225 research outputs found
Hexagons and Correlators in the Fishnet Theory
We investigate the hexagon formalism in the planar 4d conformal fishnet
theory. This theory arises from N=4 SYM by a deformation that preserves both
conformal symmetry and integrability. Based on this relation, we obtain the
hexagon form factors for a large class of states, including the BMN vacuum,
some excited states, and the Lagrangian density. We apply these form factors to
the computation of several correlators and match the results with direct
Feynman diagrammatic calculations. We also study the renormalisation of the
hexagon form factor expansion for a family of diagonal structure constants and
test the procedure at higher orders through comparison with a known universal
formula for the Lagrangian insertion.Comment: 63 page
Performance and limitations of the QAOA at constant levels on large sparse hypergraphs and spin glass models
The Quantum Approximate Optimization Algorithm (QAOA) is a general purpose
quantum algorithm designed for combinatorial optimization. We analyze its
expected performance and prove concentration properties at any constant level
(number of layers) on ensembles of random combinatorial optimization problems
in the infinite size limit. These ensembles include mixed spin models and
Max--XORSAT on sparse random hypergraphs. To enable our analysis, we prove a
generalization of the multinomial theorem which is a technical result of
independent interest. We then show that the performance of the QAOA at constant
levels for the pure -spin model matches asymptotically the ones for
Max--XORSAT on random sparse Erd\H{o}s-R\'{e}nyi hypergraphs and every
large-girth regular hypergraph. Through this correspondence, we establish that
the average-case value produced by the QAOA at constant levels is bounded away
from optimality for pure -spin models when is even. This limitation
gives a hardness of approximation result for quantum algorithms in a new regime
where the whole graph is seen.Comment: 12+46 page
OPE for all Helicity Amplitudes
We extend the Operator Product Expansion (OPE) for scattering amplitudes in
planar N=4 SYM to account for all possible helicities of the external states.
This is done by constructing a simple map between helicity configurations and
so-called charged pentagon transitions. These OPE building blocks are
generalizations of the bosonic pentagons entering MHV amplitudes and they can
be bootstrapped at finite coupling from the integrable dynamics of the color
flux tube. A byproduct of our map is a simple realization of parity in the
super Wilson loop picture.Comment: 30 page
Anomalous diffusion for neuronal growth on surfaces with controlled geometries
Geometrical cues are known to play a very important role in neuronal growth
and the formation of neuronal networks. Here, we present a detailed analysis of
axonal growth and dynamics for neuronal cells cultured on patterned
polydimethylsiloxane surfaces. We use fluorescence microscopy to image neurons,
quantify their dynamics, and demonstrate that the substrate geometrical
patterns cause strong directional alignment of axons. We quantify axonal growth
and report a general stochastic approach that quantitatively describes the
motion of growth cones. The growth cone dynamics is described by Langevin and
Fokker-Planck equations with both deterministic and stochastic contributions.
We show that the deterministic terms contain both the angular and speed
dependence of axonal growth, and that these two contributions can be separated.
Growth alignment is determined by surface geometry, and it is quantified by the
deterministic part of the Langevin equation. We combine experimental data with
theoretical analysis to measure the key parameters of the growth cone motion:
speed and angular distributions, correlation functions, diffusion coefficients,
characteristics speeds and damping coefficients. We demonstrate that axonal
dynamics displays a cross-over from Brownian motion (Ornstein-Uhlenbeck
process) at earlier times to anomalous dynamics (superdiffusion) at later
times. The superdiffusive regime is characterized by non-Gaussian speed
distributions and power law dependence of the axonal mean square length and the
velocity correlation functions. These results demonstrate the importance of
geometrical cues in guiding axonal growth, and could lead to new methods for
bioengineering novel substrates for controlling neuronal growth and
regeneration.Comment: 27 pages, 13 figures. arXiv admin note: substantial text overlap with
arXiv:1903.0133
Audiovisual integration increases the intentional step synchronization of side-by-side walkers
When people walk side-by-side, they often synchronize their steps. To achieve this, individuals might cross-modally match audiovisual signals from the movements of the partner and kinesthetic, cutaneous, visual and auditory signals from their own movements. Because signals from different sensory systems are processed with noise and asynchronously, the challenge of the CNS is to derive the best estimate based on this conflicting information. This is currently thought to be done by a mechanism operating as a Maximum Likelihood Estimator (MLE). The present work investigated whether audiovisual signals from the partner are integrated according to MLE in order to synchronize steps during walking. Three experiments were conducted in which the sensory cues from a walking partner were virtually simulated. In Experiment 1 seven participants were instructed to synchronize with human-sized Point Light Walkers and/or footstep sounds. Results revealed highest synchronization performance with auditory and audiovisual cues. This was quantified by the time to achieve synchronization and by synchronization variability. However, this auditory dominance effect might have been due to artifacts of the setup. Therefore, in Experiment 2 human-sized virtual mannequins were implemented. Also, audiovisual stimuli were rendered in real-time and thus were synchronous and co-localized. All four participants synchronized best with audiovisual cues. For three of the four participants results point toward their optimal integration consistent with the MLE model. Experiment 3 yielded performance decrements for all three participants when the cues were incongruent. Overall, these findings suggest that individuals might optimally integrate audiovisual cues to synchronize steps during side-by-side walking.info:eu-repo/semantics/publishedVersio
Parameter Setting in Quantum Approximate Optimization of Weighted Problems
Quantum Approximate Optimization Algorithm (QAOA) is a leading candidate
algorithm for solving combinatorial optimization problems on quantum computers.
However, in many cases QAOA requires computationally intensive parameter
optimization. The challenge of parameter optimization is particularly acute in
the case of weighted problems, for which the eigenvalues of the phase operator
are non-integer and the QAOA energy landscape is not periodic. In this work, we
develop parameter setting heuristics for QAOA applied to a general class of
weighted problems. First, we derive optimal parameters for QAOA with depth
applied to the weighted MaxCut problem under different assumptions on the
weights. In particular, we rigorously prove the conventional wisdom that in the
average case the first local optimum near zero gives globally-optimal QAOA
parameters. Second, for we prove that the QAOA energy landscape for
weighted MaxCut approaches that for the unweighted case under a simple
rescaling of parameters. Therefore, we can use parameters previously obtained
for unweighted MaxCut for weighted problems. Finally, we prove that for
the QAOA objective sharply concentrates around its expectation, which means
that our parameter setting rules hold with high probability for a random
weighted instance. We numerically validate this approach on general weighted
graphs and show that on average the QAOA energy with the proposed fixed
parameters is only percentage points away from that with optimized
parameters. Third, we propose a general heuristic rescaling scheme inspired by
the analytical results for weighted MaxCut and demonstrate its effectiveness
using QAOA with the XY Hamming-weight-preserving mixer applied to the portfolio
optimization problem. Our heuristic improves the convergence of local
optimizers, reducing the number of iterations by 7.2x on average
Analyzing Prospects for Quantum Advantage in Topological Data Analysis
Lloyd et al. were first to demonstrate the promise of quantum algorithms for
computing Betti numbers, a way to characterize topological features of data
sets. Here, we propose, analyze, and optimize an improved quantum algorithm for
topological data analysis (TDA) with reduced scaling, including a method for
preparing Dicke states based on inequality testing, a more efficient amplitude
estimation algorithm using Kaiser windows, and an optimal implementation of
eigenvalue projectors based on Chebyshev polynomials. We compile our approach
to a fault-tolerant gate set and estimate constant factors in the Toffoli
complexity. Our analysis reveals that super-quadratic quantum speedups are only
possible for this problem when targeting a multiplicative error approximation
and the Betti number grows asymptotically. Further, we propose a dequantization
of the quantum TDA algorithm that shows that having exponentially large
dimension and Betti number are necessary, but insufficient conditions, for
super-polynomial advantage. We then introduce and analyze specific problem
examples which have parameters in the regime where super-polynomial advantages
may be achieved, and argue that quantum circuits with tens of billions of
Toffoli gates can solve seemingly classically intractable instances.Comment: 54 pages, 7 figures. Added a number of theorems and lemmas to clarify
findings and also a discussion in the main text and new appendix about
variants of our problems with high Betti numbers that are challenging for
recent classical algorithm
Effect of MTA-based sealer on the healing of periapical lesions
Some manufacturers have recently added specific components to improve the ease of handling and insertion material properties of MTA in order to create MTA-based sealers. Objective The aim of this study was to evaluate the healing of periapical lesions in canine teeth after a single session of endodontic treatment with MTA Fillapex® compared with Sealapex® or Endo-CPM-Sealer®. Material and Methods Sixty-two root canals were performed on two 1-year-old male dogs. After coronal access and pulp extirpation, the canals were exposed to the oral cavity for 6 months in order to induce periapical lesions. The root canals were prepared, irrigated with a solution of 2.5% sodium hypochlorite and filled with gutta-percha and different sealers, according to the following groups: 1) Sealapex®; 2) Endo-CPM-Sealer®; and 3) MTA Fillapex®. Some teeth with periapical lesions were left untreated for use as positive controls. Healthy teeth were used as negative controls. After 6 months, the animals were sacrificed and serial sections from the roots were prepared for histomorphologic analysis and stained with hematoxylin and eosin and the Brown and Brenn technique. The lesions were scored according to pre-established histomorphologic parameters and the scores statistically analyzed using the Kruskal-Wallis test. Results All 3 materials produced similar patterns of healing (p>;0.05); in particular, persistent inflammation and absence of complete periapical tissue healing were consistently noted. Conclusions Preparation of the infected root canals followed by filling with the materials studied was insufficient to provide complete healing of the periapical tissues
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