9,602 research outputs found
Mach Bands: How Many Models are Possible? Recent Experiemental Findings and Modeling Attempts
Mach bands are illusory bright and dark bands seen where a luminance plateau meets a ramp, as in half-shadows or penumbras. A tremendous amount of work has been devoted to studying the psychophysics and the potential underlying neural circuitry concerning this phenomenon. A number of theoretical models have also been proposed, originating in the seminal studies of Mach himself. The present article reviews the main experimental findings after 1965 and the main recent theories of early vision that have attempted to discount for the effect. It is shown that the different theories share working principles and can be grouped in three clsses: a) feature-based; b) rule-based; and c) filling-in. In order to evaluate individual proposals it is necessary to consider them in the larger picture of visual science and to determine how they contribute to the understanding of vision in general.Air Force Office of Scientific Research (F49620-92-J-0334); Office of Naval Research (N00014-J-4100); COPPE/UFRJ, Brazi
Kitaev spin models from topological nanowire networks
We show that networks of topological nanowires can realize the physics of
exactly solvable Kitaev spin models with two-body interactions. This connection
arises from the description of the low-energy theory of both systems in terms
of a tight-binding model of Majorana modes. In Kitaev spin models the Majorana
description provides a convenient representation to solve the model, whereas in
an array of topological nanowires it arises, because the physical Majorana
modes localized at wire ends permit tunnelling between wire ends and across
different Josephson junctions. We explicitly show that an array of junctions of
three wires -- a setup relevant to topological quantum computing with nanowires
-- can realize the Yao-Kivelson model, a variant of Kitaev spin models on a
decorated honeycomb lattice. Translating the results from the latter, we show
that the network can be constructed to give rise to collective states
characterized by Chern numbers \nu = 0, +/-1 and +/-2, and that defects in an
array can be associated with vortex-like quasi-particle excitations. Finally,
we analyze the stability of the collective states as well as that of the
network as a quantum information processor. We show that decoherence inducing
instabilities, be them due to disorder or phase fluctuations, can be understood
in terms of proliferation of the vortex-like quasi-particles.Comment: 15 pages, 9 figure
An anthology of non-local QFT and QFT on noncommutative spacetime
Ever since the appearance of renormalization theory there have been several
differently motivated attempts at non-localized (in the sense of not generated
by point-like fields) relativistic particle theories, the most recent one being
at QFT on non-commutative Minkowski spacetime. The often conceptually
uncritical and historically forgetful contemporary approach to these problems
calls for a critical review the light of previous results on this subject.Comment: 33 pages tci-latex, improvements of formulations, shortening of
sentences, addition of some reference
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