bell correlations between the regions. We also show that any vector state cyclic for one of a pair of commuting non-Abelian von Neumann algebras is entangled (i.e., nonseparable) across the algebras—from which it follows that every field state with bounded energy is entangled across any two spacelike separated regions.
Abstract There are many senses in which the phenomenon of Bell correlation, originally discovered and investigated in the context of elementary nonrelativistic quantum mechanics (Bell 1987, Clauser et al. 1969), is ‘generic’ in quantum field theory models. For example, it has been shown that every pair of commuting nonabelian von Neumann algebras possesses some normal state with maximal Bell correlation (Summers 1990; see also Landau 1987b). Moreover, in most standard quantum field models, all normal states are maximally Bell correlated across spacelike separated tangent wedges
MIP*=RE implies that correlations of commuting observables on an infinite-dimensional Hilbert space (C_qc) can lie at a finite distance from every limit of finite-dimensional tensor-product correlations (C_qa). Such correlations would let a finite experiment detect an actual infinity with a modest number of rounds. In earlier work this was identified as the only route that escapes the resource bounds on certifying Hilbert-space dimension. Here we ask whether relativistic quantum field theory allows Bell correlations in this gap. Under standard structural assumptions, it does not. Suppose the theory has the split property, which follows from the Buchholz–Wichmann nuclearity condition and holds for free fields. Then the correlations of local measurements in strictly spacelike separated regions are tensor-product correlations and lie in C_qa. If the local algebras are hyperfinite, the same holds for regions that are merely spacelike, with no gap between them. Buchholz, D'Antoni and Fredenhagen showed this hyperfiniteness under a scaling-limit assumption. We give a self-contained proof based on semidiscreteness. Alice's measurements are compressed through matrix algebras by unital completely positive maps, and the resulting correlations converge to the original ones. The same applies to the type II₁ observer algebra of a de Sitter static patch, constructed by Chandrasekaran, Longo, Penington and Witten. This algebra is a corner of a crossed product of such an algebra by ℝ, so it is again injective. Within these frameworks, local measurements in spacelike separated regions cannot realise the gap found by MIP*=RE. This excludes the fourth of the logical possibilities listed by Cabello, Quintino and Kleinmann. We then bound, from the state side, how much dimension a Bell test can certify. Compressing each party to the support of its ε-smoothed reduced state reproduces all correlations to within 2(√ε_A + √ε_B) + ε_A + ε_B. Since N rounds resolve correlations only to N^(-1/2), an experiment with N rounds certifies at most the smooth max-entropy of the local state at smoothing ε ~ 1/N. We compute this effective dimension exactly for the vacuum of a lattice free scalar field. The single-particle entanglement energies form two towers, one per block endpoint, with a spacing set by the mass, so only a handful of modes are active. The effective dimension follows an area law: it is at most 9 bits for a 128-site block at ε = 10⁻⁶, against 553 bits from naive counting. As ε decreases from 10⁻¹ to 10⁻¹², it grows only from 2.3 to 13.1 bits. A product bound over thermal modes shows that the growth is polylogarithmic in 1/ε for any regularised block. In the continuum, the corresponding object is the canonical splitting type I factor of Doplicher and Longo, whose vacuum entropy coincides with the reflected entropy. Evaluating its smooth max-entropy as a function of the splitting distance is left for future work. Together with the certification budget of the companion paper, the result gives a coherent picture. Physics uses infinite-dimensional Hilbert spaces and type III algebras. Yet, within these frameworks, the correlations that local measurements can produce are reproduced to any finite precision by finite-dimensional models. The size of these models is controlled by entanglement entropy. Code, data and figures: https://github.com/Ruqing1963/qft-tsirelson-loophole
Employing mutually-commuting von Neumann algebras to represent the algebra of observables on quantum systems provides a framework for studying quantum information theory in systems with infinite degrees of freedom and quantum field theory, yielding many profound results that differ from non-relativistic quantum systems. In this paper, we establish a mutually-commuting von Neumann algebra model of quantum networks with arbitrary structures. We derive Bell-type inequalities on this model, and determine various bounds for Bell-type inequalities based on the structure of underline von Neumann algebras, and identify the algebraic structural conditions required for their violation. The conditions on the algebraic structure of observables for maximal violation of Bell-type inequalities, which we discovered in the context of von Neumann algebra models, can in turn guide the search for measurements in the non-relativistic setting.
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.
In quantum systems with infinitely many degrees of freedom, states can be infinitely entangled across a pair of subsystems. But are there different forms of infinite entanglement? To answer this, we provide a rigorous framework for studying information-theoretic properties in infinite systems, formulated in terms of von Neumann algebras. In this framework, we show that there are operational tasks that can distinguish between different forms of infinite entanglement. We find that the type classification of von Neumann algebras (types I, II, III, and their respective subtypes) is in one-to-one correspondence with operational entanglement properties. For instance, the Connes classification of type III factors corresponds to the system’s capability at the operational task of “embezzling” entanglement. Our findings promote the type classification from algebraic bookkeeping to a classification of infinite quantum systems based on their operational entanglement properties.