Maximally unitarily mixed states on a C*-algebra

Abstract

We investigate the set of maximally mixed states of a C*-algebra, extending previous work by Alberti on von Neumann algebras. We show that, unlike for von Neumann algebras, the set of maximally mixed states of a C*-algebra may fail to be weak* closed. We obtain, however, a concrete description of the weak* closure of this set, in terms of tracial states and states which factor through simple traceless quotients. For C*-algebras with the Dixmier property or with Hausdorff primitive spectrum we are able to advance our investigations further. In the latter case we obtain a concrete description of the set of maximally mixed states in terms of traces and extensions of the states of a closed two-sided ideal. We pose several questions.