9,526 research outputs found
Magnetic field stabilization system for atomic physics experiments
Atomic physics experiments commonly use millitesla-scale magnetic fields to
provide a quantization axis. As atomic transition frequencies depend on the
amplitude of this field, many experiments require a stable absolute field. Most
setups use electromagnets, which require a power supply stability not usually
met by commercially available units. We demonstrate stabilization of a field of
14.6 mT to 4.3 nT rms noise (0.29 ppm), compared to noise of 100 nT
without any stabilization. The rms noise is measured using a field-dependent
hyperfine transition in a single Ca ion held in a Paul trap at the
centre of the magnetic field coils. For the Ca "atomic clock" qubit
transition at 14.6 mT, which depends on the field only in second order, this
would yield a projected coherence time of many hours. Our system consists of a
feedback loop and a feedforward circuit that control the current through the
field coils and could easily be adapted to other field amplitudes, making it
suitable for other applications such as neutral atom traps.Comment: 6 pages, 5 figure
Probing molecular free energy landscapes by periodic loading
Single molecule pulling experiments provide information about interactions in
biomolecules that cannot be obtained by any other method. However, the
reconstruction of the molecule's free energy profile from the experimental data
is still a challenge, in particular for the unstable barrier regions. We
propose a new method for obtaining the full profile by introducing a periodic
ramp and using Jarzynski's identity for obtaining equilibrium quantities from
non-equilibrium data. Our simulated experiments show that this method delivers
significant more accurate data than previous methods, under the constraint of
equal experimental effort.Comment: 4 pages, 3 figure
funcX: A Federated Function Serving Fabric for Science
Exploding data volumes and velocities, new computational methods and
platforms, and ubiquitous connectivity demand new approaches to computation in
the sciences. These new approaches must enable computation to be mobile, so
that, for example, it can occur near data, be triggered by events (e.g.,
arrival of new data), be offloaded to specialized accelerators, or run remotely
where resources are available. They also require new design approaches in which
monolithic applications can be decomposed into smaller components, that may in
turn be executed separately and on the most suitable resources. To address
these needs we present funcX---a distributed function as a service (FaaS)
platform that enables flexible, scalable, and high performance remote function
execution. funcX's endpoint software can transform existing clouds, clusters,
and supercomputers into function serving systems, while funcX's cloud-hosted
service provides transparent, secure, and reliable function execution across a
federated ecosystem of endpoints. We motivate the need for funcX with several
scientific case studies, present our prototype design and implementation, show
optimizations that deliver throughput in excess of 1 million functions per
second, and demonstrate, via experiments on two supercomputers, that funcX can
scale to more than more than 130000 concurrent workers.Comment: Accepted to ACM Symposium on High-Performance Parallel and
Distributed Computing (HPDC 2020). arXiv admin note: substantial text overlap
with arXiv:1908.0490
On the relationship of polar mesospheric cloud ice water content, particle radius and mesospheric temperature and its use in multi-dimensional models
The distribution of ice layers in the polar summer mesosphere (called polar mesospheric clouds or PMCs) is sensitive to background atmospheric conditions and therefore affected by global-scale dynamics. To investigate this coupling it is necessary to simulate the global distribution of PMCs within a 3-dimensional (3-D) model that couples large-scale dynamics with cloud microphysics. However, modeling PMC microphysics within 3-D global chemistry climate models (GCCM) is a challenge due to the high computational cost associated with particle following (Lagrangian) or sectional microphysical calculations. By characterizing the relationship between the PMC effective radius, ice water content (<i>iwc</i>), and local temperature (<i>T</i>) from an ensemble of simulations from the sectional microphysical model, the Community Aerosol and Radiation Model for Atmospheres (CARMA), we determined that these variables can be described by a robust empirical formula. The characterized relationship allows an estimate of an altitude distribution of PMC effective radius in terms of local temperature and <i>iwc</i>. For our purposes we use this formula to predict an effective radius as part of a bulk parameterization of PMC microphysics in a 3-D GCCM to simulate growth, sublimation and sedimentation of ice particles without keeping track of the time history of each ice particle size or particle size bin. This allows cost effective decadal scale PMC simulations in a 3-D GCCM to be performed. This approach produces realistic PMC simulations including estimates of the optical properties of PMCs. We validate the relationship with PMC data from the Solar Occultation for Ice Experiment (SOFIE)
β-Catenin is necessary to keep cells of ureteric bud/Wolffian duct epithelium in a precursor state
AbstractDifferentiation is the process by which tissues/organs take on their final, physiologically functional form. This process is mediated in part by the silencing of embryonic genes and the activation of terminal, differentiation gene products. Mammalian kidney development is initiated when the Wolffian duct branches and invades the overlying metanephric mesenchyme. The newly formed epithelial bud, known as the ureteric bud, will continue to branch ultimately differentiating into the collecting duct system and ureter. Here, we show that Hoxb7-Cre mediated removal of β-catenin from the mouse Wolffian duct epithelium leads to the premature expression of gene products normally associated with the differentiated kidney collecting duct system including the water channel protein, Aquaporin-3 and the tight junction protein isoform, ZO-1α+. Mutant cells fail to maintain expression of some genes associated with embryonic development, including several mediators of branching morphogenesis, which subsequently leads to kidney aplasia or hypoplasia. Reciprocally, expression of a stabilized form of β-catenin appears to block differentiation of the collecting ducts. All of these defects occur in the absence of any effects on the adherens junctions. These data indicate a role for β-catenin in maintaining cells of the Wolffian ducts and the duct derived ureteric bud/collecting duct system in an undifferentiated or precursor state
Enabling EASEY deployment of containerized applications for future HPC systems
The upcoming exascale era will push the changes in computing architecture
from classical CPU-based systems in hybrid GPU-heavy systems with much higher
levels of complexity. While such clusters are expected to improve the
performance of certain optimized HPC applications, it will also increase the
difficulties for those users who have yet to adapt their codes or are starting
from scratch with new programming paradigms. Since there are still no
comprehensive automatic assistance mechanisms to enhance application
performance on such systems, we are proposing a support framework for future
HPC architectures, called EASEY (Enable exASclae for EverYone). The solution
builds on a layered software architecture, which offers different mechanisms on
each layer for different tasks of tuning. This enables users to adjust the
parameters on each of the layers, thereby enhancing specific characteristics of
their codes. We introduce the framework with a Charliecloud-based solution,
showcasing the LULESH benchmark on the upper layers of our framework. Our
approach can automatically deploy optimized container computations with
negligible overhead and at the same time reduce the time a scientist needs to
spent on manual job submission configurations.Comment: International Conference on Computational Science ICCS2020, 13 page
Attractive instability of oppositely charged membranes induced by charge density fluctuations
We predict the conditions under which two oppositely charged membranes show a
dynamic, attractive instability. Two layers with unequal charges of opposite
sign can repel or be stable when in close proximity. However, dynamic charge
density fluctuations can induce an attractive instability and thus facilitate
fusion. We predict the dominant instability modes and timescales and show how
these are controlled by the relative charge and membrane viscosities. These
dynamic instabilities may be the precursors of membrane fusion in systems where
artificial vesicles are engulfed by biological cells of opposite charge
Rupture of multiple parallel molecular bonds under dynamic loading
Biological adhesion often involves several pairs of specific receptor-ligand
molecules. Using rate equations, we study theoretically the rupture of such
multiple parallel bonds under dynamic loading assisted by thermal activation.
For a simple generic type of cooperativity, both the rupture time and force
exhibit several different scaling regimes. The dependence of the rupture force
on the number of bonds is predicted to be either linear, like a square root or
logarithmic.Comment: 8 pages, 2 figure
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