On Bures distance over standard form vN-algebras

Abstract

In case of a standard form vN-algebra, the Bures distance is the natural distance between the fibres of implementing vectors at normal positive linear forms. Thereby, it is well-known that to each two normal positive linear forms implementing vectors exist such that the Bures distance is attained by the metric distance of the implementing vectors in question. We discuss to which extend this can remain true if a vector in one of the fibres is considered as fixed. For each nonfinite algebra, classes of counterexamples are given and situations are analyzed where the latter type of result must fail. In the course of the paper, an account of those facts and notions is given, which can be taken as a useful minimum of basic C^*-algebraic tools needed in order to efficiently develop the fundamentals of Bures geometry over standard form vN-algebras.