9,740 research outputs found
Spin foam models and the Wheeler-DeWitt equation for the quantum 4-simplex
The asymptotics of some spin foam amplitudes for a quantum 4-simplex is known
to display rapid oscillations whose frequency is the Regge action. In this
note, we reformulate this result through a difference equation, asymptotically
satisfied by these models, and whose semi-classical solutions are precisely the
sine and the cosine of the Regge action. This equation is then interpreted as
coming from the canonical quantization of a simple constraint in Regge
calculus. This suggests to lift and generalize this constraint to the phase
space of loop quantum gravity parametrized by twisted geometries. The result is
a reformulation of the flat model for topological BF theory from the
Hamiltonian perspective. The Wheeler-de-Witt equation in the spin network basis
gives difference equations which are exactly recursion relations on the
15j-symbol. Moreover, the semi-classical limit is investigated using coherent
states, and produces the expected results. It mimics the classical constraint
with quantized areas, and for Regge geometries it reduces to the semi-classical
equation which has been introduced in the beginning.Comment: 16 pages, the new title is that of the published version (initial
title: A taste of Hamiltonian constraint in spin foam models
Summing the Instanton Series in N=2 Superconformal Large-N QCD
We consider the multi-instanton collective coordinate integration measure in
N=2 supersymmetric SU(N) gauge theory with N_F fundamental hypermultiplets. In
the large-N limit, at the superconformal point where N_F=2N and all VEVs are
turned off, the k-instanton moduli space collapses to a single copy of
AdS_5*S^1. The resulting k-instanton effective measure is proportional to
N^{1/2} g^4 Z_k^(6), where Z_k^(6) is the partition function of N=(1,0) SYM
theory in six dimensions reduced to zero dimensions. The multi-instanton can in
fact be summed in closed form. As a hint of an AdS/CFT duality, with the usual
relation between the gauge theory and string theory parameters, this precisely
matches the normalization of the charge-k D-instanton measure in type IIB
string theory compactified to six dimensions on K3 with a vanishing two-cycle.Comment: 12 pages, amslate
Gauge symmetries in spinfoam gravity: the case for "cellular quantization"
The spinfoam approach to quantum gravity rests on a "quantization" of BF
theory using 2-complexes and group representations. We explain why, in
dimension three and higher, this "spinfoam quantization" must be amended to be
made consistent with the gauge symmetries of discrete BF theory. We discuss a
suitable generalization, called "cellular quantization", which (1) is finite,
(2) produces a topological invariant, (3) matches with the properties of the
continuum BF theory, (4) corresponds to its loop quantization. These results
significantly clarify the foundations - and limitations - of the spinfoam
formalism, and open the path to understanding, in a discrete setting, the
symmetry-breaking which reduces BF theory to gravity.Comment: 6 page
Adiabatic Preparation of a Heisenberg Antiferromagnet Using an Optical Superlattice
We analyze the possibility to prepare a Heisenberg antiferromagnet with cold
fermions in optical lattices, starting from a band insulator and adiabatically
changing the lattice potential. The numerical simulation of the dynamics in 1D
allows us to identify the conditions for success, and to study the influence
that the presence of holes in the initial state may have on the protocol. We
also extend our results to two-dimensional systems.Comment: 5 pages, 4 figures + Supplementary Material (5 pages, 6 figures),
published versio
Incidental jurisdiction in the award in “The ‘Enrica Lexie’ Incident (Italy v. India)”: Part II
Exploiting Points and Lines in Regression Forests for RGB-D Camera Relocalization
Camera relocalization plays a vital role in many robotics and computer vision
tasks, such as global localization, recovery from tracking failure and loop
closure detection. Recent random forests based methods exploit randomly sampled
pixel comparison features to predict 3D world locations for 2D image locations
to guide the camera pose optimization. However, these image features are only
sampled randomly in the images, without considering the spatial structures or
geometric information, leading to large errors or failure cases with the
existence of poorly textured areas or in motion blur. Line segment features are
more robust in these environments. In this work, we propose to jointly exploit
points and lines within the framework of uncertainty driven regression forests.
The proposed approach is thoroughly evaluated on three publicly available
datasets against several strong state-of-the-art baselines in terms of several
different error metrics. Experimental results prove the efficacy of our method,
showing superior or on-par state-of-the-art performance.Comment: published as a conference paper at 2018 IEEE/RSJ International
Conference on Intelligent Robots and Systems (IROS
Micromagnetic Simulation of Nanoscale Films with Perpendicular Anisotropy
A model is studied for the theoretical description of nanoscale magnetic
films with high perpendicular anisotropy. In the model the magnetic film is
described in terms of single domain magnetic grains with Ising-like behavior,
interacting via exchange as well as via dipolar forces. Additionally, the model
contains an energy barrier and a coupling to an external magnetic field.
Disorder is taken into account in order to describe realistic domain and domain
wall structures. The influence of a finite temperature as well as the dynamics
can be modeled by a Monte Carlo simulation.
Many of the experimental findings can be investigated and at least partly
understood by the model introduced above. For thin films the magnetisation
reversal is driven by domain wall motion. The results for the field and
temperature dependence of the domain wall velocity suggest that for thin films
hysteresis can be described as a depinning transition of the domain walls
rounded by thermal activation for finite temperatures.Comment: Revtex, Postscript Figures, to be published in J. Appl.Phy
Heavy-Quark Potentials and AdS/QCD
We give an example of modeling phenomenological heavy-quark potentials in a
five-dimensional framework nowadays known as AdS/QCD. In particular we
emphasize the absence of infrared renormalons.Comment: 10 pages, 3 figures; v3: some corrections, to appear in PR
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