Entanglement in C^*-algebras: tensor products of state spaces

We analyze the Namioka-Phelps minimal and maximal tensor products of compact convex sets arising as state spaces of C^*-algebras, and, relatedly, study entanglement in (infinite dimensional) C^*-algebras. The minimal Namioka-Phelps tensor product of the state spaces of two C^*-algebras is shown to correspond to the set of separable = un-entangled states on the tensor product of the C^*-algebras. We show that these maximal and minimal tensor product of the state spaces agree precisely when one of the two C^*-algebras is commutative. This confirms an old conjecture by Barker in the case where the compact convex sets are state spaces of C^*-algebras. The Namioka-Phelps tensor product of the trace simplexes of two C^*-algebras is shown always to be the trace simplex of the tensor product of the C^*-algebras. This can be used, for example, to show that the trace simplex of (any) tensor product of C^*-algebras is the Poulsen simplex if and only if the trace simplex of each of the C^*-algebras is the Poulsen simplex or trivial (and not all trivial).