Quantum mass functions (QMFs), which are tightly related to decoherence functionals, were introduced by Loeliger and Vontobel [IEEE Trans. Inf. Theory, 2017, 2020] as a generalization of probability mass functions toward modeling quantum information processing setups in terms of factor graphs. Simple quantum mass functions (SQMFs) are a special class of QMFs that do not explicitly model classical random variables. Nevertheless, classical random variables appear implicitly in an SQMF if some marginals of the SQMF satisfy some conditions; variables of the SQMF corresponding to these “emerging” random variables are called classicable variables. Of particular interest are jointly classicable variables. In this paper we initiate the characterization of the set of marginals given by the collection of jointly classicable variables of a graphical model and compare them with other concepts associated with graphical models like the sets of realizable marginals and the local marginal polytope. In order to further characterize this set of marginals given by the collection of jointly classicable variables, we generalize the CHSH inequality based on the Pearson correlation coefficients, and thereby prove a conjecture proposed by Pozsgay et al. A crucial feature of this inequality is its nonlinearity, which poses difficulties in the proof.
Quantum mass functions (QMFs), which are tightly related to decoherence functionals, were introduced by Loeliger and Vontobel [IEEE Trans. Inf. Theory, 2017, 2020] as a generalization of probability mass functions toward modeling quantum information processing setups in terms of factor graphs. Simple quantum mass functions (SQMFs) are a special class of QMFs that do not explicitly model classical random variables. Nevertheless, classical random variables appear implicitly in an SQMF if some marginals of the SQMF satisfy some conditions; variables of the SQMF corresponding to these “emerging” random variables are called classicable variables. Of particular interest are jointly classicable variables. In this paper we initiate the characterization of the set of marginals given by the collection of jointly classicable variables of a graphical model and compare them with other concepts associated with graphical models like the sets of realizable marginals and the local marginal polytope. In order to further characterize this set of marginals given by the collection of jointly classicable variables, we generalize the CHSH inequality based on the Pearson correlation coefficients, and thereby prove a conjecture proposed by Pozsgay et al. A crucial feature of this inequality is its nonlinearity, which poses difficulties in the proof.
Bell inequalities characterize the boundary of the local-realist correlation polytope -- the set of joint probability distributions achievable by classical hidden-variable models. Quantum mechanics exceeds this boundary through non-commutativity, reaching the Tsirelson bound 22 for CHSH. We show that this polytope structure is not specific to quantum foundations: it appears identically in the causal inference literature, where the instrumental inequality, the Balke--Pearl linear programming bounds, and the Tian--Pearl probabilities of causation all arise as facets of the same marginal compatibility polytope. Fine's theorem -- that CHSH inequalities hold if and only if a joint distribution exists -- is precisely the pivot: the instrumental variable model in causal inference is structurally equivalent to the Bell local hidden-variable model, with the instrument playing the role of the measurement setting and the latent confounder playing the role of the hidden variable λ. We develop this correspondence in detail, extending it to algorithmic (Kolmogorov complexity) and entropic formulations of Bell inequalities, the NPA semidefinite programming hierarchy, and the MIP∗=RE undecidability result. We further show that the Born-rule / Bayes-rule duality underlying quantum Bayesian computation exploits the same non-commutativity that enables Bell violation, providing polynomial speedups for posterior inference. The framework yields a concrete dictionary between quantum information theory, causal econometrics, and Bayesian computation, and suggests new directions including NPA-based quantum causal inference algorithms and quantum architectures for function approximation.
In prior work, we have shown how the basic concepts and terms of quantum mechanics relate to factorizations and marginals of complex-valued quantum mass functions, which are generalizations of joint probability mass functions. In this paper, using quantum mass functions, we discuss the realization of measurements in terms of unitary interactions and marginalizations. It follows that classical measurement results strictly belong to local models, i.e., marginals of more detailed models. Classical variables that are created by marginalization do not exist in the unmarginalized model, and different marginalizations may yield incompatible classical variables. These observations are illustrated by the Frauchiger-Renner paradox, which is analyzed (and resolved) in terms of quantum mass functions. Throughout, the paper uses factor graphs to represent quantum systems/models with multiple measurements at different points in time.
We give a set of necessary conditions for locality in bipartite systems, which include and generalize known Bell's inequalities. Each condition corresponds to a specific order of the expansion of random variables defined on graphs, in terms of genuinely n-variable correlation functions. The first non-trivial order leads to known Bell inequalities, while higher orders produce additional, non-equivalent conditions. In particular, in CHSH settings, we obtain at least two additional tight conditions which do not reduce to the CHSH inequality. This shows that tight Bell inequalities are sufficient but not necessary conditions for the non-locality of bipartite quantum correlations.
Abstract In this paper, we study random instances of the classical marginal problem. We encode the problem in a graph, where the vertices have assigned fixed binary probability distributions, and edges have assigned random bivariate distributions having the incident vertex distributions as marginals. We provide estimates on the probability that a joint distribution on the graph exists, having the bivariate edge distributions as marginals. Our study is motivated by Fine’s theorem in quantum mechanics. We study in great detail the graphs corresponding to CHSH and Bell–Wigner scenarios providing rations of volumes between the local and non-signaling polytopes.
FINDING: Bell/CHSH inequalities define the boundary between classical local-hidden-variable theories and quantum correlations; recent work (Buscemi) reformulates them as a unified family of "rules" covering entanglement and nonlocality, and new physical platforms (ballistic electrons in semiconductor quantum wires) are proposed as testbeds. MATH: - CHSH inequality (classical bound): |E(a,b) − E(a,b′) + E(a′,b) + E(a′,b′)| ≤ 2 - Quantum (Tsirelson) bound: same expression ≤ 2√2 ≈ 2.828 - Quantum/classical ratio: 2√2 / 2 = √2 ≈ 1.4142 - Maximal violation angle for singlet state: correlations E(a,b) = −a·b; optimal settings at 45° increments give S = 2√2 - General Bell functional: S = Σ_{x,y} w_{xy} P(a,b|x,y); local polytope bounded by facet inequalities; quantum set Q strictly contains local polytope L, strictly contained in no-signaling set NS - Buscemi's unification: inequalities expressed via conditional min-/max-entropy constraints — "new rules" generalize CHSH to entropy-based Bell Author: Andrew Stewart Caldin, Independent Researcher, UK. Part of the E8 Intelligence Research series. Platform: e8intelligence.com
The Clauser-Horne-Shimony-Holt (CHSH) inequalities are the most famous examples of Bell inequalities. Cabello, Severini, and Winter came up with a graph approach to noncontextuality inequalities, which connects some graph-theoretic concepts to quantum and classical correlations. For example, the theta body of the exclusivity graph can be associated with the set of correlations achieved by quantum theory. Following the Cabello-Severini-Winter (CSW) approach, one may think that the theta body of the CHSH graph TH(GCHSH) is equal to the quantum set of the CHSH Bell inequality QCHSH, but is this really true? All assumptions about the CHSH inequalities come from Bell scenarios, while the CSW approach only demands the exclusivity structure of a noncontextuality (NC) scenario. To deal with the extra structure related to the presence of different players in a Bell scenario like the CHSH one, the colored-graph approach was introduced. Does it make any difference to think about the CHSH scenario as a Bell one or a more general NC scenario? The Bell CHSH inequality is represented by a bicolored graph G and the NC CHSH inequality by a simple graph G, which is the shadow of the colored graph G. In general, we have that the theta body of the colored graph ${\ensuremath{\theta}}_{c}(\mathcal{G})$ is a subset of the theta body of its shadow graph TH(G) in the same way that the Lov\'asz number, which corresponds to the quantum bound, of the simple graph $\ensuremath{\vartheta}(G)$ is greater than or equal to the Lov\'asz number of the colored graph ${\ensuremath{\vartheta}}_{c}(\mathcal{G})$. In the case of the CHSH inequality, we have that $\ensuremath{\vartheta}(G)={\ensuremath{\vartheta}}_{c}(\mathcal{G})$. Does this accident also hold for the corresponding quantum sets? Is it true that $\mathrm{TH}(G)={\ensuremath{\theta}}_{c}(\mathcal{G})$, which would mean that every correlation reached by quantum theory applied to the CHSH NC scenario could also be obtained at the in principle more restrictive CHSH Bell scenario? In this paper our answer to such a question is negative. We show that ${\ensuremath{\theta}}_{c}(\mathcal{G})\ensuremath{\nsubseteq}\mathrm{TH}(G)$ and therefore that there are quantum correlations which cannot be obtained under Bell restrictions.