1,876 research outputs found
Polynomial-time algorithm for simulation of weakly interacting quantum spin systems
We describe an algorithm that computes the ground state energy and
correlation functions for 2-local Hamiltonians in which interactions between
qubits are weak compared to single-qubit terms. The running time of the
algorithm is polynomial in the number of qubits and the required precision.
Specifically, we consider Hamiltonians of the form , where
H_0 describes non-interacting qubits, V is a perturbation that involves
arbitrary two-qubit interactions on a graph of bounded degree, and
is a small parameter. The algorithm works if is below a certain
threshold value that depends only upon the spectral gap of H_0, the maximal
degree of the graph, and the maximal norm of the two-qubit interactions. The
main technical ingredient of the algorithm is a generalized Kirkwood-Thomas
ansatz for the ground state. The parameters of the ansatz are computed using
perturbative expansions in powers of . Our algorithm is closely
related to the coupled cluster method used in quantum chemistry.Comment: 27 page
Analysis of path integrals at low temperature : Box formula, occupation time and ergodic approximation
We study the low temperature behaviour of path integrals for a simple
one-dimensional model. Starting from the Feynman-Kac formula, we derive a new
functional representation of the density matrix at finite temperature, in terms
of the occupation times of Brownian motions constrained to stay within boxes
with finite sizes. From that representation, we infer a kind of ergodic
approximation, which only involves double ordinary integrals. As shown by its
applications to different confining potentials, the ergodic approximation turns
out to be quite efficient, especially in the low-temperature regime where other
usual approximations fail
A Multiscale Approach to Determination of Thermal Properties and Changes in Free Energy: Application to Reconstruction of Dislocations in Silicon
We introduce an approach to exploit the existence of multiple levels of
description of a physical system to radically accelerate the determination of
thermodynamic quantities. We first give a proof of principle of the method
using two empirical interatomic potential functions. We then apply the
technique to feed information from an interatomic potential into otherwise
inaccessible quantum mechanical tight-binding calculations of the
reconstruction of partial dislocations in silicon at finite temperature. With
this approach, comprehensive ab initio studies at finite temperature will now
be possible.Comment: 5 pages, 3 figure
Microstructural evolution under low shear rates during Rheo processing of LM25 alloy
© ASM InternationalMicrostructural features of LM25 alloy processed by two different routes: (1) conventional casting, and(2)shear casting based on inclined heated surface are studied. The microstructures of the primary phase for the shear-cast samples show rosette or ellipsoidal morphologies. Heat transfer of contacting melt with the
inclined tube surface and shear stress exerted on the layers of the melt as result of gravitational force are crucial parameters for the microstructural evolution. Compared to those produced by conventional casting, shear-cast samples have a much improved tensile strength and ductility due to globular microstructure
Ozone loss derived from balloon-borne tracer measurements in the 1999/2000 Arctic winter
Balloon-borne measurements of CFC11 (from the DIRAC in situ gas chromatograph and the DESCARTES grab sampler), ClO and O3 were made during the 1999/2000 Arctic winter as part of the SOLVE-THESEO 2000 campaign, based in Kiruna (Sweden). Here we present the CFC11 data from nine flights and compare them first with data from other instruments which flew during the campaign and then with the vertical distributions calculated by the SLIMCAT 3D CTM. We calculate ozone loss inside the Arctic vortex between late January and early March using the relation between CFC11 and O3 measured on the flights. The peak ozone loss (~1200ppbv) occurs in the 440-470K region in early March in reasonable agreement with other published empirical estimates. There is also a good agreement between ozone losses derived from three balloon tracer data sets used here. The magnitude and vertical distribution of the loss derived from the measurements is in good agreement with the loss calculated from SLIMCAT over Kiruna for the same days
High real-space resolution measurement of the local structure of Ga_1-xIn_xAs using x-ray diffraction
High real-space resolution atomic pair distribution functions (PDF)s from the
alloy series Ga_1-xIn_xAs have been obtained using high-energy x-ray
diffraction. The first peak in the PDF is resolved as a doublet due to the
presence of two nearest neighbor bond lengths, Ga-As and In-As, as previously
observed using XAFS. The widths of nearest, and higher, neighbor pairs are
analyzed by separating the strain broadening from the thermal motion. The
strain broadening is five times larger for distant atomic neighbors as compared
to nearest neighbors. The results are in agreement with model calculations.Comment: 4 pages, 5 figure
The second law and beyond in microscopic quantum setups
The Clausius inequality (CI) is one of the most versatile forms of the second
law. Although it was originally conceived for macroscopic steam engines, it is
also applicable to quantum single particle machines. Moreover, the CI is the
main connecting thread between classical microscopic thermodynamics and
nanoscopic quantum thermodynamics. In this chapter, we study three different
approaches for obtaining the CI. Each approach shows different aspects of the
CI. The goals of this chapter are: (i) To show the exact assumptions made in
various derivations of the CI. (ii) To elucidate the structure of the second
law and its origin. (iii) To discuss the possibilities each approach offers for
finding additional second-law like inequalities. (iv) To pose challenges
related to the second law in nanoscopic setups. In particular, we introduce and
briefly discuss the notions of exotic heat machines (X machines), and "lazy
demons".Comment: As a chapter of: F. Binder, L. A. Correa, C. Gogolin, J. Anders, and
G. Adesso (eds.), "Thermodynamics in the quantum regime - Recent Progress and
Outlook", (Springer International Publishing). v1 does not include references
to other book chapter
No Evidence That Genetic Variation at the Klotho Locus Is Associated With Longevity in Caucasians from the Newcastle 85+ Study and the UK Biobank
Copyright © The Author(s) 2021. The demographics of Western populations are changing, with an increase in the proportion of older adults. There is evidence to suggest that genetic factors may influence the aging process: studying these may lead to interventions to help individuals live a longer and healthier life. Evidence from several groups indicates that Klotho (KL), a gene encoding a single-pass transmembrane protein that acts as an FGF23 co-receptor, may be associated with longevity and healthy aging. We aimed to explore this area further by comparing the genotype counts in 642 long-lived individuals from the Newcastle 85+ Study with 18 295 middle-aged Newcastle-based controls from the UK Biobank to test whether variants at the KL gene locus are over- or under-represented in older individuals. If KL is associated with longevity, then we would expect the genotype counts to differ between the 2 cohorts. We found that the rs2283368 CC genotype and the rs9536338 C allele, but not the KL-VS haplotype, were associated with reaching very old age. However, these associations did not replicate in the remainder of the UK Biobank cohort. Thus, our results do not reliably support the role of KL as a longevity factor.Calico LLC (South San Francisco, California, United States); HAA is the recipient of a PhD studentship from the College of Health, Medical and Life Sciences, Brunel University London
Cytogerontology since 1881: A reappraisal of August Weismann and a review of modern progress
Cytogerontology, the science of cellular ageing, originated in 1881 with the prediction by August Weismann that the somatic cells of higher animals have limited division potential. Weismann's prediction was derived by considering the role of natural selection in regulating the duration of an organism's life. For various reasons, Weismann's ideas on ageing fell into neglect following his death in 1914, and cytogerontology has only reappeared as a major research area following the demonstration by Hayflick and Moorhead in the early 1960s that diploid human fibroblasts are restricted to a finite number of divisions in vitro.
In this review we give a detailed account of Weismann's theory, and we reveal that his ideas were both more extensive in their scope and more pertinent to current research than is generally recognised. We also appraise the progress which has been made over the past hundred years in investigating the causes of ageing, with particular emphasis being given to (i) the evolution of ageing, and (ii) ageing at the cellular level. We critically assess the current state of knowledge in these areas and recommend a series of points as primary targets for future research
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