780 research outputs found
Magnetic structure of free cobalt clusters studied with Stern-Gerlach deflection experiments
We have studied the magnetic properties of free cobalt clusters in two
semi-independent Stern-Gerlach deflection experiments at temperatures between
60 and 307 K. We find that clusters consisting of 13 to 200 cobalt atoms
exhibit behavior that is entirely consistent with superparamagnetism, though
complicated by finite-system fluctuations in cluster temperature. By fitting
the data to the Langevin function, we report magnetic moments per atom for each
cobalt cluster size and compare the results of our two measurements and all
those performed previously. In addition to a gradual decrease in moment per
atom with increasing size, there are oscillations that appear to be caused by
geometrical shell structure. We discuss our observations in light of the two
competing models for Langevin-like magnetization behavior in free clusters,
superparamagnetism and adiabatic magnetization, and conclude that the evidence
strongly supports the superparamagnetic model
Joint Elastic Side-Scattering Lidar and Raman Lidar Measurements of Aerosol Optical Properties in South East Colorado
We describe an experiment, located in south-east Colorado, USA, that measured
aerosol optical depth profiles using two Lidar techniques. Two independent
detectors measured scattered light from a vertical UV laser beam. One detector,
located at the laser site, measured light via the inelastic Raman
backscattering process. This is a common method used in atmospheric science for
measuring aerosol optical depth profiles. The other detector, located
approximately 40km distant, viewed the laser beam from the side. This detector
featured a 3.5m2 mirror and measured elastically scattered light in a bistatic
Lidar configuration following the method used at the Pierre Auger cosmic ray
observatory. The goal of this experiment was to assess and improve methods to
measure atmospheric clarity, specifically aerosol optical depth profiles, for
cosmic ray UV fluorescence detectors that use the atmosphere as a giant
calorimeter. The experiment collected data from September 2010 to July 2011
under varying conditions of aerosol loading. We describe the instruments and
techniques and compare the aerosol optical depth profiles measured by the Raman
and bistatic Lidar detectors.Comment: 34 pages, 16 figure
Climatologies of nighttime upper thermospheric winds measured by groundâbased FabryâPerot interferometers during geomagnetically quiet conditions: 1. Local time, latitudinal, seasonal, and solar cycle dependence
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94633/1/jgra18559.pd
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Fusion reactor design studies
This progress report will give a detailed breakdown of the work accomplished for ARIES-III during the contract period, November 1, 1990 to October 31, 1991. The areas of effort discussed are: Neutronics; First-Wall; Shield; Safety; Systems; Startup and Shutdown; Energy Conversion; Ripple Loss; and Fuel Resources
Quantum Computation by Communication
We present a new approach to scalable quantum computing--a ``qubus
computer''--which realises qubit measurement and quantum gates through
interacting qubits with a quantum communication bus mode. The qubits could be
``static'' matter qubits or ``flying'' optical qubits, but the scheme we focus
on here is particularly suited to matter qubits. There is no requirement for
direct interaction between the qubits. Universal two-qubit quantum gates may be
effected by schemes which involve measurement of the bus mode, or by schemes
where the bus disentangles automatically and no measurement is needed. In
effect, the approach integrates together qubit degrees of freedom for
computation with quantum continuous variables for communication and
interaction.Comment: final published versio
Atmospheric Density Uncertainty Quantification for Satellite Conjunction Assessment
Conjunction assessment requires knowledge of the uncertainty in the predicted
orbit. Errors in the atmospheric density are a major source of error in the
prediction of low Earth orbits. Therefore, accurate estimation of the density
and quantification of the uncertainty in the density is required. Most
atmospheric density models, however, do not provide an estimate of the
uncertainty in the density. In this work, we present a new approach to quantify
uncertainties in the density and to include these for calculating the
probability of collision Pc. For this, we employ a recently developed dynamic
reduced-order density model that enables efficient prediction of the
thermospheric density. First, the model is used to obtain accurate estimates of
the density and of the uncertainty in the estimates. Second, the density
uncertainties are propagated forward simultaneously with orbit propagation to
include the density uncertainties for Pc calculation. For this, we account for
the effect of cross-correlation in position uncertainties due to density errors
on the Pc. Finally, the effect of density uncertainties and cross-correlation
on the Pc is assessed. The presented approach provides the distinctive
capability to quantify the uncertainty in atmospheric density and to include
this uncertainty for conjunction assessment while taking into account the
dependence of the density errors on location and time. In addition, the results
show that it is important to consider the effect of cross-correlation on the
Pc, because ignoring this effect can result in severe underestimation of the
collision probability.Comment: 15 pages, 6 figures, 5 table
Superradiance and Phase Multistability in Circuit Quantum Electrodynamics
By modeling the coupling of multiple superconducting qubits to a single
cavity in the circuit-quantum electrodynamics (QED) framework we find that it
should be possible to observe superradiance and phase multistability using
currently available technology. Due to the exceptionally large couplings
present in circuit-QED we predict that superradiant microwave pulses should be
observable with only a very small number of qubits (just three or four), in the
presence of energy relaxation and non-uniform qubit-field coupling strengths.
This paves the way for circuit-QED implementations of superradiant state
readout and decoherence free subspace state encoding in subradiant states. The
system considered here also exhibits phase multistability when driven with
large field amplitudes, and this effect may have applications for collective
qubit readout and for quantum feedback protocols.Comment: Published Versio
Predicting Cell Cycle Regulated Genes by Causal Interactions
The fundamental difference between classic and modern biology is that technological innovations allow to generate high-throughput data to get insights into molecular interactions on a genomic scale. These high-throughput data can be used to infer gene networks, e.g., the transcriptional regulatory or signaling network, representing a blue print of the current dynamical state of the cellular system. However, gene networks do not provide direct answers to biological questions, instead, they need to be analyzed to reveal functional information of molecular working mechanisms. In this paper we propose a new approach to analyze the transcriptional regulatory network of yeast to predict cell cycle regulated genes. The novelty of our approach is that, in contrast to all other approaches aiming to predict cell cycle regulated genes, we do not use time series data but base our analysis on the prior information of causal interactions among genes. The major purpose of the present paper is to predict cell cycle regulated genes in S. cerevisiae. Our analysis is based on the transcriptional regulatory network, representing causal interactions between genes, and a list of known periodic genes. No further data are used. Our approach utilizes the causal membership of genes and the hierarchical organization of the transcriptional regulatory network leading to two groups of periodic genes with a well defined direction of information flow. We predict genes as periodic if they appear on unique shortest paths connecting two periodic genes from different hierarchy levels. Our results demonstrate that a classical problem as the prediction of cell cycle regulated genes can be seen in a new light if the concept of a causal membership of a gene is applied consequently. This also shows that there is a wealth of information buried in the transcriptional regulatory network whose unraveling may require more elaborate concepts than it might seem at first
Bose-Einstein condensation on a superconducting atom chip
We have produced a Bose-Einstein condensate (BEC) on an atom chip using only
superconducting wires in a cryogenic environment. We observe the onset of
condensation for 10^4 atoms at a temperature of 100 nK. This result opens the
way for studies of atom losses and decoherence in a BEC interacting with a
superconducting surface. Studies of dipole-blockade with long-lived Rydberg
atoms in a small and dense atomic sample are underway.Comment: 4 pages, 4 figures. Accepted for publication in Europhysics Letter
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