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Experimental entanglement of six photons in graph states
Chao-Yang Lu1, Xiao-Qi Zhou1, Otfried Gühne2, Wei-Bo Gao1, Jin Zhang1, Zhen-Sheng Yuan1, Alexander Goebel3, Tao Yang1, and Jian-Wei Pan1,3
1) Hefei National Laboratory for Physical Sciences at Microscale and Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China. 2)Institut für Quantenoptik und Quanteninformation, Österreichische Akademie der Wissenschaften, Technikerstraβe 21A, A-6020 Innsbruck, Austria. 3) Physikalisches Institut, Universität Heidelberg, Philosophenweg 12, D-69120 Heidelberg, Germany

Graph states1-3 are special kinds of multipartite entangled states that correspond to mathematical graphs where the vertices take the role of quantum spinsystems and the edges represent interactions. They not only provide an efficient model to study multiparticle entanglement1, but also find wide applications in quantum error correction3, multi-party quantum communication4 and most prominently, serve as the central resource in one-way quantum computation5. Here we report the creation of two special instances of graph states, the six-photonGreenberger-Horne-Zeilinger states6 -the largest photonic Schrödinger cat, and the six-photon cluster states2-- a state-of-theart one-way quantum computer. Flexibly, slight modifications of our method allow creation of many other graph states. Thus we have demonstrated the ability of entangling six photons and engineering multiqubit graph states, and created a test-bed for investigations of one-way quantumcomputation7-15 and studies of multiparticle entanglement as well as foundational issues such as nonlocality16,17 and decoherence18. Entanglement lies at the heart of quantum mechanics and plays a crucial role in quantum information processing. Many efforts have been undertaken to create especially multipartite entangled states in different physical systems19-22. In recent years, special kinds ofmultipartite entangled states, the graph states1-3, have moved into the center of attention. They

can be associated with graphs where each vertex represents a qubit prepared in the state
1 2

(| 0〉+ |1〉 ) and each edge represents a controlled phase gate having been applied between

the two connected qubits. An interesting feature is that many entanglement properties of graph states areclosely related to their underlying graphs. Besides their thought-provoking theoretical structure1, the graph states also have shed new insights in studies of nonlocality16,17 and decoherence18 and served as essential resource for various quantum information tasks3,4, most prominently as the exceptionally universal resource for one-way quantum computation5. Encouraging progresses7-14,20 have beenachieved in this direction, especially in the linear optics regime15. Yet a major challenge ahead lies in the experimental generation of multiqubit graph states. Of special interest in the graph state family are the Greenberger-Horne-Zeilinger (GHZ) states and the cluster states. Experimentally, six-atom GHZ states21 and four-photon cluster states20 have been realized. Here we report the creationof six-photon GHZ states and cluster state with verifiable six-partite entanglement. To do so, we start from three EinsteinPodolsky-Rosen (EPR) entangled photon pairs in the state | Φ + 〉 ij = 1 ( | H 〉i | H 〉 j + | V 〉i | V 〉 j ) , 2

where H and V denote horizontal and vertical polarization, and i and j label the spatial modes of the photons (Fig. 1a). We superpose photons in mode 2 and 3 (4and 5) at polarizing beam splitters (PBS). Since the PBS transmits H and reflects V polarization, only if both incoming photons have the same polarization can they go to different outputs. Thus a coincidence detection of all the six outputs corresponds to the state | G6 〉 = 1 ( | H 〉1 | H 〉 2 | H 〉 3 | H 〉 4 | H 〉 5 | H 〉 6 + | V 〉1 | V 〉 2 | V 〉 3 | V 〉 4 | V 〉 5 | V 〉 6 ) , 2

which is a...
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