15,788 research outputs found
Localized gap soliton trains of Bose-Einstein condensates in an optical lattice
We develop a systematic analytical approach to study the linear and nonlinear
solitary excitations of quasi-one-dimensional Bose-Einstein condensates trapped
in an optical lattice. For the linear case, the Bloch wave in the energy
band is a linear superposition of Mathieu's functions and ;
and the Bloch wave in the band gap is a linear superposition of
and . For the nonlinear case, only solitons inside the band gaps are
likely to be generated and there are two types of solitons -- fundamental
solitons (which is a localized and stable state) and sub-fundamental solitons
(which is a lacalized but unstable state). In addition, we find that the
pinning position and the amplitude of the fundamental soliton in the lattice
can be controlled by adjusting both the lattice depth and spacing. Our
numerical results on fundamental solitons are in quantitative agreement with
those of the experimental observation [Phys. Rev. Lett. {\bf92}, 230401
(2004)]. Furthermore, we predict that a localized gap soliton train consisting
of several fundamental solitons can be realized by increasing the length of the
condensate in currently experimental conditions.Comment: 9 pages, 6 figures, accepted for publicaiton in PR
Reconstructed Intentions in Collaborative Problem Solving Dialogues
We provide evidence that speech act recognition, is 1) difficult for humans to do and 2) likely to misidentify proposals involving reconstructed intentions. We examine the reliability of coding for speech acts in collaborative dialogues and we present an approach for recognizing reconstructed proposals using domain context and other more easily recognized features. 1 Introduction Speech act recognition plays a prominent role in dialogue understanding, in traditional approaches that infer a plan using plan construction operators [PA80], [LA90], [LC91, LC92], and in more recent techniques relying on statistical correlations or finite state machines [RM95, QDL + 97]. Both approaches recognize surface speech acts, using surface form and information provided by the discourse context and the discourse operators, or by a finite state approximation of the planning information. These approaches assume that it is (relatively) simple to recognize speech acts, and that speech acts are a requi..
The world-sheet description of A and B branes revisited
We give a manifest supersymmetric description of A and B branes on Kahler
manifolds using a completely local N=2 superspace formulation of the
world-sheet nonlinear sigma-model in the presence of a boundary. In particular,
we show that an N=2 superspace description of type A boundaries is possible, at
least when the background is Kahler. This leads to an elegant and concrete
setting for studying coisotropic A branes. Here, apgesan important role is
played by the boundary potential, whose precise physical meaning remains to be
fully understood. Duality transformations relating A and B branes in the
presence of isometries are studied as well.Comment: LaTeX, 32 page
Fast quantum algorithm for numerical gradient estimation
Given a blackbox for f, a smooth real scalar function of d real variables,
one wants to estimate the gradient of f at a given point with n bits of
precision. On a classical computer this requires a minimum of d+1 blackbox
queries, whereas on a quantum computer it requires only one query regardless of
d. The number of bits of precision to which f must be evaluated matches the
classical requirement in the limit of large n.Comment: additional references and minor clarifications and corrections to
version
(Never) Mind your p's and q's: Von Neumann versus Jordan on the Foundations of Quantum Theory
In two papers entitled "On a new foundation [Neue Begr\"undung] of quantum
mechanics," Pascual Jordan (1927b,g) presented his version of what came to be
known as the Dirac-Jordan statistical transformation theory. As an alternative
that avoids the mathematical difficulties facing the approach of Jordan and
Paul A. M. Dirac (1927), John von Neumann (1927a) developed the modern Hilbert
space formalism of quantum mechanics. In this paper, we focus on Jordan and von
Neumann. Central to the formalisms of both are expressions for conditional
probabilities of finding some value for one quantity given the value of
another. Beyond that Jordan and von Neumann had very different views about the
appropriate formulation of problems in quantum mechanics. For Jordan, unable to
let go of the analogy to classical mechanics, the solution of such problems
required the identication of sets of canonically conjugate variables, i.e., p's
and q's. For von Neumann, not constrained by the analogy to classical
mechanics, it required only the identication of a maximal set of commuting
operators with simultaneous eigenstates. He had no need for p's and q's. Jordan
and von Neumann also stated the characteristic new rules for probabilities in
quantum mechanics somewhat differently. Jordan (1927b) was the first to state
those rules in full generality. Von Neumann (1927a) rephrased them and, in a
subsequent paper (von Neumann, 1927b), sought to derive them from more basic
considerations. In this paper we reconstruct the central arguments of these
1927 papers by Jordan and von Neumann and of a paper on Jordan's approach by
Hilbert, von Neumann, and Nordheim (1928). We highlight those elements in these
papers that bring out the gradual loosening of the ties between the new quantum
formalism and classical mechanics.Comment: New version. The main difference with the old version is that the
introduction has been rewritten. Sec. 1 (pp. 2-12) in the old version has
been replaced by Secs. 1.1-1.4 (pp. 2-31) in the new version. The paper has
been accepted for publication in European Physical Journal
Symmetric three-particle motion in Stokes flow: equilibrium for heavy spheres in contrast to "end-of-world" for point forces
A stationary stable solution of the Stokes equations for three identical
heavy solid spheres falling in a vertical plane is found. It has no analog in
the point-particle approximation. Three spheres aligned horizontally at equal
distances evolve towards the equilibrium relative configuration while the point
particles collapse onto a single point in a finite time.Comment: 4 pages, 7 figure
Isolation and characterisation of microsatellite loci from Galapagos lava lizards (Microlophus spp.)
Homogenization induced by chaotic mixing and diffusion in an oscillatory chemical reaction
A model for an imperfectly mixed batch reactor with the chlorine dioxide-iodine-malonic acid (CDIMA) reaction, with the mixing being modelled by chaotic advection, is considered. The reactor is assumed to be operating in oscillatory mode and the way in which an initial spatial perturbation becomes homogenized is examined. When the kinetics are such that the only stable homogeneous state is oscillatory then the perturbation is always entrained into these oscillations. The rate at which this occurs is relatively insensitive to the chemical effects, measured by the Damkohler number, and is comparable to the rate of homogenization of a passive contaminant. When both steady and oscillatory states are stable, spatially homogeneous states, two possibilities can occur. For the smaller Damkohler numbers, a localized perturbation at the steady state is homogenized within the background oscillations. For larger Damkohler numbers, regions of both oscillatory and steady behavior can co-exist for relatively long times before the system collapses to having the steady state everywhere. An interpretation of this behavior is provided by the one-dimensional Lagrangian filament model, which is analyzed in detail
Coulomb Drag between One-Dimensional Wigner Crystal Rings
We consider the Coulomb drag between two metal rings in which the long range
Coulomb interaction leads to the formation of a Wigner crystal. The first ring
is threaded by an Ahranov Bohm flux creating a persistent current J_0. The
second ring is brought in close proximity to the second and due to the Coulomb
interaction between the two rings a drag current J_D is produced in the second.
We investigate this system at zero temperature for perfect rings as well as the
effects of impurities. We show that the Wigner crystal state can in principle
lead to a higher ratio of drag current to drive current J_D/J_0 than in weakly
interacting electron systems.Comment: 12 pages, 10 figure
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