Cluster state
In quantum information and quantum computing, a cluster state[1] is a type of highly entangled state of multiple qubits. Cluster states are generated in lattices of qubits with Ising type interactions. A cluster C is a connected subset of a ddimensional lattice, and a cluster state is a pure state of the qubits located on C. They are different from other types of entangled states such as GHZ states or W states in that it is more difficult to eliminate quantum entanglement (via projective measurements) in the case of cluster states. Another way of thinking of cluster states is as a particular instance of graph states, where the underlying graph is a connected subset of a ddimensional lattice. Cluster states are especially useful in the context of the oneway quantum computer. For a comprehensible introduction to the topic see.[2]
Formally, cluster states are states which obey the set eigenvalue equations:
where are the correlation operators
with and being Pauli matrices, denoting the neighbourhood of and being a set of binary parameters specifying the particular instance of a cluster state.
Examples for 2, 3 and 4 qubits
Here are some examples of onedimensional cluster states (d=1), for , where is the number of qubits. We take for all , which means the cluster state is the unique simultaneous eigenstate that has corresponding eigenvalue 1 under all correlation operators. In each example the set of correlation operators and the corresponding cluster state is listed.
This is an EPRpair (up to local transformations).

This is the GHZstate (up to local transformations).
 .
 This is not a GHZstate and can not be converted to a GHZstate with local operations.
In all examples is the identity operator, and tensor products are omitted. The states above can be obtained from the all zero state by first applying a Hadamard gate to every qubit, and then a controlledZ gate between all qubits that are adjacent to each other.
Experimental creation of cluster states
Cluster states have been realized experimentally. They have been obtained in photonic experiments using parametric downconversion. [3] [4] In such systems, the horizontal and vertical polarizations of the photons code the qubit. Cluster states have been created also in optical lattices of cold atoms.[5]
Entanglement criteria and Bell inequalities for cluster states
After the cluster states is cerated in an experiment, it is important to verify that indeed, an entangled quantum state has been cerated and obtain the fidelity with respect to an ideal cluster state. There are efficient conditions to detect entanglement close to cluster states, that need only the minimal two local measurement settings [6]. Similar conditions can also be used to estimate the fidelity with respect to an ideal cluster state. [7] Bell inequalities have also been developed for cluster states. [8] [9]
See also
References
 H. J. Briegel; R. Raussendorf (2001). "Persistent Entanglement in arrays of Interacting Particles". Physical Review Letters. 86 (5): 910–3. arXiv:quantph/0004051. Bibcode:2001PhRvL..86..910B. doi:10.1103/PhysRevLett.86.910. PMID 11177971.
 Briegel, Hans J. "Cluster States". In Greenberger, Daniel; Hentschel, Klaus & Weinert, Friedel (eds.). Compendium of Quantum Physics  Concepts, Experiments, History and Philosophy. Springer. pp. 96–105. ISBN 9783540706229.
 P. Walther, K. J. Resch, T. Rudolph, E. Schenck, H. Weinfurter, V. Vedral, M. Aspelmeyer and A. Zeilinger (2005). "Experimental oneway quantum computing". Nature. 434 (7030): 169–76. arXiv:quantph/0503126. Bibcode:2005Natur.434..169W. doi:10.1038/nature03347. PMID 15758991.CS1 maint: multiple names: authors list (link)
 N. Kiesel; C. Schmid; U. Weber; G. Tóth; O. Gühne; R. Ursin; H. Weinfurter (2005). "Experimental Analysis of a 4Qubit Cluster State". Phys. Rev. Lett. 95 (21): 210502. arXiv:quantph/0508128. Bibcode:2005PhRvL..95u0502K. doi:10.1103/PhysRevLett.95.210502. PMID 16384122.
 O. Mandel; M. Greiner; A. Widera; T. Rom; T. W. Hänsch; I. Bloch (2003). "Controlled collisions for multiparticle entanglement of optically trapped atoms". Nature. 425 (6961): 937–940. arXiv:quantph/0308080. Bibcode:2003Natur.425..937M. doi:10.1038/nature02008. PMID 14586463.
 Tóth, Géza; Gühne, Otfried (17 February 2005). "Detecting Genuine Multipartite Entanglement with Two Local Measurements". Physical Review Letters. 94 (6). doi:10.1103/PhysRevLett.94.060501.
 Tóth, Géza; Gühne, Otfried (29 August 2005). "Entanglement detection in the stabilizer formalism". Physical Review A. 72 (2). doi:10.1103/PhysRevA.72.022340.
 Scarani, Valerio; Acín, Antonio; Schenck, Emmanuel; Aspelmeyer, Markus (18 April 2005). "Nonlocality of cluster states of qubits". Physical Review A. 71 (4). doi:10.1103/PhysRevA.71.042325.
 Gühne, Otfried; Tóth, Géza; Hyllus, Philipp; Briegel, Hans J. (14 September 2005). "Bell Inequalities for Graph States". Physical Review Letters. 95 (12). doi:10.1103/PhysRevLett.95.120405.