  Along with the vast progress in experimental quantum technologies there is an
increasing demand for the quantification of entanglement between three or more
quantum systems. Theory still does not provide adequate tools for this purpose.
The objective is, besides the quest for exact results, to develop operational
methods that allow for efficient entanglement quantification. Here we put
forward an analytical approach that serves both these goals. We provide a
simple procedure to quantify Greenberger-Horne-Zeilinger--type multipartite
entanglement in arbitrary three-qubit states. For two qubits this method is
equivalent to Wootters' seminal result [Phys. Rev. Lett. 80, 2245 (1998)]. It
establishes a close link between entanglement quantification and entanglement
detection by witnesses, and can be generalised both to higher dimensions and to
more than three parties.
