From vanishing pairwise entanglement to global separability:Entanglement structure and measures in the W subspace
The paper establishes a separability criterion for states in the W subspace: if all reduced two-qubit subsystems are separable, the entire state is separable. It proposes the sum of two-tangles as a valid entanglement measure for pure W states, noting it remains constant at one across all $n$ qubits. It also introduces the sum of $\pi$-tangles to quantify entanglement in the large-$n$ limit where standard measures fail.