Greenberger-Horne-Zeilinger-type violation of local realism by mixed   states
  Cluster states are multi-particle entangled states with special entanglement properties particularly suitable for quantum computation. It has been shown that cluster states can exhibit Greenberger-Horne-Zeilinger (GHZ)-type non-locality even when some of their qubits have been lost. In the present work, we generated a four-photon mixed state, which is equivalent to the partial, qubit-loss state of an N-qubit cluster state up to some local transformations. By using this mixed state, we then realize a GHZ-type violation of local realism. Our results not only demonstrate a mixed state's GHZ-type non-locality but also exhibit the robustness of cluster states under qubit-loss conditions. 
X.-Q. Zhou, C.-Y. Lu, W.-B. Gao, J. Zhang, Z.-B. Chen, T. Yang, J.-W.   Pan