Physics: GHZ state (1989): |GHZ⟩ = (|000⟩+|111⟩)/√2. If all three measure X: ⟨XXX⟩ = +1 (always correlated). If two measure Y and one X: ⟨XYY⟩ = ⟨YXY⟩ = ⟨YYX⟩ = −1. Local realism requires product ⟨XXX⟩·⟨XYY⟩·⟨YXY⟩·⟨YYX⟩ = (+1)·(−1)·(−1)·(−1) = −1, but must be +1 classically → contradiction with certainty, no statistics needed. 4-qubit cluster states enable one-way quantum computing. GHZ fragility: decoheres exponentially fast with particle number n. Bouwmeester 1999: photonic GHZ. Quantum networks: GHZ state enables multi-party quantum secret sharing.