25,948 research outputs found

    Aspects of Two-Level Systems under External Time Dependent Fields

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    The dynamics of two-level systems in time-dependent backgrounds is under consideration. We present some new exact solutions in special backgrounds decaying in time. On the other hand, following ideas of Feynman, Vernon and Hellwarth, we discuss in detail the possibility to reduce the quantum dynamics to a classical Hamiltonian system. This, in particular, opens the possibility to directly apply powerful methods of classical mechanics (e.g. KAM methods) to study the quantum system. Following such an approach, we draw conclusions of relevance for ``quantum chaos'' when the external background is periodic or quasi-periodic in time.Comment: To appear in J. Phys. A. Mathematical and Genera

    The non-integrability of the Zipoy-Voorhees metric

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    The low frequency gravitational wave detectors like eLISA/NGO will give us the opportunity to test whether the supermassive compact objects lying at the centers of galaxies are indeed Kerr black holes. A way to do such a test is to compare the gravitational wave signals with templates of perturbed black hole spacetimes, the so-called bumpy black hole spacetimes. The Zipoy-Voorhees (ZV) spacetime (known also as the Îł\gamma spacetime) can be included in the bumpy black hole family, because it can be considered as a perturbation of the Schwarzschild spacetime background. Several authors have suggested that the ZV metric corresponds to an integrable system. Contrary to this integrability conjecture, in the present article it is shown by numerical examples that in general ZV belongs to the family of non-integrable systems.Comment: 10 pages, 13 figure

    Palestinians, Israel and the Quartet: Pulling Back from the Brink

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    Throughout years of uprising and Israeli military actions, siege of West Bank cities and President Arafat's de facto house arrest, it was hard to imagine the situation getting worse for Palestinians. It has. On all fronts -- Palestinian/Palestinian, Palestinian/Israeli and Palestinian/ international -- prevailing dynamics are leading to a dangerous breakdown. Subjected to the cumulative effects of a military occupation in its 40th year and now what is effectively an international sanctions regime, the Hamas-led Palestinian Authority (PA) government cannot pay salaries or deliver basic services. Diplomacy is frozen, with scant prospect of thaw -- and none at all of breakthrough. And Hamas's electoral victory and the reactions it provoked among Fatah loyalists have intensified chaos and brought the nation near civil war. There is an urgent need for all relevant players to pragmatically reassess their positions. This report is also available in Arabic at http://www.crisisgroup.org/home/index.cfm?id=4171&l=6

    Iterative Decoding and Turbo Equalization: The Z-Crease Phenomenon

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    Iterative probabilistic inference, popularly dubbed the soft-iterative paradigm, has found great use in a wide range of communication applications, including turbo decoding and turbo equalization. The classic approach of analyzing the iterative approach inevitably use the statistical and information-theoretical tools that bear ensemble-average flavors. This paper consider the per-block error rate performance, and analyzes it using nonlinear dynamical theory. By modeling the iterative processor as a nonlinear dynamical system, we report a universal "Z-crease phenomenon:" the zig-zag or up-and-down fluctuation -- rather than the monotonic decrease -- of the per-block errors, as the number of iteration increases. Using the turbo decoder as an example, we also report several interesting motion phenomenons which were not previously reported, and which appear to correspond well with the notion of "pseudo codewords" and "stopping/trapping sets." We further propose a heuristic stopping criterion to control Z-crease and identify the best iteration. Our stopping criterion is most useful for controlling the worst-case per-block errors, and helps to significantly reduce the average-iteration numbers.Comment: 6 page

    High Clear Bell of Morning by Ann Eriksson

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    Review of Ann Eriksson\u27s High Clear Bell of Morning

    Quantum spin liquid at finite temperature: proximate dynamics and persistent typicality

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    Quantum spin liquids are long-range entangled states of matter with emergent gauge fields and fractionalized excitations. While candidate materials, such as the Kitaev honeycomb ruthenate α\alpha-RuCl3_3, show magnetic order at low temperatures TT, here we demonstrate numerically a dynamical crossover from magnon-like behavior at low TT and frequencies ω\omega to long-lived fractionalized fermionic quasiparticles at higher TT and ω\omega. This crossover is akin to the presence of spinon continua in quasi-1D spin chains. It is further shown to go hand in hand with persistent typicality down to very low TT. This aspect, which has also been observed in the spin-1/2 kagome Heisenberg antiferromagnet, is a signature of proximate spin liquidity and emergent gauge degrees of freedom more generally, and can be the basis for the numerical study of many finite-TT properties of putative spin liquids.Comment: 13 pages, 11 figures, accepted versio
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