422 research outputs found

    Transiently Consistent SDN Updates: Being Greedy is Hard

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    The software-defined networking paradigm introduces interesting opportunities to operate networks in a more flexible, optimized, yet formally verifiable manner. Despite the logically centralized control, however, a Software-Defined Network (SDN) is still a distributed system, with inherent delays between the switches and the controller. Especially the problem of changing network configurations in a consistent manner, also known as the consistent network update problem, has received much attention over the last years. In particular, it has been shown that there exists an inherent tradeoff between update consistency and speed. This paper revisits the problem of updating an SDN in a transiently consistent, loop-free manner. First, we rigorously prove that computing a maximum (greedy) loop-free network update is generally NP-hard; this result has implications for the classic maximum acyclic subgraph problem (the dual feedback arc set problem) as well. Second, we show that for special problem instances, fast and good approximation algorithms exist

    Tracing the First Steps of American Sturgeon Pioneers in Europe

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    Background: A Baltic population of Atlantic sturgeon was founded ~1,200 years ago by migrantsfrom North America, but after centuries of persistence, the population was extirpated in the 1960s,mainly as a result of over-harvest and habitat alterations. As there are four genetically distinctgroups of Atlantic sturgeon inhabiting North American rivers today, we investigated the geneticprovenance of the historic Baltic population by ancient DNA analyses using mitochondrial andnuclear markers.Results: The phylogeographic signal obtained from multilocus microsatellite DNA genotypes andmitochondrial DNA control region haplotypes, when compared to existing baseline datasets fromextant populations, allowed for the identification of the region-of-origin of the North AmericanAtlantic sturgeon founders. Moreover, statistical and simulation analyses of the multilocusgenotypes allowed for the calculation of the effective number of individuals that originally foundedthe European population of Atlantic sturgeon. Our findings suggest that the Baltic population of A.oxyrinchus descended from a relatively small number of founders originating from the northernextent of the species\u27 range in North America.Conclusion: These results demonstrate that the most northerly distributed North American A.oxyrinchus colonized the Baltic Sea ~1,200 years ago, suggesting that Canadian specimens should bethe primary source of broodstock used for restoration in Baltic rivers. This study illustrates thegreat potential of patterns obtained from ancient DNA to identify population-of-origin toinvestigate historic genotype structure of extinct populations

    Genetic Assessment of Remnant Sub-Populations of Sterlet (Acipenser ruthenus Linnaeus, 1758) in the Upper Danube

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    While the potamodromous sterlet was common in the past throughout the Upper Danube in Germany and Austria, it nearly vanished in the second half of the 20th century. Until recently, only one small and isolated reproductive sub-population is known from the German–Austrian border. However, isolated remnants in another section downstream of Vienna, near the Austrian–Slovakian border, were discovered in 2014. An assessment of the population size is one of the most important prerequisites for conservation management. This study aims to assess the population sizes at both sites, using genetic pedigrees and comparison to mark–recapture data. A total of 193 samples collected from these populations between 2011 and 2021 have been investigated. In addition, 59 samples from captive stocks, 38 wild fish from downstream, and 247 genetic profiles from previous studies were used for comparison. Results show close relationships and intermittent reproduction on one site. Estimated populations based upon genetic pedigree are very small, and are consistent with mark–recapture results. Small population sizes of remnant populations have only limited, sporadic reproduction, as well as continual losses to outmigration support conservation actions for sturgeons in the Upper Danube, including the restoration of functional migration corridors.LIFE-Sterlet and LIFE-Boat 4 Sturgeons projectLIFE-Program of the European UnionDer Sterlet im Oberen DonautalPeer Reviewe

    Asymmetry of charge relaxation times in quantum dots: The influence of degeneracy

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    Using time-resolved transconductance spectroscopy, we study the tunneling dynamics between a two-dimensional electron gas (2DEG) and self-assembled quantum dots (QDs), embedded in a field-effect transistor structure. We find that the tunneling of electrons from the 2DEG into the QDs is governed by a different time constant than the reverse process, i.e., tunneling from the QDs to the 2DEG. This asymmetry is a clear signature of Coulomb interaction and makes it possible to determine the degeneracy of the quantum dot orbitals even when the individual states cannot be resolved energetically because of inhomogeneous broadening. Our experimental data can be qualitatively explained within a master-equation approach

    Optical Visualization of Radiative Recombination at Partial Dislocations in GaAs

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    Individual dislocations in an ultra-pure GaAs epilayer are investigated with spatially and spectrally resolved photoluminescence imaging at 5~K. We find that some dislocations act as strong non-radiative recombination centers, while others are efficient radiative recombination centers. We characterize luminescence bands in GaAs due to dislocations, stacking faults, and pairs of stacking faults. These results indicate that low-temperature, spatially-resolved photoluminescence imaging can be a powerful tool for identifying luminescence bands of extended defects. This mapping could then be used to identify extended defects in other GaAs samples solely based on low-temperature photoluminescence spectra.Comment: 4 pages, 4 figure

    A linear triple quantum dot system in isolated configuration

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    The scaling up of electron spin qubit based nanocircuits has remained challenging up to date and involves the development of efficient charge control strategies. Here we report on the experimental realization of a linear triple quantum dot in a regime isolated from the reservoir. We show how this regime can be reached with a fixed number of electrons. Charge stability diagrams of the one, two and three electron configurations where only electron exchange between the dots is allowed are observed. They are modelled with established theory based on a capacitive model of the dot systems. The advantages of the isolated regime with respect to experimental realizations of quantum simulators and qubits are discussed. We envision that the results presented here will make more manipulation schemes for existing qubit implementations possible and will ultimately allow to increase the number of tunnel coupled quantum dots which can be simultaneously controlled
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