1,327 research outputs found
Resummation of Nonalternating Divergent Perturbative Expansions
A method for the resummation of nonalternating divergent perturbation series
is described. The procedure constitutes a generalization of the Borel-Pad\'{e}
method. Of crucial importance is a special integration contour in the complex
plane. Nonperturbative imaginary contributions can be inferred from the purely
real perturbative coefficients. A connection is drawn from the quantum field
theoretic problem of resummation to divergent perturbative expansions in other
areas of physics.Comment: 5 pages, LaTeX, 2 tables, 1 figure; discussion of the Carleman
criterion added; version to appear in Phys. Rev.
Witnessing eigenstates for quantum simulation of Hamiltonian spectra
The efficient calculation of Hamiltonian spectra, a problem often intractable
on classical machines, can find application in many fields, from physics to
chemistry. Here, we introduce the concept of an "eigenstate witness" and
through it provide a new quantum approach which combines variational methods
and phase estimation to approximate eigenvalues for both ground and excited
states. This protocol is experimentally verified on a programmable silicon
quantum photonic chip, a mass-manufacturable platform, which embeds entangled
state generation, arbitrary controlled-unitary operations, and projective
measurements. Both ground and excited states are experimentally found with
fidelities >99%, and their eigenvalues are estimated with 32-bits of precision.
We also investigate and discuss the scalability of the approach and study its
performance through numerical simulations of more complex Hamiltonians. This
result shows promising progress towards quantum chemistry on quantum computers.Comment: 9 pages, 4 figures, plus Supplementary Material [New version with
minor typos corrected.
Structure and Composition of Two Transitional Circumstellar Disks in Corona Australis
The late stages of evolution of the primordial circumstellar disks
surrounding young stars are poorly understood, yet vital to constrain theories
of planet formation. We consider basic structural models for the disks around
two ~10 Myr-old members of the nearby RCrA association, RX J1842.9-3532 and RX
J1852.3-3700. We present new arcsecond-resolution maps of their 230 GHz
continuum emission from the Submillimeter Array and unresolved CO(3-2) spectra
from the Atacama Submillimeter Telescope Experiment. By combining these data
with broadband fluxes from the literature and infrared fluxes and spectra from
the catalog of the Formation and Evolution of Planetary Systems (FEPS) Legacy
program on the Spitzer Space Telescope, we assemble a multiwavelength data set
probing the gas and dust disks. Using the Monte Carlo radiative transfer code
RADMC to model simultaneously the SED and millimeter continuum visibilities, we
derive basic dust disk properties and identify an inner cavity of radius 16 AU
in the disk around RX J1852.3-3700. We also identify an optically thin 5 AU
cavity in the disk around RX J1842.9-3532, with a small amount of optically
thick material close to the star. The molecular line observations suggest an
intermediate disk inclination in RX J1842.9-3532, consistent with the continuum
emission. In combination with the dust models, the molecular data allow us to
derive a lower CO content than expected, suggesting that the process of gas
clearing is likely underway in both systems, perhaps simultaneously with planet
formation.Comment: 11 pages, 5 figures, accepted for publication in A
Media Witnessing: Exploring the Audience of Distant Suffering
This article aims at demonstrating the relevance of the concept of ‘media witnessing’ as an analytical lens for the study of audience engagement with media reports of distant suffering. Drawing upon existing theoretical work on the concept, the article approaches media witnessing as a distinct modality of audience experience and constructs an analytical framework for its study. Applying this framework on an empirical study of Greek audiences, the article provides a typology of witnessing, consisting of four different types of audience engagement with media stories of human suffering. This typology illustrates the complexities inherent in the practice of watching suffering on television, as well as the limitations of mediated cosmopolitan imagination
Resummation of the Divergent Perturbation Series for a Hydrogen Atom in an Electric Field
We consider the resummation of the perturbation series describing the energy
displacement of a hydrogenic bound state in an electric field (known as the
Stark effect or the LoSurdo-Stark effect), which constitutes a divergent formal
power series in the electric field strength. The perturbation series exhibits a
rich singularity structure in the Borel plane. Resummation methods are
presented which appear to lead to consistent results even in problematic cases
where isolated singularities or branch cuts are present on the positive and
negative real axis in the Borel plane. Two resummation prescriptions are
compared: (i) a variant of the Borel-Pade resummation method, with an
additional improvement due to utilization of the leading renormalon poles (for
a comprehensive discussion of renormalons see [M. Beneke, Phys. Rep. vol. 317,
p. 1 (1999)]), and (ii) a contour-improved combination of the Borel method with
an analytic continuation by conformal mapping, and Pade approximations in the
conformal variable. The singularity structure in the case of the LoSurdo-Stark
effect in the complex Borel plane is shown to be similar to (divergent)
perturbative expansions in quantum chromodynamics.Comment: 14 pages, RevTeX, 3 tables, 1 figure; numerical accuracy of results
enhanced; one section and one appendix added and some minor changes and
additions; to appear in phys. rev.
Evidence for Evolution Among Primordial Disks in the 5 Myr Old Upper Scorpius OB Association
Moderate-resolution, near-infrared spectra between 0.8 and 5.2 microns were
obtained for 12 late-type (K0-M3) disk-bearing members of the ~5 Myr old Upper
Scorpius OB association using SpeX on the NASA Infrared Telescope Facility. For
most sources, continuum excess emission first becomes apparent between ~2.2 and
4.5 microns and is consistent with that produced by single-temperature
blackbodies having characteristic temperatures ranging from ~500 to 1300 K. The
near-infrared spectra for 5 of 12 Upper Scorpius sources exhibit Pa-gamma,
Pa-beta and Br-gamma emission, indicators of disk accretion. Using a
correlation between Pa-beta and Br-gamma emission line luminosity and accretion
luminosity, mass accretion rates (Mdot) are derived for these sources that
range from Mdot = 3.5 X 10^{-10} to 1.5 X 10^{-8} MSun per yr. Merging the SpeX
observations with Spitzer Space Telescope mid-infrared (5.4-37.0 micron)
spectroscopy and 24 and 70 micron broadband photometry, the observed spectral
energy distributions are compared with those predicted by two-dimensional,
radiative transfer accretion disk models. Of the 9 Upper Scorpius sources
examined in this analysis, 3 exhibit spectral energy distributions that are
most consistent with models having inner disk radii that substantially exceed
their respective dust sublimation radii. The remaining Upper Scorpius members
possess spectral energy distributions that either show significant dispersion
among predicted inner disk radii or are best described by models having inner
disk rims coincident with the dust sublimation radius.Comment: 35 pages, 5 figures, accepted for publication in the Astronomical
Journa
Perturbative approach to the hydrogen atom in strong magnetic field
The states of hydrogen atom with principal quantum number n <= 3 and zero
magnetic quantum number in constant homogeneous magnetic field H are
considered. The perturbation theory series is summed with the help of Borel
transformation and conformal mapping of the Borel variable. Convergence of
approximate energy eigenvalues and their agreement with corresponding existing
results are observed for external fields up to n^3 H ~ 5. The possibility of
restoring the asymptotic behaviour of energy levels using perturbation theory
coefficients is also discussed.Comment: LaTeX, 8 pages with 5 eps figure
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