Reduction of free independence to tensor independence

Abstract

We show how to reduce free independence to tensor independence in the strong sense. We construct a suitable unital *-algebra of closed operators `affiliated' with a given unital *-algebra and call the associated closure `monotone'. Then we prove that monotone closed operators of the form $$ X'= \sum_{k=1}^{\infty}X(k)\bar{\otimes} p_{k}, X''=\sum_{k=1}^{\infty} p_{k}\bar{\otimes}X(k) $$ are free with respect to a tensor product state, where $X(k)$ are tensor independent copies of a random variable $X$ and $(p_{k})$ is a sequence of orthogonal projections. For unital free *-algebras, we construct a monotone closed analog of a unital *-bialgebra called a `monotone closed quantum semigroup' which implements the additive free convolution, without using the concept of dual groups.