1,489 research outputs found
THE AMUNDSEN SEA LOW Variability, Change, and Impact on Antarctic Climate
The Amundsen Sea low (ASL) is a climatological low pressure center that exerts considerable influence on the climate of West Antarctica. Its potential to explain important recent changes in Antarctic climate, for example, in temperature and sea ice extent, means that it has become the focus of an increasing number of studies. Here, the authors summarize the current understanding of the ASL, using reanalysis datasets to analyze recent variability and trends, as well as ice-core chemistry and climate model projections, to examine past and future changes in the ASL, respectively. The ASL has deepened in recent decades, affecting the climate through its influence on the regional meridional wind field, which controls the advection of moisture and heat into the continent. Deepening of the ASL in spring is consistent with observed West Antarctic warming and greater sea ice extent in the Ross Sea. Climate model simulations for recent decades indicate that this deepening is mediated by tropical variability while climate model projections through the twenty-first century suggest that the ASL will deepen in some seasons in response to greenhouse gas concentration increases
Encoded Recoupling and Decoupling: An Alternative to Quantum Error Correcting Codes, Applied to Trapped Ion Quantum Computation
A recently developed theory for eliminating decoherence and design
constraints in quantum computers, ``encoded recoupling and decoupling'', is
shown to be fully compatible with a promising proposal for an architecture
enabling scalable ion-trap quantum computation [D. Kielpinski et al., Nature
417, 709 (2002)]. Logical qubits are encoded into pairs of ions. Logic gates
are implemented using the Sorensen-Molmer (SM) scheme applied to pairs of ions
at a time. The encoding offers continuous protection against collective
dephasing. Decoupling pulses, that are also implemented using the SM scheme
directly to the encoded qubits, are capable of further reducing various other
sources of qubit decoherence, such as due to differential dephasing and due to
decohered vibrational modes. The feasibility of using the relatively slow SM
pulses in a decoupling scheme quenching the latter source of decoherence
follows from the observed 1/f spectrum of the vibrational bath.Comment: 12 pages, no figure
Some exact results for a trapped quantum gas at finite temperature
We present closed analytical expressions for the particle and kinetic energy
spatial densities at finite temperatures for a system of noninteracting
fermions (bosons) trapped in a d-dimensional harmonic oscillator potential. For
d=2 and 3, exact expressions for the N-particle densities are used to calculate
perturbatively the temperature dependence of the splittings of the energy
levels in a given shell due to a very weak interparticle interaction in a
dilute Fermi gas. In two dimensions, we obtain analytically the surprising
result that the |l|-degeneracy in a harmonic oscillator shell is not lifted in
the lowest order even when the exact, rather than the Thomas-Fermi expression
for the particle density is used. We also demonstrate rigorously (in two
dimensions) the reduction of the exact zero-temperature fermionic expressions
to the Thomas-Fermi form in the large-N limit.Comment: 14 pages, 4 figures include
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Measurements of cross sections and resonance structures following electron-impact excitation/ionization of Na-like Kr and Xe
We report high-resolution measurements of electron impact excitation and ionization cross sections for the Na-like ions Kr{sup 25+} and Xe{sup 43+}. Ions with ionization states centered on the Na-like configurations were produced in an electron beam ion trap (EBIT) using electrons with energies below the L shell ionization thresholds. The Na-like ions were exposed to an electron beam with an energy between 3 and 7 keV. The Na- and Ne-like ions were then extracted and their intensities measured as a function of the electron beam energy. Theoretical ionization cross sections were calculated using relativistic distorted wave methods. Complex resonance structures that appear in the computed cross sections are observed in the experimental results. These results are the first experimental observation of resonant-excitation-double-autoionization (REDA) in highly charged high-Z ions
Motion Robust Magnetic Susceptibility and Field Inhomogeneity Estimation Using Regularized Image Restoration Techniques for fMRI
In functional MRI, head motion may cause dynamic nonlinear field-inhomogeneity changes, especially with large out-of-plane rotations. This may lead to dynamic geometric distortion or blurring in the time series, which may reduce activation detection accuracy. The use of image registration to estimate dynamic field inhomogeneity maps from a static field map is not sufficient in the presence of such rotations. This paper introduces a retrospective approach to estimate magnetic susceptibility induced field maps of an object in motion, given a static susceptibility induced field map and the associated object motion parameters. It estimates a susceptibility map from a static field map using regularized image restoration techniques, and applies rigid body motion to the former. The dynamic field map is then computed using susceptibility voxel convolution. The method addresses field map changes due to out-of-plane rotations during time series acquisition and does not involve real time field map acquisitions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/85944/1/Fessler233.pd
Anisotropic conductivity of Nd_{1.85}Ce_{0.15}CuO_{4-\delta} films at submillimeter wavelengths
The anisotropic conductivity of thin NdCeCuO
films was measured in the frequency range 8 cm 40 cm and
for temperatures 4 K K. A tilted sample geometry allowed to extract
both, in-plane and c-axis properties. The in-plane quasiparticle scattering
rate remains unchanged as the sample becomes superconducting. The temperature
dependence of the in-plane conductivity is reasonably well described using the
Born limit for a d-wave superconductor. Below T_{{\rm C}%} the c-axis
dielectric constant changes sign at the screened c-axis plasma
frequency. The temperature dependence of the c-axis conductivity closely
follows the linear in T behavior within the plane.Comment: 4 pages, 4 figure
Precision spectroscopy with two correlated atoms
We discuss techniques that allow for long coherence times in laser
spectroscopy experiments with two trapped ions. We show that for this purpose
not only entangled ions prepared in decoherence-free subspaces can be used but
also a pair of ions that are not entangled but subject to the same kind of
phase noise. We apply this technique to a measurement of the electric
quadrupole moment of the 3d D5/2 state of 40Ca+ and to a measurement of the
linewidth of an ultrastable laser exciting a pair of 40Ca+ ions
Ideal Gases in Time-Dependent Traps
We investigate theoretically the properties of an ideal trapped gas in a
time-dependent harmonic potential. Using a scaling formalism, we are able to
present simple analytical results for two important classes of experiments:
free expansion of the gas upon release of the trap; and the response of the gas
to a harmonic modulation of the trapping potential is investigated. We present
specific results relevant to current experiments on trapped Fermions.Comment: 5 pages, 3 eps figure
Radiative and interelectronic-interaction corrections to the hyperfine splitting in highly charged B-like ions
The ground-state hyperfine splitting values of high-Z boronlike ions are
calculated. Calculation of the interelectronic-interaction contribution is
based on a combination of the 1/Z perturbation theory and the large-scale
configuration-interaction Dirac-Fock-Sturm method. The screened QED corrections
are evaluated utilizing an effective screening potential approach. Total
hyperfine splitting energies are presented for several B-like ions of
particular interest: {}^{45}Sc{}^{16+}, {}^{57}Fe{}^{21+}, {}^{207}Pb{}^{77+},
and {}^{209}Bi{}^{78+}. For lead and bismuth the experimental values of the 1s
hyperfine splitting are employed to improve significantly the theoretical
results by reducing the uncertainty due to the nuclear effects.Comment: 12 pages, 2 figures, 3 table
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