title: Entangled three-qubit states without concurrence and three-tangle.
abstract: We provide a complete analysis of mixed three-qubit states composed of a Greenberger-Horne-Zeilinger state and a W state orthogonal to the former. We present optimal decompositions and convex roofs for the three-tangle. Further, we provide an analytical method to decide whether or not an arbitrary rank-2 state of three qubits has vanishing three-tangle. These results highlight intriguing differences compared to the properties of two-qubit mixed states, and may serve as a quantitative reference for future studies of entanglement in multipartite mixed states. By studying the Coffman-Kundu-Wootters inequality we find that, while the amounts of inequivalent entanglement types strictly add up for pure states, this "monogamy" can be lifted for mixed states by virtue of vanishing tangle measures.
authors.affiliation: Institut für Theoretische Physik, Universität Regensburg, D-93040 Regensburg, Germany.
authors.forename: Robert
authors.initials: R
authors.lastname: Lohmayer
authors.forename: Andreas
authors.initials: A
authors.lastname: Osterloh
authors.forename: Jens
authors.initials: J
authors.lastname: Siewert
authors.forename: Armin
authors.initials: A
authors.lastname: Uhlmann