Gleason's theorem for composite systems

TL;DR

This paper generalizes Gleason's theorem to composite quantum systems, incorporating dilations and dynamical consistency, thereby extending its foundational role in quantum mechanics.

Contribution

It introduces a novel extension of Gleason's theorem to bipartite systems, including dilations and dynamical consistency conditions.

Findings

The theorem holds for composite systems with the new conditions.

Dilation and dynamical consistency are necessary for the generalization.

The single system case remains unaffected by these conditions.

Abstract

Gleason's theorem [A. Gleason, J. Math. Mech., \textbf{6}, 885 (1957)] is an important result in the foundations of quantum mechanics, where it justifies the Born rule as a mathematical consequence of the quantum formalism. Formally, it presents a key insight into the projective geometry of Hilbert spaces, showing that finitely additive measures on the projection lattice $\PH$ extend to positive linear functionals on the algebra of bounded operators $\BH$. Over many years, and by the effort of various authors, the theorem has been broadened in its scope from type I to arbitrary von Neumann algebras (without type $\text{I}_2$ factors). Here, we prove a generalisation of Gleason's theorem to composite systems. To this end, we strengthen the original result in two ways: first, we extend its scope to dilations in the sense of Naimark [M. A. Naimark, C. R. (Dokl.) Acad. Sci. URSS, n. Ser., \textbf{41}, 359 (1943)] and Stinespring [W. F. Stinespring, Proc. Am. Math. Soc., \textbf{6}, 211 (1955)] and second, we require consistency with respect to dynamical correspondences on the respective (local) algebras in the composition [E. M. Alfsen and F. W. Shultz, Commun. Math. Phys., \textbf{194}, 87 (1998)]. We show that neither of these conditions changes the result in the single system case, yet both are necessary to obtain a generalisation to bipartite systems.