A note on versions:The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher's version. Please see the repository url above for details on accessing the published version and note that access may require a subscription. Understanding how quantum resources can be quantified and distributed over many parties has profound applications in quantum communication. As one of the most intriguing features of quantum mechanics, Einstein-Podolsky-Rosen (EPR) steering is a useful resource for secure quantum networks. By reconstructing the covariance matrix of a continuous variable four-mode square Gaussian cluster state subject to asymmetric loss, we quantify the amount of bipartite steering with a variable number of modes per party, and verify recently introduced monogamy relations for Gaussian steerability, which establish quantitative constraints on the security of information shared among different parties. We observe a very rich structure for the steering distribution, and demonstrate one-way EPR steering of the cluster state under Gaussian measurements, as well as one-to-multimode steering. Our experiment paves the way for exploiting EPR steering in Gaussian cluster states as a valuable resource for multiparty quantum information tasks.

Quantum entanglement swapping is one of the most promising ways to realize the quantum connection among local quantum nodes. In this Letter, we present an experimental demonstration of the entanglement swapping between two independent multipartite entangled states, each of which involves a tripartite Greenberger-Horne-Zeilinger (GHZ) entangled state of an optical field. The entanglement swapping is implemented deterministically by means of a joint measurement on two optical modes coming from the two multipartite entangled states respectively and the classical feedforward of the measurement results. After entanglement swapping the two independent multipartite entangled states are merged into a large entangled state in which all unmeasured quantum modes are entangled. The entanglement swapping between a tripartite GHZ state and an Einstein-PodolskyRosen entangled state is also demonstrated and the dependence of the resultant entanglement on transmission loss is investigated. The presented experiment provides a feasible technical reference for constructing more complicated quantum networks.PACS numbers: 03.67. Hk, 03.67.Bg, 42.50.Ex, 42.50.Lc Multipartite entangled states play essential roles in quantum computation and quantum networks. Cluster states, a type of multipartite entangled states, are basic quantum resources for one-way quantum computation [1,2]. Based on a prepared large scale cluster state, one-way quantum computation can be implemented by measurement and feedforward of the measured results [3][4][5][6]. It has been demonstrated that a local quantum network can be built by distributing a multipartite entangled state among quantum nodes [7][8][9][10]. If we have two space-separated local quantum networks built by two independent multipartite entangled states, respectively, how can we establish entanglement between the quantum nodes in the two local quantum networks? It has been proposed that a large scale cluster state can be generated by the fusion of small scale cluster states, which is completed by means of linear optical elements [11]. The shaping of a larger cluster state to a smaller one according to the requirement for one-way quantum computation has been demonstrated [12]. Another feasible method of merging two multipartite entangled states into one larger multipartite entangled state is quantum entanglement swapping [13], which has been proposed to build a global quantum network of clocks that may allow the construction of a real-time single international time scale (world clock) with unprecedented stability and accuracy [14].Quantum teleportation enables transportation of an unknown quantum state to a remote station by using an entangled state as the quantum resource. Up to now, long distance quantum teleportation of single pho- * Electronic address: suxl@sxu.edu.cn † Electronic address: kcpeng@sxu.edu.cn tons over 100 km has been experimentally demonstrated [15][16][17]. Quantum entanglement swapping, which makes two independent quantum entangled states become entangled without direct int...

Einstein-Podolsky-Rosen (EPR) steering exhibits an inherent asymmetric feature that differs from both entanglement and Bell nonlocality, which leads to one-way EPR steering. Although this one-way EPR steering phenomenon has been experimentally observed, the schemes to manipulate the direction of EPR steering have not been investigated thoroughly. In this paper, we propose and experimentally demonstrate three schemes to manipulate the direction of EPR steering, either by varying the noise on one party of a two-mode squeezed state (TMSS) or transmitting the TMSS in a noisy channel. The dependence of the direction of EPR steering on the noise and transmission efficiency in the quantum channel is analyzed. The experimental results show that the direction of EPR steering of the TMSS can be changed in the presented schemes. Our work is helpful in understanding the fundamental asymmetry of quantum nonlocality and has potential applications in future asymmetric quantum information processing

A quantum communication network can be constructed by distributing a multipartite entangled state to space-separated nodes. Entangled optical beams with highest flying speed and measurable brightness can be used as carriers to convey information in quantum communication networks. Losses and noises existing in real communication channels will reduce or even totally destroy entanglement. The phenomenon of disentanglement will result in the complete failure of quantum communication. Here, we present the experimental demonstrations on the disentanglement and the entanglement revival of tripartite entangled optical beams used in a quantum network. We experimentally demonstrate that symmetric tripartite entangled optical beams are robust in pure lossy but noiseless channels. In a noisy channel, the excess noise will lead to the disentanglement and the destroyed entanglement can be revived by the use of a correlated noisy channel (non-Markovian environment). The presented results provide useful technical references for establishing quantum networks.

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