Extremal marginal tracial states in coupled systems
  Let \Gamma be the convex set consisting of all marginal tracial states on the tensor product B \otimes B of the algebra B of nxn matrices over the complex numbers. We find necessary and sufficient conditions for such a state to be extremal in \Gamma. We also give a characterization of those extreme points in \Gamma which are pure states. We conjecture that all extremal marginal tracial states are pure states. 
Geoffrey L. Price and Shoichuro Sakai