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An entanglement witness gives W=1/2−∣⟨Φ+∣ψ⟩∣2W = 1/2 - |\langle\Phi^+|\psi\rangle|^2 for the target Bell state ∣Φ+⟩|\Phi^+\rangle. The test state is exactly ∣ψ⟩=∣Φ+⟩|\psi\rangle = |\Phi^+\rangle, so the overlap ∣⟨Φ+∣ψ⟩∣2=1|\langle\Phi^+|\psi\rangle|^2 = 1. What is the witness value WW?