Quantum measurable cardinals
  We investigate states on von Neumann algebras which are not normal but enjoy various forms of infinite additivity, and show that these exist on $B(H)$ if and only if the cardinality of an orthonormal basis of $H$ satisfies various large cardinal conditions. For instance, there is a singular countably additive pure state on $B(l^2(\kappa))$ if and only if $\kappa$ is Ulam measurable, and there is a singular ${<}\,\kappa$-additive pure state on $B(l^2(\kappa))$ if and only if $\kappa$ is measurable. The proofs make use of Farah and Weaver's theory of quantum filters. Applications to Ueda's peak set theorem for von Neumann algebras are discussed in the final section. 
David P. Blecher and Nik Weaver
math.OA math.FA math.LO