Measurement-based one-way quantum computation using cluster states as resources provides an efficient model to perform computation and information processing of quantum codes. Arbitrary Gaussian quantum computation can be implemented sufficiently by long single-mode and two-mode gate sequences. However, continuous variable gate sequences have not been realized so far due to an absence of cluster states larger than four submodes. Here we present the first continuous variable gate sequence consisting of a single-mode squeezing gate and a two-mode controlled-phase gate based on a six-mode cluster state. The quantum property of this gate sequence is confirmed by the fidelities and the quantum entanglement of two output modes, which depend on both the squeezing and controlled-phase gates. The experiment demonstrates the feasibility of implementing Gaussian quantum computation by means of accessible gate sequences.

The preparation of multipartite entangled states is the prerequisite for exploring quantum information networks and quantum computation. In this letter, we present the first experimental demonstration of eight-partite spatially separated CV entangled states. The initial resource quantum states are eight squeezed states of light, through the linearly optical transformation of which two types of the eight-partite cluster entangled states are prepared, respectively. The generated eight entangled photonic qumodes are spatially separated, which provide valuable quantum resources to implement more complicated quantum information task.PACS numbers: 03.67. Bg, 03.67.Lx, 03.65.Ud, 42.50.Dv Developing quantum information (QI) science have exhibited unusual potentiality [1, 2]. Optical QI based on exploiting discrete-variable (DV) of single-photon states (photonic qubits) and continuous-variable (CV) of optical modes (photonic qumodes) plays important role in QI development. The one-way quantum computation(QC) based on multipartite cluster entanglement is initially proposed by Raussendorf and Briegel in the DV model [3], then it is extended to the CV regime by Menicucci et al [4]. For one-way QC model the qubits (qumodes) are initialized in a multipartite cluster entangled state firstly, then a variety of quantum logical operations can be achieved only via the single-qubit (qumode) projective measurement and the classical feedforward of the measured outcomes, in which the order and choices of measurements are determined by the required algorithm [3]. The basic logical operations of one-way DVQC has been experimentally demonstrated by several groups [5][6][7].Parallelly, the theoretical and experimental explorations on one-way CVQC were also proceeding continually [8][9][10][12][13][14][15]. In contrast of the probabilistic generation of photonic qubits in most cases, CV cluster states are produced in an unconditional fashion and thus the one-way QC with CV cluster entangled photonic qumodes can be implemented deterministically [12][13][14][15][16][17][18][19]. Following the theoretical proposals on one-way CVQC the principally experimental demonstrations of various one-way QC logical operations over CVs were achieved by utilizing bipartite and four-partite cluster entangled photonic qumodes, respectively [12][13][14][15]. To develop more complicated QC larger cluster states with more numbers of entangled qubits (qumodes) are desired. However, the numbers of spatially separable entangled qumodes generated by experiments still stay below four-partites, so far [16][17][18]. In the paper, we present the first experimental achievement on producing CV eight-partite entangled states for photonic qumodes. Using eight squeezed states of light to be the initial resource quantum states and passing through the linearly optical transformation on a specially designed beam-splitter network, the eight-partite linear and two-diamond shape cluster states for photonic qumodes are prepared, respectively. The entanglement feature among the ...

Single-mode squeezing and Fourier transformation operations are two essential logical gates in continuous-variable quantum computation, which have been experimentally implemented by means of an optical four-mode cluster state. In this paper, we present a simpler and more efficient protocol based on the use of Einstein-Podolsky-Rosen two-mode entangled states to realize the same operations. The theoretical calculations and the experimental results demonstrate that the presented scheme not only decreases the requirement to the resource quantum states at the largest extent but also enhances significantly the squeezing degree and the fidelity of the resultant modes under an identical resource condition. That is because in our system the influence of the excess noises deriving from the imperfect squeezing of the resource states is degraded. The gate operations applying two-mode entanglement can be utilized as a basic element in a future quantum computer involving a large-scale cluster state.

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Quantum error correction protects the quantum state against noise and decoherence in quantum communication and quantum computation, which enables one to perform fault-torrent quantum information processing. We experimentally demonstrate a quantum error correction scheme with a five-wave-packet code against a single stochastic error, the original theoretical model of which was firstly proposed by S. L. Braunstein and T. A. Walker. Five submodes of a continuous variable cluster entangled state of light are used for five encoding channels. Especially, in our encoding scheme the information of the input state is only distributed on three of the five channels and thus any error appearing in the remained two channels never affects the output state, i.e. the output quantum state is immune from the error in the two channels. The stochastic error on a single channel is corrected for both vacuum and squeezed input states and the achieved fidelities of the output states are beyond the corresponding classical limit.The transmission of quantum states with high fidelity is an essential requirement for implementing quantum information processing with high quality. However, losses and noises in channels inevitably lead to errors into transmitted quantum states and thus make the distortion of resultant states. The aim of quantum error correction (QEC) is to eliminate or, at least, reduce the hazards resulting from the imperfect channels and to ensure transmission of quantum states with high fidelity 1 . A variety of discrete variable QEC protocols, such as nine-qubit code 2 , five-qubit code 3 , topological code 4,5 , have been suggested and the experiments of QEC have been realized in different physical systems, such as nuclear magnetic resonance 6-8 , ionic 9,10 , photonic 11,12 , superconducting systems 13,14 and Rydberg atoms 15 .Besides quantum information with discrete variables, quantum information with continuous variables (CV) is also promptly developing [16][17][18][19][20][21][22][23] . Different types of CV QEC codes for correcting single non-Gaussian error have been proposed, such as nine-wave-packet code 24,25 , five-wave-packet code 26,27 , entanglement-assisted code 28 and erasure-correcting code 29 . A CV QEC scheme against Gaussian noise with a non-Gaussian operation of photon counting has been also theoretically analyzed 30 . The CV QEC schemes of the nine-wave-packet code 31 , erasure-correcting code against photon loss 32 and the correcting code with the correlated noisy channels 33 have been experimentally demonstrated.According to the no-go theorem proved in ref. 34, Gaussian errors are impossible to be corrected with pure Gaussian operations. However, non-Gaussian stochastic errors, which frequently occur in free-space channels with atmospheric fluctuations for example [35][36][37] , can be corrected by Gaussian schemes since the no-go theorem does not apply in this case. Generally, the stochastic error model is described by 38