title: Experimental violation of a cluster state bell inequality.
abstract: Cluster states are a new type of multiqubit entangled states with entanglement properties exceptionally well suited for quantum computation. In the present work, we experimentally demonstrate that correlations in a four-qubit linear cluster state cannot be described by local realism. This exploration is based on a recently derived Bell-type inequality [V. Scarani et al., Phys. Rev. A 71, 042325 (2005)] which is tailored, by using a combination of three- and four-particle correlations, to be maximally violated by cluster states but not violated at all by GHZ states. We observe a cluster-state Bell parameter of 2.59+/-0.08, which is more than 7sigma larger than the threshold of 2 imposed by local realism.
authors.affiliation: Institut für Experimentalphysik, Universität Wien, Boltzmanngasse 5, A-1090 Wien, Austria.
authors.forename: Philip
authors.initials: P
authors.lastname: Walther
authors.forename: Markus
authors.initials: M
authors.lastname: Aspelmeyer
authors.forename: Kevin J
authors.initials: KJ
authors.lastname: Resch
authors.forename: Anton
authors.initials: A
authors.lastname: Zeilinger