Fidelity threshold for long-range entanglement in quantum networks

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

Hypergraphic LP Relaxations for Steiner Trees

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

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

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

The China's Rise as an International Trading Power

This paper undertakes a detailed review of the policies that have shaped China's explosion of a global supply of exports, and examines long trend statistics on the evolution of China's trading partners and the goods it trades in the post-reform period. This review notes common characteristics in China's trade experience with those of earlier successful export-based economies of East Asia, such as South Korea and Japan. The survey finds that China's pattern of trade and trading partners are similar to those of more market-based Asian economies, but that the Chinese economy's orientation toward foreign trade is considerably greater than expected for an economy of its size and level of development. The authors argue that China still has a long way to go in terms of its export boom, especially if compared to the experiences of South Korea, Japan, and Taiwan. This suggests that China is on track to become one of the world's most formidable trading powers and its export policies and export performance will exert increasing influence on how the global trade regime evolves in the future.

Thermodynamic depressions within cavities and cavitation inception in liquid hydrogen and liquid nitrogen Final report, 15 Jul. 1964 - 15 Dec. 1967

Thermodynamic depressions within cavities and cavitation inception in liquid hydrogen and nitrogen in transparent plastic venturi tube

Node-balancing by edge-increments

Suppose you are given a graph $G=(V,E)$ with a weight assignment $w:V\rightarrow\mathbb{Z}$ and that your objective is to modify $w$ using legal steps such that all vertices will have the same weight, where in each legal step you are allowed to choose an edge and increment the weights of its end points by $1$. In this paper we study several variants of this problem for graphs and hypergraphs. On the combinatorial side we show connections with fundamental results from matching theory such as Hall's Theorem and Tutte's Theorem. On the algorithmic side we study the computational complexity of associated decision problems. Our main results are a characterization of the graphs for which any initial assignment can be balanced by edge-increments and a strongly polynomial-time algorithm that computes a balancing sequence of increments if one exists.Comment: 10 page

Cavitation in liquid cryogens

Cavitation in liquid hydrogen and nitrogen flowing in transparent plastic ventur

Incipient and developed cavitation in liquid cryogens

Cavitational flow of liquid nitrogen and liquid hydrogen in Venturi tub