Quantum Correlations in the Minimal Scenario

Quantum 7, 947 (2023).

https://doi.org/10.22331/q-2023-03-16-947

In the minimal scenario of quantum correlations, two parties can choose from two observables with two possible outcomes each. Probabilities are specified by four marginals and four correlations. The resulting four-dimensional convex body of correlations, denoted $mathcal{Q}$, is fundamental for quantum information theory. We review and systematize what is known about $mathcal{Q}$, and add many details, visualizations, and complete proofs. In particular, we provide a detailed description of the boundary, which consists of three-dimensional faces isomorphic to elliptopes and sextic algebraic manifolds of exposed extreme points. These patches are separated by cubic surfaces of non-exposed extreme points. We provide a trigonometric parametrization of all extreme points, along with their exposing Tsirelson inequalities and quantum models. All non-classical extreme points (exposed or not) are self-testing, i.e., realized by an essentially unique quantum model.
Two principles, which are specific to the minimal scenario, allow a quick and complete overview: The first is the pushout transformation, i.e., the application of the sine function to each coordinate. This transforms the classical correlation polytope exactly into the correlation body $mathcal{Q}$, also identifying the boundary structures. The second principle, self-duality, is an isomorphism between $mathcal{Q}$ and its polar dual, i.e., the set of affine inequalities satisfied by all quantum correlations (“Tsirelson inequalities”). The same isomorphism links the polytope of classical correlations contained in $mathcal{Q}$ to the polytope of no-signalling correlations, which contains $mathcal{Q}$.
We also discuss the sets of correlations achieved with fixed Hilbert space dimension, fixed state or fixed observables, and establish a new non-linear inequality for $mathcal{Q}$ involving the determinant of the correlation matrix.