12,916 research outputs found
Real time plasma equilibrium reconstruction in a Tokamak
The problem of equilibrium of a plasma in a Tokamak is a free boundary
problemdescribed by the Grad-Shafranov equation in axisymmetric configurations.
The right hand side of this equation is a non linear source, which represents
the toroidal component of the plasma current density. This paper deals with the
real time identification of this non linear source from experimental
measurements. The proposed method is based on a fixed point algorithm, a finite
element resolution, a reduced basis method and a least-square optimization
formulation
Optical investigations of noncrystalline semiconductors
Three areas of investigation into the properties of amorphous silicon and boron are reported: (1) optical properties of elemental amorphous semiconductors; (2) Mossbauer studies of disordered systems; and (3) theoretical aspects of disordered semiconductors
Matrix product states approach to the Heisenberg ferrimagnetic spin chains
We propose a new version of the matrix product (MP) states approach to the
description of quantum spin chains, which allows one to construct MP states
with certain total spin and its z-projection. We show that previously known MP
wavefunctions for integer-spin antiferromagnetic chains and ladders correspond
to some particular cases of our general ansatz. Our method allows to describe
systems with spontaneously broken rotational symmetry, like quantum
ferrimagnetic chains whose ground state has nonzero total spin. We apply this
approach to describe the ground state properties of the isotropic ferrimagnetic
Heisenberg chain with alternating spins 1 and 1/2 and compare our variational
results with the high-precision numerical data obtained by means of the quantum
Monte Carlo (QMC) method. For both the ground state energy and the correlation
functions we obtain very good agreement between the variational results and the
QMC data.Comment: 4 pages, RevTeX, uses psfig.sty, submitted to Phys. Rev.
Calculation of the neutron electric dipole moment with two dynamical flavors of domain wall fermions
We present a study of the neutron electric dipole moment () within
the framework of lattice QCD with two flavors of dynamical lig ht quarks. The
dipole moment is sensitive to the topological structure of the gaug e fields,
and accuracy can only be achieved by using dynamical, or sea quark, calc
ulations. However, the topological charge evolves slowly in these calculations,
le ading to a relatively large uncertainty in . It is shown, using
quenched configurations, that a better sampling of the charge d istribution
reduces this problem, but because the CP even part of the fermion determinant
is absent, both the topological charge dis tribution and are
pathological in the chiral limit. We discuss the statistical and systematic
uncertainties arising from the topological charge distr ibution and unphysical
size of the quark mass in our calculations and prospects fo r eliminating them.
Our calculations employ the RBC collaboration two flavor domain wall fermion
and DBW2 gauge action lattices with inverse lattice spacing 1.7
GeV, physical volume fm), and light quark mass roughly equal
to the strange quark mass ( and 0.04). We determine a value of
the electric dipole moment that is zero withi n (statistical) errors, e--fm at the smaller sea quark mass. Satisfactory
results for the magnetic and electric form factors of the proton and neutron
are also obtained and presented.Comment: 46 pages. Changed one author addres
BMSSM Implications for Cosmology
The addition of non-renormalizable terms involving the Higgs fields to the
MSSM (BMSSM) ameliorates the little hierarchy problem of the MSSM. We analyze
in detail the two main cosmological issues affected by the BMSSM: dark matter
and baryogenesis. The regions for which the relic abundance of the LSP is
consistent with WMAP and collider constraints are identified, showing that the
bulk region and other previously excluded regions are now permitted. Requiring
vacuum stability limits the allowed regions. Based on a two-loop finite
temperature effective potential analysis, we show that the electroweak phase
transition can be sufficiently first order in regions that for the MSSM are
incompatible with the LEP Higgs mass bound, including parameter values of
\tan\beta \lsim 5, m_{\tilde{t}_{1}} > m_t, m_Q << TeV.Comment: 28 pages, 4 figures. References adde
Berry-phase blockade in single-molecule magnets
We formulate the problem of electron transport through a single-molecule
magnet (SMM) in the Coulomb blockade regime taking into account topological
interference effects for the tunneling of the large spin of a SMM. The
interference originates from spin Berry phases associated with different
tunneling paths. We show that in the case of incoherent spin states it is
essential to place the SMM between oppositely spin-polarized source and drain
leads in order to detect the spin tunneling in the stationary current, which
exhibits topological zeros as a function of the transverse magnetic field.Comment: 4 pages, Revtex 4, 4 EPS figure
Optimized design of universal two-qubit gates
We construct optimized implementations of the CNOT and other universal
two-qubit gates that, unlike many of the previously proposed protocols, are
carried out in a single step. The new protocols require tunable inter-qubit
couplings but, in return, show a significant improvements in the quality of
gate operations. Our optimization procedure can be further extended to the
combinations of elementary two-qubit as well as irreducible many-qubit gates.Comment: 6 pages, 2 figure
Optimal copying of entangled two-qubit states
We investigate the problem of copying pure two-qubit states of a given degree
of entanglement in an optimal way. Completely positive covariant quantum
operations are constructed which maximize the fidelity of the output states
with respect to two separable copies. These optimal copying processes hint at
the intricate relationship between fundamental laws of quantum theory and
entanglement.Comment: 13 pages, 7 figure
Active Semi-Supervised Learning Using Sampling Theory for Graph Signals
We consider the problem of offline, pool-based active semi-supervised
learning on graphs. This problem is important when the labeled data is scarce
and expensive whereas unlabeled data is easily available. The data points are
represented by the vertices of an undirected graph with the similarity between
them captured by the edge weights. Given a target number of nodes to label, the
goal is to choose those nodes that are most informative and then predict the
unknown labels. We propose a novel framework for this problem based on our
recent results on sampling theory for graph signals. A graph signal is a
real-valued function defined on each node of the graph. A notion of frequency
for such signals can be defined using the spectrum of the graph Laplacian
matrix. The sampling theory for graph signals aims to extend the traditional
Nyquist-Shannon sampling theory by allowing us to identify the class of graph
signals that can be reconstructed from their values on a subset of vertices.
This approach allows us to define a criterion for active learning based on
sampling set selection which aims at maximizing the frequency of the signals
that can be reconstructed from their samples on the set. Experiments show the
effectiveness of our method.Comment: 10 pages, 6 figures, To appear in KDD'1
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