27,822 research outputs found
Polychromatic X-ray CT Image Reconstruction and Mass-Attenuation Spectrum Estimation
We develop a method for sparse image reconstruction from polychromatic
computed tomography (CT) measurements under the blind scenario where the
material of the inspected object and the incident-energy spectrum are unknown.
We obtain a parsimonious measurement-model parameterization by changing the
integral variable from photon energy to mass attenuation, which allows us to
combine the variations brought by the unknown incident spectrum and mass
attenuation into a single unknown mass-attenuation spectrum function; the
resulting measurement equation has the Laplace integral form. The
mass-attenuation spectrum is then expanded into first order B-spline basis
functions. We derive a block coordinate-descent algorithm for constrained
minimization of a penalized negative log-likelihood (NLL) cost function, where
penalty terms ensure nonnegativity of the spline coefficients and nonnegativity
and sparsity of the density map. The image sparsity is imposed using
total-variation (TV) and norms, applied to the density-map image and
its discrete wavelet transform (DWT) coefficients, respectively. This algorithm
alternates between Nesterov's proximal-gradient (NPG) and limited-memory
Broyden-Fletcher-Goldfarb-Shanno with box constraints (L-BFGS-B) steps for
updating the image and mass-attenuation spectrum parameters. To accelerate
convergence of the density-map NPG step, we apply a step-size selection scheme
that accounts for varying local Lipschitz constant of the NLL. We consider
lognormal and Poisson noise models and establish conditions for biconvexity of
the corresponding NLLs. We also prove the Kurdyka-{\L}ojasiewicz property of
the objective function, which is important for establishing local convergence
of the algorithm. Numerical experiments with simulated and real X-ray CT data
demonstrate the performance of the proposed scheme
Evolution of String-Wall Networks and Axionic Domain Wall Problem
We study the cosmological evolution of domain walls bounded by strings which
arise naturally in axion models. If we introduce a bias in the potential, walls
become metastable and finally disappear. We perform two dimensional lattice
simulations of domain wall networks and estimate the decay rate of domain
walls. By using the numerical results, we give a constraint for the bias
parameter and the Peccei-Quinn scale. We also discuss the possibility to probe
axion models by direct detection of gravitational waves produced by domain
walls.Comment: 19 pages, 7 figures; revised version of the manuscript, accepted for
publication in JCA
Transmission lines and resonators based on quantum Hall plasmonics: electromagnetic field, attenuation and coupling to qubits
Quantum Hall edge states have some characteristic features that can prove
useful to measure and control solid state qubits. For example, their high
voltage to current ratio and their dissipationless nature can be exploited to
manufacture low-loss microwave transmission lines and resonators with a
characteristic impedance of the order of the quantum of resistance . The high value of the impedance guarantees that the
voltage per photon is high and for this reason high impedance resonators can be
exploited to obtain larger values of coupling to systems with a small charge
dipole, e.g. spin qubits. In this paper, we provide a microscopic analysis of
the physics of quantum Hall effect devices capacitively coupled to external
electrodes. The electrical current in these devices is carried by edge
magnetoplasmonic excitations and by using a semiclassical model, valid for a
wide range of quantum Hall materials, we discuss the spatial profile of the
electromagnetic field in a variety of situations of interest. Also, we perform
a numerical analysis to estimate the lifetime of these excitations and, from
the numerics, we extrapolate a simple fitting formula which quantifies the
factor in quantum Hall resonators. We then explore the possibility of reaching
the strong photon-qubit coupling regime, where the strength of the interaction
is higher than the losses in the system. We compute the Coulomb coupling
strength between the edge magnetoplasmons and singlet-triplet qubits, and we
obtain values of the coupling parameter of the order ;
comparing these values to the estimated attenuation in the resonator, we find
that for realistic qubit designs the coupling can indeed be strong
A Convex Reconstruction Model for X-ray Tomographic Imaging with Uncertain Flat-fields
Classical methods for X-ray computed tomography are based on the assumption
that the X-ray source intensity is known, but in practice, the intensity is
measured and hence uncertain. Under normal operating conditions, when the
exposure time is sufficiently high, this kind of uncertainty typically has a
negligible effect on the reconstruction quality. However, in time- or
dose-limited applications such as dynamic CT, this uncertainty may cause severe
and systematic artifacts known as ring artifacts. By carefully modeling the
measurement process and by taking uncertainties into account, we derive a new
convex model that leads to improved reconstructions despite poor quality
measurements. We demonstrate the effectiveness of the methodology based on
simulated and real data sets.Comment: Accepted at IEEE Transactions on Computational Imagin
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