Parametrization and distillability of three-qubit entanglement

There is an ongoing effort to quantify entanglement of quantum pure states for systems with more than two subsystems. We consider three approaches to this problem for three-qubit states: choosing a basis which puts the state into a standard form, enumerating "local invariants", and using operational quantities such as the number of maximally entangled states which can be distilled. In this Letter we evaluate a particular standard form, the Schmidt form, which is a generalization of the Schmidt decomposition for bipartite pure states. We show how the coefficients in this case can be parametrized in terms of five physically meaningful local invariants; we use this form to prove the efficacy of a particular distillation technique for GHZ triplets; and we relate the yield of GHZs to classes of states with unusual entanglement properties, showing that these states represent extremes of distillability as functions of two local invariants. (C) 2001 Elsevier Science B.V. All rights reserved.