6,218 research outputs found
Complexity without chaos: Plasticity within random recurrent networks generates robust timing and motor control
It is widely accepted that the complex dynamics characteristic of recurrent
neural circuits contributes in a fundamental manner to brain function. Progress
has been slow in understanding and exploiting the computational power of
recurrent dynamics for two main reasons: nonlinear recurrent networks often
exhibit chaotic behavior and most known learning rules do not work in robust
fashion in recurrent networks. Here we address both these problems by
demonstrating how random recurrent networks (RRN) that initially exhibit
chaotic dynamics can be tuned through a supervised learning rule to generate
locally stable neural patterns of activity that are both complex and robust to
noise. The outcome is a novel neural network regime that exhibits both
transiently stable and chaotic trajectories. We further show that the recurrent
learning rule dramatically increases the ability of RRNs to generate complex
spatiotemporal motor patterns, and accounts for recent experimental data
showing a decrease in neural variability in response to stimulus onset
Supersymmetric Boundaries and Junctions in Four Dimensions
We make a comprehensive study of (rigid) N=1 supersymmetric sigma-models with
general K\"ahler potentials K and superpotentials w on four-dimensional
space-times with boundaries. We determine the minimal (non-supersymmetric)
boundary terms one must add to the standard bulk action to make it off-shell
invariant under half the supersymmetries without imposing any boundary
conditions. Susy boundary conditions do arise from the variational principle
when studying the dynamics. Upon including an additional boundary action that
depends on an arbitrary real boundary potential B one can generate very general
susy boundary conditions. We show that for any set of susy boundary conditions
that define a Lagrangian submanifold of the K\"ahler manifold, an appropriate
boundary potential B can be found. Thus the non-linear sigma-model on a
manifold with boundary is characterised by the tripel (K,B,w). We also discuss
the susy coupling to new boundary superfields and generalize our results to
supersymmetric junctions between completely different susy sigma-models, living
on adjacent domains and interacting through a "permeable" wall. We obtain the
supersymmetric matching conditions that allow us to couple models with
different K\"ahler potentials and superpotentials on each side of the wall.Comment: 38 pages, 1 figur
Autoantibodies against oxidized low-density lipoprotein and lipid profile in patients with chronic periaortitis: case–control study
Polarization due to rotational distortion in the bright star Regulus
This is the full published article (retrieved from the 6 months post-publication posting on arXiv) including the Methods and Supplementary Information sections: 33 pages, 10 figures, 8 tablesPolarization in stars was first predicted by Chandrasekhar [1] who calculated a substantial linear polarization at the stellar limb for a pure electron-scattering atmosphere. This polarization will average to zero when integrated over a spherical star but could be detected if the symmetry is broken, for example by the eclipse of a binary companion. Nearly 50 years ago, Harrington and Collins [2] modeled another way of breaking the symmetry and producing net polarization - the distortion of a rapidly rotating hot star. Here we report the first detection of this effect. Observations of the linear polarization of Regulus, with two different high-precision polarimeters, range from +42 parts-per-million (ppm) at a wavelength of 741 nm to -22 ppm at 395 nm. The reversal from red to blue is a distinctive feature of rotation-induced polarization. Using a new set of models for the polarization of rapidly rotating stars we find that Regulus is rotating at 96.5(+0.6/-0.8)% of its critical angular velocity for breakup, and has an inclination greater than 76.5 degrees. The rotation axis of the star is at a position angle of 79.5+/-0.7 degrees. The conclusions are independent of, but in good agreement with, the results of previously published interferometric observations of Regulus [3]. The accurate measurement of rotation in early-type stars is important for understanding their stellar environments [4], and course of their evolution [5].Peer reviewedFinal Accepted Versio
Searching for plasticity in dissociated cortical cultures on multi-electrode arrays
We attempted to induce functional plasticity in dense cultures of cortical cells using stimulation through extracellular electrodes embedded in the culture dish substrate (multi-electrode arrays, or MEAs). We looked for plasticity expressed in changes in spontaneous burst patterns, and in array-wide response patterns to electrical stimuli, following several induction protocols related to those used in the literature, as well as some novel ones. Experiments were performed with spontaneous culture-wide bursting suppressed by either distributed electrical stimulation or by elevated extracellular magnesium concentrations as well as with spontaneous bursting untreated. Changes concomitant with induction were no larger in magnitude than changes that occurred spontaneously, except in one novel protocol in which spontaneous bursts were quieted using distributed electrical stimulation
Efimov effect in quantum magnets
Physics is said to be universal when it emerges regardless of the underlying
microscopic details. A prominent example is the Efimov effect, which predicts
the emergence of an infinite tower of three-body bound states obeying discrete
scale invariance when the particles interact resonantly. Because of its
universality and peculiarity, the Efimov effect has been the subject of
extensive research in chemical, atomic, nuclear and particle physics for
decades. Here we employ an anisotropic Heisenberg model to show that collective
excitations in quantum magnets (magnons) also exhibit the Efimov effect. We
locate anisotropy-induced two-magnon resonances, compute binding energies of
three magnons and find that they fit into the universal scaling law. We propose
several approaches to experimentally realize the Efimov effect in quantum
magnets, where the emergent Efimov states of magnons can be observed with
commonly used spectroscopic measurements. Our study thus opens up new avenues
for universal few-body physics in condensed matter systems.Comment: 7 pages, 5 figures; published versio
Topologically protected quantum bits from Josephson junction arrays
All physical implementations of quantum bits (qubits), carrying the
information and computation in a putative quantum computer, have to meet the
conflicting requirements of environmental decoupling while remaining
manipulable through designed external signals. Proposals based on quantum
optics naturally emphasize the aspect of optimal isolation, while those
following the solid state route exploit the variability and scalability of
modern nanoscale fabrication techniques. Recently, various designs using
superconducting structures have been successfully tested for quantum coherent
operation, however, the ultimate goal of reaching coherent evolution over
thousands of elementary operations remains a formidable task. Protecting qubits
from decoherence by exploiting topological stability, a qualitatively new
proposal due to Kitaev, holds the promise for long decoherence times, but its
practical physical implementation has remained unclear so far. Here, we show
how strongly correlated systems developing an isolated two-fold degenerate
quantum dimer liquid groundstate can be used in the construction of
topologically stable qubits and discuss their implementation using Josephson
junction arrays.Comment: 6 pages, 4 figure
S-duality and 2d Topological QFT
We study the superconformal index for the class of N=2 4d superconformal
field theories recently introduced by Gaiotto. These theories are defined by
compactifying the (2,0) 6d theory on a Riemann surface with punctures. We
interpret the index of the 4d theory associated to an n-punctured Riemann
surface as the n-point correlation function of a 2d topological QFT living on
the surface. Invariance of the index under generalized S-duality
transformations (the mapping class group of the Riemann surface) translates
into associativity of the operator algebra of the 2d TQFT. In the A_1 case, for
which the 4d SCFTs have a Lagrangian realization, the structure constants and
metric of the 2d TQFT can be calculated explicitly in terms of elliptic gamma
functions. Associativity then holds thanks to a remarkable symmetry of an
elliptic hypergeometric beta integral, proved very recently by van de Bult.Comment: 25 pages, 11 figure
Metabolic analysis of the interaction between plants and herbivores
Insect herbivores by necessity have to deal with a large arsenal of plant defence metabolites. The levels of defence compounds may be increased by insect damage. These induced plant responses may also affect the metabolism and performance of successive insect herbivores. As the chemical nature of induced responses is largely unknown, global metabolomic analyses are a valuable tool to gain more insight into the metabolites possibly involved in such interactions. This study analyzed the interaction between feral cabbage (Brassica oleracea) and small cabbage white caterpillars (Pieris rapae) and how previous attacks to the plant affect the caterpillar metabolism. Because plants may be induced by shoot and root herbivory, we compared shoot and root induction by treating the plants on either plant part with jasmonic acid. Extracts of the plants and the caterpillars were chemically analysed using Ultra Performance Liquid Chromatography/Time of Flight Mass Spectrometry (UPLCT/MS). The study revealed that the levels of three structurally related coumaroylquinic acids were elevated in plants treated on the shoot. The levels of these compounds in plants and caterpillars were highly correlated: these compounds were defined as the ‘metabolic interface’. The role of these metabolites could only be discovered using simultaneous analysis of the plant and caterpillar metabolomes. We conclude that a metabolomics approach is useful in discovering unexpected bioactive compounds involved in ecological interactions between plants and their herbivores and higher trophic levels.
A valley-spin qubit in a carbon nanotube
Although electron spins in III-V semiconductor quantum dots have shown great
promise as qubits, a major challenge is the unavoidable hyperfine decoherence
in these materials. In group IV semiconductors, the dominant nuclear species
are spinless, allowing for qubit coherence times that have been extended up to
seconds in diamond and silicon. Carbon nanotubes are a particularly attractive
host material, because the spin-orbit interaction with the valley degree of
freedom allows for electrical manipulation of the qubit. In this work, we
realise such a qubit in a nanotube double quantum dot. The qubit is encoded in
two valley-spin states, with coherent manipulation via electrically driven spin
resonance (EDSR) mediated by a bend in the nanotube. Readout is performed by
measuring the current in Pauli blockade. Arbitrary qubit rotations are
demonstrated, and the coherence time is measured via Hahn echo. Although the
measured decoherence time is only 65 ns in our current device, this work offers
the possibility of creating a qubit for which hyperfine interaction can be
virtually eliminated
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