We propose an entanglement measure to quantify three-qubit entanglement in terms of negativity. A monogamy inequality analogous to the Coffman-Kundu-Wootters inequality is established. This consequently leads to a definition of residual entanglement, which is referred to as the three-$\pi$ in order to distinguish it from the three-tangle. The three-$\pi$ is proved to be a natural entanglement measure. By contrast to the three-tangle, it is shown that the three-$\pi$ always gives greater than zero values for pure states belonging to the $W$ and Greenberger-Horne-Zeilinger classes, implying that three-way entanglement always exists for them; the three-tangle generally underestimates the three-way entanglement of a given system. This investigation will offer an alternative tool to understand genuine multipartite entanglement.

• Received 13 February 2007

DOI:https://doi.org/10.1103/PhysRevA.75.062308