Quantum integrals

Abstract

We first solve in ZFC a problem that the first author and Weaver posed ten years ago: a state on a von Neumann algebra is countably additive on orthogonal projections if and only if it is sequentially weak* continuous, equivalently, sequentially normal. Such states may be considered the quantum (countably additive) probability measures, or rather their extension to a noncommutative integral. The principal tools for this are known ideas from direct integral theory from the 1970s. Indeed we prove a disintegration theorem for states on products of sigma-finite von Neumann algebras. A decomposition adapted to a given sequence reduces the continuity assertion in our main result to the classical dominated convergence theorem. The argument needs no separability hypothesis. We give several applications. For example we give a new variant of Gleason theorem, describing the countably additive projection measures for any von Neumann algebra with no type $I_2$ direct summand. We then turn to the weight case, discussing sequentially normal weights and the famous related Haagerup's Problem 1.11, giving some partial results (which may conceivably may turn out to be best possible in a certain sense). For example we solve Haagerup's problem if our weight is strongly or strictly semifinite, or with no restrictions for certain classes of von Neumann algebras. We give several applications of our main result to weights. In forthcoming work we consider many applications to `quantum measure and integration theory', such as variants of Lebesgue's dominated convergence theorem for von Neumann algebras.