67 research outputs found
Practical quantum realization of the ampere from the electron charge
One major change of the future revision of the International System of Units
(SI) is a new definition of the ampere based on the elementary charge \emph{e}.
Replacing the former definition based on Amp\`ere's force law will allow one to
fully benefit from quantum physics to realize the ampere. However, a quantum
realization of the ampere from \emph{e}, accurate to within in
relative value and fulfilling traceability needs, is still missing despite many
efforts have been spent for the development of single-electron tunneling
devices. Starting again with Ohm's law, applied here in a quantum circuit
combining the quantum Hall resistance and Josephson voltage standards with a
superconducting cryogenic amplifier, we report on a practical and universal
programmable quantum current generator. We demonstrate that currents generated
in the milliampere range are quantized in terms of
( is the Josephson frequency) with a measurement uncertainty of
. This new quantum current source, able to deliver such accurate
currents down to the microampere range, can greatly improve the current
measurement traceability, as demonstrated with the calibrations of digital
ammeters. Beyond, it opens the way to further developments in metrology and in
fundamental physics, such as a quantum multimeter or new accurate comparisons
to single electron pumps.Comment: 15 pages, 4 figure
The Lyman Alpha Forest in the Spectra of QSOs
Observations of redshifted Lyman alpha forest absorption in the spectra of
quasistellar objects (QSOs) provide a highly sensitive probe of the
distribution of gaseous matter in the universe. Over the past two decades
optical spectroscopy with large ground-based telescopes, and more recently
ultraviolet spectroscopy from space have yielded a wealth of information on
what appears to be a gaseous, photoionized intergalactic medium, partly
enriched by the products of stellar nucleosynthesis, residing in coherent
structures over many hundreds of kiloparsecs. Recent progress with cosmological
hydro-simulations based on hierarchical structure formation models has led to
important insights into the physical structures giving rise to the forest. If
these ideas are correct, a truely inter- and proto-galactic medium [at high
redshift (z ~ 3), the main repository of baryons] collapses under the influence
of dark matter gravity into flattened or filamentary structures, which are seen
in absorption against background QSOs. With decreasing redshift, galaxies
forming in the denser regions, may contribute an increasing part of the Lyman
alpha absorption cross-section. Comparisons between large data samples from the
new generation of telescopes and artificial Lyman alpha forest spectra from
cosmological simulations promise to become a useful cosmological tool.Comment: latex plus three postscript figures, uses psfig,sty; Annual Review of
Astronomy and Astrophysics 1998, vol. 36 (in press
Global snow mass measurements and the effect of stratigraphic detail on inversion of microwave brightness temperatures
Snow provides large seasonal storage of freshwater, and information about the distribution of snow mass as Snow Water Equivalent (SWE) is important for hydrological planning and detecting climate change impacts. Large regional disagreements remain between estimates from reanalyses, remote sensing and modelling. Assimilating passive microwave information improves SWE estimates in many regions but the assimilation must account for how microwave scattering depends on snow stratigraphy. Physical snow models can estimate snow stratigraphy, but users must consider the computational expense of model complexity versus acceptable errors. Using data from the National Aeronautics and Space Administration Cold Land Processes Experiment (NASA CLPX) and the Helsinki University of Technology (HUT) microwave emission model of layered snowpacks, it is shown that simulations of the brightness temperature difference between 19 GHz and 37 GHz vertically polarised microwaves are consistent with Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) and Special Sensor Microwave Imager (SSM/I) retrievals once known stratigraphic information is used. Simulated brightness temperature differences for an individual snow profile depend on the provided stratigraphic detail. Relative to a profile defined at the 10 cm resolution of density and temperature measurements, the error introduced by simplification to a single layer of average properties increases approximately linearly with snow mass. If this brightness temperature error is converted into SWE using a traditional retrieval method then it is equivalent to ±13 mm SWE (7% of total) at a depth of 100 cm. This error is reduced to ±5.6 mm SWE (3 % of total) for a two-layer model
Transcriptional Profiling of the Dose Response: A More Powerful Approach for Characterizing Drug Activities
The dose response curve is the gold standard for measuring the effect of a drug treatment, but is rarely used in genomic scale transcriptional profiling due to perceived obstacles of cost and analysis. One barrier to examining transcriptional dose responses is that existing methods for microarray data analysis can identify patterns, but provide no quantitative pharmacological information. We developed analytical methods that identify transcripts responsive to dose, calculate classical pharmacological parameters such as the EC50, and enable an in-depth analysis of coordinated dose-dependent treatment effects. The approach was applied to a transcriptional profiling study that evaluated four kinase inhibitors (imatinib, nilotinib, dasatinib and PD0325901) across a six-logarithm dose range, using 12 arrays per compound. The transcript responses proved a powerful means to characterize and compare the compounds: the distribution of EC50 values for the transcriptome was linked to specific targets, dose-dependent effects on cellular processes were identified using automated pathway analysis, and a connection was seen between EC50s in standard cellular assays and transcriptional EC50s. Our approach greatly enriches the information that can be obtained from standard transcriptional profiling technology. Moreover, these methods are automated, robust to non-optimized assays, and could be applied to other sources of quantitative data
Gut microbiota and sirtuins in obesity-related inflammation and bowel dysfunction
Obesity is a chronic disease characterized by persistent low-grade inflammation with alterations in gut motility. Motor abnormalities suggest that obesity has effects on the enteric nervous system (ENS), which controls virtually all gut functions. Recent studies have revealed that the gut microbiota can affect obesity and increase inflammatory tone by modulating mucosal barrier function. Furthermore, the observation that inflammatory conditions influence the excitability of enteric neurons may add to the gut dysfunction in obesity. In this article, we discuss recent advances in understanding the role of gut microbiota and inflammation in the pathogenesis of obesity and obesity-related gastrointestinal dysfunction. The potential contribution of sirtuins in protecting or regulating the circuitry of the ENS under inflamed states is also considered
Focal-plane detector system for the KATRIN experiment
The focal-plane detector system for the KArlsruhe TRItium Neutrino (KATRIN)
experiment consists of a multi-pixel silicon p-i-n-diode array, custom readout
electronics, two superconducting solenoid magnets, an ultra high-vacuum system,
a high-vacuum system, calibration and monitoring devices, a scintillating veto,
and a custom data-acquisition system. It is designed to detect the low-energy
electrons selected by the KATRIN main spectrometer. We describe the system and
summarize its performance after its final installation.Comment: 28 pages. Two figures revised for clarity. Final version published in
Nucl. Inst. Meth.
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