title: Quantum discord and the geometry of Bell-diagonal states.
abstract: The set of Bell-diagonal states for two qubits can be depicted as a tetrahedron in three dimensions. We consider the level surfaces of entanglement and quantum discord for Bell-diagonal states. This provides a complete picture of the structure of entanglement and discord for this simple case and, in particular, of their nonanalytic behavior under decoherence. The pictorial approach also indicates how to show that discord is neither convex nor concave.
authors.affiliation: Center for Quantum Information and Control, University of New Mexico, MSC07-4220, Albuquerque, New Mexico 87131-0001, USA. mlang@unm.edu
authors.forename: Matthias D
authors.initials: MD
authors.lastname: Lang
authors.forename: Carlton M
authors.initials: CM
authors.lastname: Caves