The $W$ subspace is defined as the subspace of the $n$-qubit Hilbert space spanned by states containing at most one excitation. In this work, a separability criterion is established for pure and mixed states supported in this subspace: if all reduced two-qubit subsystems are separable, then the entire $n$-qubit state is necessarily separable. This result motivates the identification of the sum of two-tangles as an entanglement measure for pure states supported in the $W$ subspace. Furthermore, it is shown that the commonly used π-tangle becomes ineffective for the $n$-qubit $W$ state as the system size increases, and vanishes in the large-$n$ limit. To address this limitation, the sum of π-tangles is introduced, which, like the sum of two-tangles, successfully quantifies the entanglement of the $n$-qubit $W$ state in the large-$n$ limit. In addition, a condition for entanglement measures is introduced, which facilitates the formulation of a well-behaved and physically meaningful entanglement measure.

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• Entanglement detection
• Entanglement measures
• Quantum entanglement