Optimal detection of entanglement in Greenberger-Horne-Zeilinger states
Optimal detection of entanglement in Greenberger-Horne-Zeilinger states
We present a broad class of $N$-qubit Greenberger-Horne-Zeilinger (GHZ)-diagonal states such that nonpositivity under the partial transpose operation is necessary and sufficient for the presence of entanglement, including many naturally arising instances such as dephased GHZ states. Furthermore, our proof directly leads to an entanglement witness which saturates this bound. …