25,948 research outputs found
Aspects of Two-Level Systems under External Time Dependent Fields
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
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 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
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
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
Review of Ann Eriksson\u27s High Clear Bell of Morning
Quantum spin liquid at finite temperature: proximate dynamics and persistent typicality
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 -RuCl, show magnetic order at low
temperatures , here we demonstrate numerically a dynamical crossover from
magnon-like behavior at low and frequencies to long-lived
fractionalized fermionic quasiparticles at higher and . 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 . 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- properties of putative spin liquids.Comment: 13 pages, 11 figures, accepted versio
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