18,330 research outputs found

    Modelling of hydrological response to climate change in glacierized Central Asian catchments

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    The arid lowlands of Central Asia are highly dependent on the water supplied by the Tien Shan mountains. Snow and ice storage make large contributions to current runoff, particularly in summer. Two runoff models with different temporal resolutions, HBV-ETH and OEZ, were applied in three glaciated catchments of the Tien Shan mountains. Scenario runs were produced for a climate change caused by the doubling of atmospheric CO2 as predicted by the GISS global circulation model and assuming a 50% reduction of glaciation extent, as well as a complete loss of glaciation. Agreement of the results was best for runs based on 50% glaciation loss, where both models predict an increase in spring and summer runoff compared to current levels. Scenarios for complete loss of glaciation predict an increase in spring runoff levels, followed by lower runoff levels for July and August. Model predictions differ concerning the degree of reduction of late summer runoff. These scenarios are sensitive to model simulation of basin precipitation, as well as to reduction of glaciation extent

    Measuring the Solar Radius from Space during the 2003 and 2006 Mercury Transits

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    The Michelson Doppler Imager (MDI) aboard the Solar and Heliospheric Observatory observed the transits of Mercury on 2003 May 7 and 2006 November 8. Contact times between Mercury and the solar limb have been used since the 17th century to derive the Sun's size but this is the first time that high-quality imagery from space, above the Earth's atmosphere, has been available. Unlike other measurements this technique is largely independent of optical distortion. The true solar radius is still a matter of debate in the literature as measured differences of several tenths of an arcsecond (i.e., about 500 km) are apparent. This is due mainly to systematic errors from different instruments and observers since the claimed uncertainties for a single instrument are typically an order of magnitude smaller. From the MDI transit data we find the solar radius to be 960".12 +/- 0".09 (696,342 +/- 65 km). This value is consistent between the transits and consistent between different MDI focus settings after accounting for systematic effects.Comment: Accepted for publication in The Astrophysical Journal (2012 March 5

    Optimizing photon indistinguishability in the emission from incoherently-excited semiconductor quantum dots

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    Most optical quantum devices require deterministic single-photon emitters. Schemes so far demonstrated in the solid state imply an energy relaxation which tends to spoil the coherent nature of the time evolution, and with it the photon indistinguishability. We focus our theoretical investigation on semiconductor quantum dots embedded in microcavities. Simple and general relations are identified between the photon indistinguishability and the collection efficiency. The identification of the key parameters and of their interplay provides clear indications for the device optimization

    Electroweak 2 -> 2 amplitudes for electron-positron annihilation at TeV energies

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    The non-radiative scattering amplitudes for electron-positron annihilation into quark and lepton pairs in the TeV energy range are calculated in the double-logarithmic approximation. The expressions for the amplitudes are obtained using infrared evolution equations with different cut-offs for virtual photons and for W and Z bosons, and compared with previous results obtained with an universal cut-off.Comment: Revtex4, 17 pages, 7 figures. Some minor changes made, more refs adde

    Phonon-induced decoherence for a quantum dot spin qubit operated by Raman passage

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    We study single-qubit gates performed via stimulated Raman adiabatic passage (STIRAP) on a spin qubit implemented in a quantum dot system in the presence of phonons. We analyze the interplay of various kinds of errors resulting from the carrier-phonon interaction as well as from quantum jumps related to nonadiabaticity and calculate the fidelity as a function of the pulse parameters. We give quantitative estimates for an InAs/GaAs system and identify the parameter values for which the error is considerably minimized, even to values below 10410^{-4} per operation.Comment: Final version; considerable extensions; 18 pages, 7 figure

    Stochastic Analysis of a Churn-Tolerant Structured Peer-to-Peer Scheme

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    We present and analyze a simple and general scheme to build a churn (fault)-tolerant structured Peer-to-Peer (P2P) network. Our scheme shows how to "convert" a static network into a dynamic distributed hash table(DHT)-based P2P network such that all the good properties of the static network are guaranteed with high probability (w.h.p). Applying our scheme to a cube-connected cycles network, for example, yields a O(logN)O(\log N) degree connected network, in which every search succeeds in O(logN)O(\log N) hops w.h.p., using O(logN)O(\log N) messages, where NN is the expected stable network size. Our scheme has an constant storage overhead (the number of nodes responsible for servicing a data item) and an O(logN)O(\log N) overhead (messages and time) per insertion and essentially no overhead for deletions. All these bounds are essentially optimal. While DHT schemes with similar guarantees are already known in the literature, this work is new in the following aspects: (1) It presents a rigorous mathematical analysis of the scheme under a general stochastic model of churn and shows the above guarantees; (2) The theoretical analysis is complemented by a simulation-based analysis that validates the asymptotic bounds even in moderately sized networks and also studies performance under changing stable network size; (3) The presented scheme seems especially suitable for maintaining dynamic structures under churn efficiently. In particular, we show that a spanning tree of low diameter can be efficiently maintained in constant time and logarithmic number of messages per insertion or deletion w.h.p. Keywords: P2P Network, DHT Scheme, Churn, Dynamic Spanning Tree, Stochastic Analysis

    Sch 29482, laboratory evaluation of a new penem antibiotic

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    The antibacterial activity of a new penem antibiotic, Sch 29482 (SCH), was examined in comparison with appropriate cephalosporins and penicillins. The drug inhibited penicillinase-positive and negative staphylococci equally well, being 2-5 times more active than cephalothin or cefamandole and 10-20 times more active than methicillin. Staphylococci resistant to methicillin were susceptible to SCH in agar dilution tests. Staphylococci tolerant to methicillin were also tolerant to SCH. Streptococci and pneumococci were highly susceptible to the drug. The agent was of only moderate activity against enterococci, especially Streptococcus faecium strains. MICs of ampicillin and penicillin G against enterococci were 4-8 times lower than those of SCH. SCH was bactericidal. Neither the choice of the method used for susceptibility testing, nor the size of the inoculum nor various test media influenced the in-vitro activity of this drug against a representative collection of Gram-negative and Gram-positive bacteri
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