7,489 research outputs found

    Fidelity threshold for long-range entanglement in quantum networks

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    We present a strategy to generate long-range entanglement in noisy quantum networks. We consider a cubic lattice whose bonds are partially entangled mixed states of two qubits, and where quantum operations can be applied perfectly at the nodes. In contrast to protocols designed for one- or two-dimensional regular lattices, we find that entanglement can be created between arbitrarily distant qubits if the fidelity of the bonds is higher than a critical value, independent of the system size. Therefore, we show that a constant overhead of local resources, together with connections of finite fidelity, is sufficient to achieve long-distance quantum communication in noisy networks.Comment: published versio

    Cavitation in liquid cryogens

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    Cavitation in liquid hydrogen and nitrogen flowing in transparent plastic ventur

    Incipient and developed cavitation in liquid cryogens

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    Cavitational flow of liquid nitrogen and liquid hydrogen in Venturi tub

    Hypergraphic LP Relaxations for Steiner Trees

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    We investigate hypergraphic LP relaxations for the Steiner tree problem, primarily the partition LP relaxation introduced by Koenemann et al. [Math. Programming, 2009]. Specifically, we are interested in proving upper bounds on the integrality gap of this LP, and studying its relation to other linear relaxations. Our results are the following. Structural results: We extend the technique of uncrossing, usually applied to families of sets, to families of partitions. As a consequence we show that any basic feasible solution to the partition LP formulation has sparse support. Although the number of variables could be exponential, the number of positive variables is at most the number of terminals. Relations with other relaxations: We show the equivalence of the partition LP relaxation with other known hypergraphic relaxations. We also show that these hypergraphic relaxations are equivalent to the well studied bidirected cut relaxation, if the instance is quasibipartite. Integrality gap upper bounds: We show an upper bound of sqrt(3) ~ 1.729 on the integrality gap of these hypergraph relaxations in general graphs. In the special case of uniformly quasibipartite instances, we show an improved upper bound of 73/60 ~ 1.216. By our equivalence theorem, the latter result implies an improved upper bound for the bidirected cut relaxation as well.Comment: Revised full version; a shorter version will appear at IPCO 2010

    1/N_c Corrections in Meson-Baryon Scattering

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    Corrections to meson/ground-state baryon scattering amplitudes in the 1/N_c expansion of QCD have previously been shown to be controlled by the t-channel difference |I_t - J_t| of isospin and angular momentum and by the change of hypercharge Y_t. Here we derive the corresponding expressions in the original scattering s channel, allowing for nonzero meson spin and nontrivial SU(3) flavor quantum numbers, and provide explicit examples of the crossing relevant for pi N --> rho N and K N scattering.Comment: Addition of a physical example, minor changes to clarify certain issues and add one reference. 18 pages, no figure

    Response of river-dominated delta channel networks to permanent changes in river discharge

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    Using numerical experiments, we investigate how river-dominated delta channel networks are likely to respond to changes in river discharge predicted to occur over the next century as a result of environmental change. Our results show for a change in discharge up to 60% of the initial value, a decrease results in distributary abandonment in the delta, whereas an increase does not significantly affect the network. However, an increase in discharge beyond a threshold of 60% results in channel creation and an increase in the density of the distributary network. This behavior is predicted by an analysis of an individual bifurcation subject to asymmetric water surface slopes in the bifurcate arms. Given that discharge in most river basins will change by less than 50% in the next century, our results suggest that deltas in areas of increased drought will be more likely to experience significant rearrangement of the delta channel network. Copyright 2010 by the American Geophysical Union

    Status of woodland caribou in Alberta

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    A recent review of woodland caribou {Rangifer tarandus caribou) status in Alberta estimated that there are between 3600 and 6700 caribou occupying 113 000 km2 of habitat. There are two ecotypes of caribou in Alberta; the mountain ecotype in the west central region and the boreal ecotype primarily in the north. Mountain caribou populations are stable or declining and boreal populations, where data are available, appear to be stable or declining slowly. A major initiative in caribou management in Alberta has been the development of the Woodland Caribou Conservation Strategy. This document was developed over two and a half years by a committee of multi-stakeholder representatives. The past five years has seen an increase in baseline inventory and applied research jointly funded by government, industry and universities, addressing a wide range of management issues from caribou response to logging to interactions of moose, wolves and caribou in the boreal ecosystem. Land use conflicts on caribou range remain high with timber harvesting, oil and gas development, peat moss extraction, coal mining, agricultural expansion and increasing road access overlapping. Cumulative effects of these disturbances are poorly understood and have received little attention to date
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