Graph states are special kinds of multipartite entangled states that correspond to mathematical graphs where the vertices take the role of quantum spin systems and the edges represent interactions. They not only provide an efficient model to study multiparticle entanglement, but also find wide applications in quantum error correction, multi-party quantum communication and most prominently, serve as the central resource in one-way quantum computation. Here we report the creation of two special instances of graph states, the six-photon Greenberger-Horne-Zeilinger states -- the largest photonic Schr\"{o}dinger cat, and the six-photon cluster states-- a state-of-the-art 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 quantum computation and studies of multiparticle entanglement as well as foundational issues such as nonlocality and decoherence

Coherent manipulation of a large number of qubits and the generation of entangled states between them has been an important goal and benchmark in quantum information science, leading to various applications such as measurement-based quantum computing 1 and high-precision quantum metrology 2 . However, the experimental preparation of multiparticle entanglement remains challenging. Using atoms 3,4 , entangled states of up to eight qubits have been created, and up to six photons 5 have been entangled. Here, by exploiting both the photons' polarization and momentum degrees of freedom, we experimentally generate hyper-entangled six-, eight-and ten-qubit Schrödinger cat states with verified genuine multi-qubit entanglement. We also demonstrate super-resolving phase measurements enhanced by entanglement, with a precision to beat the standard quantum limit. Modifications of the experimental set-up would enable the generation of other graph states up to ten qubits. Our method offers a way of expanding the effective Hilbert space and should provide a versatile test-bed for various quantum applications.Control of single photonic qubits using linear optics has been an appealing approach to implementing quantum computing 6 . Experiments in recent years have demonstrated the photons' extremely long decoherence time 7 , fast clock speed 8 , a series of controlled quantum logic gates 9,10 and algorithms 8,11,12 and the generation of multiqubit entangled states 5 . A significant challenge, however, lies in making experimentally accessible sources of photonic multi-qubit states. This is because, on the one hand, the probabilistic nature of spontaneous parametric down-conversion 13 represents a bottleneck with regard to the attainable brightness and fidelity of multiphoton states based on it: manipulating seven photons or more seems an insurmountable challenge with present technology. On the other hand, triggered single-photon sources from independent quantum dots or other emitters still suffer from spectral and temporal distinguishability, which prevents up-scaling.There is, however, a way to experimentally control more effectively qubits, by exploiting hyper-entanglement 14 -the simultaneous entanglement in the multiple degrees of freedom that naturally exist for various physical systems. For instance, one can encode quantum information not only in the polarization of a single photon, but also in its spatial modes 15 Although the largest hyper-entangled state 15 realized so far has expanded the Hilbert space up to 144 dimensions, it is a product state of two-party entangled states and does not involve multipartite entanglement. Other schemes 20,21 for creating hyperentanglement have been limited by the technical problem of photonic subwavelength phase stability and seem infeasible to generate larger states than the two-photon four-qubit ones. Here, we will describe a method that overcomes these limitations, and the experimental generation of hyper-entangled six-, eight-and ten-qubit photonic Schrödinger cat states.The Schröding...

The heralded generation of entangled states is a long-standing goal in quantum information processing, because it is indispensable for a number of quantum protocols. Polarization entangled photon pairs are usually generated through spontaneous parametric down-conversion, but the emission is probabilistic. Their applications are generally accompanied by post-selection and destructive photon detection. Here, we report a source of entanglement generated in an event-ready manner by conditioned detection of auxiliary photons. This scheme benefits from the stable and robust properties of spontaneous parametric down-conversion and requires only modest experimental efforts. It is flexible and allows the preparation efficiency to be significantly improved by using beamsplitters with different transmission ratios. We have achieved a fidelity better than 87% and a state preparation efficiency of 45% for the source. This could offer promise in essential photonics-based quantum information tasks, and particularly in enabling optical quantum computing by reducing dramatically the computational overhead.Comment: 24 pages, 4 figures, 1 tabl

We report an experimental realization of one-way quantum computing on a two-photon four-qubit cluster state. This is accomplished by developing a two-photon cluster state source entangled both in polarization and spatial modes. With this special source, we implemented a highly efficient Grover's search algorithm and high-fidelity two qubits quantum gates. Our experiment demonstrates that such cluster states could serve as an ideal source and a building block for rapid and precise optical quantum computation.PACS numbers: 03.67. Lx, 03.67.Mn, 42.50.Dv Highly entangled multipartite states, so-called cluster states, have recently raised enormous interest in quantum information processing (QIP). These sorts of states are crucial as a fundamental resource and a building block aimed at one-way universal quantum computing [1]. They are also essential elements for various quantum error correction codes and quantum communication protocols [2]. Moreover, the entanglements are shown to be robust against decoherence [3], and persistent against loss of qubits [1], and thus are exceptionally well suited for quantum computing and many tasks [1,2]. Considerable efforts have been made toward generating and characterizing cluster state in linear optics [4,5,6,7,8,9]. Recently the principal feasibility of a one-way quantum computing model has been experimentally demonstrated through 4-photon cluster states successfully [7,8,10].So far, preparing photonic cluster state still suffers from several serious limitations. Due to the probabilistic nature and Poissonian distribution of the parametric down-conversion process, the generation rate of 4-photon cluster states is quite low [5,6,7,8], and largely restricts speed of computing. Besides, the quality and fidelity of prepared cluster states are relatively low [6,7,8], which are difficult to be improved substantially. These disadvantages consequently impose great challenges of advancement even for few-qubit quantum computing.Fortunately, motivated by the progress that an important type of states termed hyper-entangled states have been experimentally generated [11,12,13,14], we have the possibility to produce a new type of cluster state (2-photon 4-qubit cluster state) with nearly perfect fidelity and high generation rate. The hyper-entangled states have been used to test "All-Versus-Nothing" (AVN) quantum nonlocality [11,12,15], and are shown to lead to an enhancing violation of local realism [16,17]. The states also enable to perform complete deterministic Bell state analysis [18] as demonstrated in [14,19].In this Letter we report an experimental realization of one-way quantum computing with such a 2-photon 4-qubit cluster state. The key idea is to develop and employ a bright source which produces a 2-photon state entangled both in polarization and spatial modes. We are thus able to implement the Grover's algorithm and quantum gates with excellent performances. The genuine four-partite entanglement and high fidelity of better than 88% are characterized by an optimal entanglement w...

Quantum teleportation 1 , a way to transfer the state of a quantum system from one location to another, is central to quantum communication 2 and plays an important role in a number of quantum computation protocols 3-5 . Previous experimental demonstrations have been implemented with photonic 6-8 or ionic qubits 9,10 . Very recently long-distance teleportation 11,12 and open-destination teleportation 13 have also been realized. Until now, previous experiments 6-13 have only been able to teleport single qubits. However, since teleportation of single qubits is insufficient for a large-scale realization of quantum communication and computation 2-5 , teleportation of a composite system containing two or more qubits has been seen as a long-standing goal in quantum information science.Here, we present the experimental realization of quantum teleportation of a two-qubit composite system. In the experiment, we develop and exploit a six-photon interferometer to teleport an arbitrary polarization state of two photons. The observed teleportation fidelities for different initial states are all