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.
The notion of W state has been generalized for qubits and then refers to the quantum superpostion with equal expansion coefficients of all possible pure states in which exactly one of the qubits in an "excited state" , while all other ones are in the "ground state"
Both the robustness against particle loss and the LOCC-inequivalence with the (generalized) GHZ state also hold for the -qubit W state.
In systems in which a single qubit is stored in an ensemble of many two level systems the logical "1" is often represented by the W state while the logical "0" is represented by the state . Here the W state's robustness against particle loss is a very beneficial property ensuring good storage properties of these ensemble based quantum memories.
- A pure state of parties is called biseparable, if one can find a partition of the parties in two disjoint subsets and with such that , i.e. is a product state with respect to he partition .
- W. Dür, G. Vidal, and J.I. Cirac (2000). "Three qubits can be entangled in two inequivalent ways". Phys. Rev. A 62: 062314. arXiv:quant-ph/0005115. Bibcode:2000PhRvA..62f2314D. doi:10.1103/PhysRevA.62.062314.
- A bipartition of the three qubits is any grouping and in which two qubits are considered to belong to the same party. The three qubit state can then be considered as a state on and studied with bipartite entanglement measures.
- M. Fleischhauer and M. D. Lukin (2002). "Quantum memory for photons: Dark-state polaritons" 65. p. 022314. arXiv:quant-ph/0106066. Bibcode:2002PhRvA..65b2314F. doi:10.1103/PhysRevA.65.022314.