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Conference on Lasers and Electro-Optics

Unconditional Continuous Variable Entanglement Cascading

Xueyuan Hu, Ying Gu, Qihuang Gong, Guang‐Can Guo

Published Jan 1, 2009• 0 citations
Publisher

Abstract

A sequence of continuous-variable entanglement swapping is studied. We obtain the upper-bound of Gaussian cascaded entanglement and propose an unconditional protocol achieving it. We demonstrate entanglement can be unconditionally cascaded for any nonzero-squeezed entangled sources.
DOI
10.1364/cleo.2009.jwa86

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We investigate the continuous-variable entanglement swapping protocol in a non-Gaussian setting, with non- Gaussian states employed either as entangled inputs and/or as swapping resources. The quality of the swapping protocol is assessed in terms of the teleportation fidelity achievable when using the swapped states as shared entangled resources in a teleportation protocol. We thus introduce a two-step cascaded quantum communication scheme that includes a swapping protocol followed by a teleportation protocol. The swapping protocol is fed by a general class of tunable non-Gaussian states, the squeezed Bell states, which, by means of controllable free parameters, allows for a continuous morphing from Gaussian twin beams up to maximally non-Gaussian squeezed number states. In the realistic instance, taking into account the effects of losses and imperfections, we show that as the input two-mode squeezing increases, optimized non-Gaussian swapping resources allow for a monotonically increasing enhancement of the fidelity compared to the corresponding Gaussian setting. This result implies that the use of non-Gaussian resources is necessary to guarantee the success of continuous-variable entanglement swapping in the presence of decoherence.
A sequence of entanglement swapping of continuous variables is considered. It is classified into one-way entanglement swapping and two-way entanglement swapping, where the former (the latter) uses one-way (two-way) classical communication. When resources of quantum entanglement are bipartite Gaussian states, it is shown that the one-way entanglement swapping is superior to the two-way entanglement swapping. This means that although the entanglement swapping is performed the same number of times, there is a case that the one-way entanglement swapping can yield an entangled state while the two-way entanglement swapping cannot.
We study the entanglement distillation in continuous-variable systems when a photon replacement protocol is employed. A cascaded protocol is studied and we find that the resultant entanglement increases by increasing the number of repetitions. Interestingly, the entanglement enhancement is not sensitive to the asymmetry of the protocol and gives the same result for any arrangement in the absence of loss. The non-Gaussianity of the outcome state is also studied and it is found that the non-Gaussianity of the state dramatically depends on the experimental arrangements. We also consider the case of imperfect detectors and show that the main results are valid provided high-efficiency detectors are employed. By providing practical information on photon replacement operation, this work is one step towards the realization of universal quantum computation; in particular, in setups where de-Gaussifying protocols are only applicable to one of the parties.
The unconditional entanglement swapping for continuous variables is experimentally demonstrated. Two initial entangled states are produced from two nondegenerate optical parametric amplifiers operating at de-amplification. Through implementing the direct measurement of the Bell-state between two optical beams from each amplifier the remaining two optical beams, which have never directly interacted with each other, are entangled. The quantum correlation degrees of 1.23 and 1.12 dB below the shot noise limit for the amplitude and phase quadratures resulting from the entanglement swapping are measured straightly.