Properties
The W state is the representative of one of the two non-biseparable classes of three-qubit states (the other being the GHZ state) which cannot be transformed (not even probabilistically) into each other by local quantum operations. Thus and represent two very different kinds of tripartite entanglement.
This difference is, for example, illustrated by the following interesting property of the W state: if one of the three qubits is lost, the state of the remaining 2-qubit system is still entangled. This robustness of W-type entanglement contrasts strongly with the Greenberger-Horne-Zeilinger state which is fully separable after loss of one qubit.
The states in the W class can be distinguished from all other three-qubit states by means of multipartite entanglement measures. In particular, W states have non-zero entanglement across any bipartition while the 3-tangle vanishes, which is also non-zero for GHZ-type states.
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