16,336 research outputs found
Implementing Quantum Gates by Optimal Control with Doubly Exponential Convergence
We introduce a novel algorithm for the task of coherently controlling a
quantum mechanical system to implement any chosen unitary dynamics. It performs
faster than existing state of the art methods by one to three orders of
magnitude (depending on which one we compare to), particularly for quantum
information processing purposes. This substantially enhances the ability to
both study the control capabilities of physical systems within their coherence
times, and constrain solutions for control tasks to lie within experimentally
feasible regions. Natural extensions of the algorithm are also discussed.Comment: 4+2 figures; to appear in PR
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New <sup>40</sup>Ar/<sup>39</sup>Ar dating of the Grande Ronde lavas, Columbia River Basalts, USA: Implications for duration of flood basalt eruption episodes
Grande Ronde Basalt (GRB) lavas represent the most voluminous eruptive pulse of the Columbia River-Snake River-Yellowstone hotspot volcanism. With an estimated eruptive volume of 150,000 km3, GRB lavas form at least 66% of the total volume of the Columbia River Basalt Group. New 40Ar/39Ar dates for GRB lavas reveal they were emplaced within a maximum period of 0.42 ± 0.18 My. A well-documented stratigraphy indicates at least 110 GRB flow fields (or individual eruptions), and on this basis suggests an average inter-eruption hiatus of less than 4,000 years. Isotopic age-dating cannot resolve time gaps between GRB eruptions, and it is difficult to otherwise form a picture of the durations of eruptions because of non-uniform weathering in the top of flow fields and a general paucity of sediments between GR lavas. Where sediment has formed on top of the GRB, it varies in thickness from zero to 20-30 cm of silty to fine-sandy material, with occasional diatomaceous sediment. Individual GRB eruptions varied considerably in volume but many were greater than 1000 km3 in size. Most probably eruptive events were not equally spaced in time; some eruptions may have followed short periods of volcanic repose (perhaps 102 to 103 of yrs), whilst others could have been considerably longer (many 1000 s to > 104 yrs). Recent improvements in age-dating for other continental flood basalt (CFB) lava sequences have yielded estimates of total eruptive durations of less than 1 My for high-volume pulses of lava production. The GRB appears to be a similar example, where the main pulse occupied a brief period. Even allowing for moderate to long-duration pahoehoe flow field production, the amount of time the system spends in active lava-producing mode is small – less than c. 2.6% (based on eruption durations of approximately 10,000 yrs, as compared to the duration of the entire eruptive pulse of c. 420,000 yrs). A review of available 40Ar/39Ar data for the major voluminous phases of the Columbia River Basalt Group suggests that activity of the Steens Basalt-Imnaha Basalt-GRB may have, at times, been simultaneous, with obvious implications for climatic effects. Resolving intervals between successive eruptions during CFB province construction, and durations of main eruptive pulses, remains vital to determining the environmental impact of these huge eruptions
Quantifying stretching and rearrangement in epithelial sheet migration
Although understanding the collective migration of cells, such as that seen
in epithelial sheets, is essential for understanding diseases such as
metastatic cancer, this motion is not yet as well characterized as individual
cell migration. Here we adapt quantitative metrics used to characterize the
flow and deformation of soft matter to contrast different types of motion
within a migrating sheet of cells. Using a Finite-Time Lyapunov Exponent (FTLE)
analysis, we find that - in spite of large fluctuations - the flow field of an
epithelial cell sheet is not chaotic. Stretching of a sheet of cells (i.e.,
positive FTLE) is localized at the leading edge of migration. By decomposing
the motion of the cells into affine and non-affine components using the metric
D, we quantify local plastic rearrangements and describe the motion
of a group of cells in a novel way. We find an increase in plastic
rearrangements with increasing cell densities, whereas inanimate systems tend
to exhibit less non-affine rearrangements with increasing density.Comment: 21 pages, 7 figures This is an author-created, un-copyedited version
of an article accepted for publication in the New Journal of Physics. IOP
Publishing Ltd is not responsible for any errors or omissions in this version
of the manuscript or any version derived from it. The Version of Record is
available online at doi:10.1088/1367-2630/15/2/02503
Multipole strength function of deformed superfluid nuclei made easy
We present an efficient method for calculating strength functions using the
finite amplitude method (FAM) for deformed superfluid heavy nuclei within the
framework of the nuclear density functional theory. We demonstrate that FAM
reproduces strength functions obtained with the fully self-consistent
quasi-particle random-phase approximation (QRPA) at a fraction of computational
cost. As a demonstration, we compute the isoscalar and isovector monopole
strength for strongly deformed configurations in Pu by considering huge
quasi-particle QRPA spaces. Our approach to FAM, based on Broyden's iterative
procedure, opens the possibility for large-scale calculations of strength
distributions in well-bound and weakly bound nuclei across the nuclear
landscape.Comment: 5 pages, 3 figure
Stillbirth should be given greater priority on the global health agenda
Stillbirths are largely excluded from international measures of mortality and morbidity. Zeshan Qureshi and colleagues argue that stillbirth should be higher on the global health agenda
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