897 research outputs found
Photoionization of Co and electron-impact excitation of Co using the Dirac R-matrix method
Modelling of massive stars and supernovae (SNe) plays a crucial role in
understanding galaxies. From this modelling we can derive fundamental
constraints on stellar evolution, mass-loss processes, mixing, and the products
of nucleosynthesis. Proper account must be taken of all important processes
that populate and depopulate the levels (collisional excitation, de-excitation,
ionization, recombination, photoionization, bound-bound processes). For the
analysis of Type Ia SNe and core collapse SNe (Types Ib, Ic and II) Fe group
elements are particularly important. Unfortunately little data is currently
available and most noticeably absent are the photoionization cross-sections for
the Fe-peaks which have high abundances in SNe. Important interactions for both
photoionization and electron-impact excitation are calculated using the
relativistic Dirac Atomic -matrix Codes (DARC) for low ionization stages of
cobalt. All results are calculated up to photon energies of 45 eV and electron
energies up to 20 eV. The wavefunction representation of Co III has been
generated using GRASP0 by including the dominant 3d, 3d[4s, 4p],
3p3d and 3p3d configurations, resulting in 292 fine structure
levels. Electron-impact collision strengths and Maxwellian averaged effective
collision strengths across a wide range of astrophysically relevant
temperatures are computed for Co III. In addition, statistically weighted
level-resolved ground and metastable photoionization cross-sections are
presented for Co II and compared directly with existing work.Comment: 11 pages, 8 figures and 4 table
Probing Qubit Memory Errors at the Part-per-Million Level
Robust qubit memory is essential for quantum computing, both for near-term
devices operating without error correction, and for the long-term goal of a
fault-tolerant processor. We directly measure the memory error for
a Ca trapped-ion qubit in the small-error regime and find
for storage times t\lesssim50\,\mbox{ms}. This exceeds
gate or measurement times by three orders of magnitude. Using randomized
benchmarking, at t=1\,\mbox{ms} we measure ,
around ten times smaller than that extrapolated from the time,
and limited by instability of the atomic clock reference used to benchmark the
qubit.Comment: 8 pages, 5 figure
High-fidelity quantum logic gates using trapped-ion hyperfine qubits
We demonstrate laser-driven two-qubit and single-qubit logic gates with
fidelities 99.9(1)% and 99.9934(3)% respectively, significantly above the
approximately 99% minimum threshold level required for fault-tolerant quantum
computation, using qubits stored in hyperfine ground states of calcium-43 ions
held in a room-temperature trap. We study the speed/fidelity trade-off for the
two-qubit gate, for gate times between 3.8s and 520s, and develop a
theoretical error model which is consistent with the data and which allows us
to identify the principal technical sources of infidelity.Comment: 1 trap, 2 ions, 3 nines. Detailed write-up of arXiv:1406.5473
including single-qubit gate data als
Atomic data and spectral model for Fe III
peer reviewe
Single-photon single ionization of W ions: experiment and theory
Experimental and theoretical results are reported for photoionization of
Ta-like (W) tungsten ions. Absolute cross sections were measured in the
energy range 16 to 245 eV employing the photon-ion merged-beam setup at the
Advanced Light Source in Berkeley. Detailed photon-energy scans at 100 meV
bandwidth were performed in the 16 to 108 eV range. In addition, the cross
section was scanned at 50 meV resolution in regions where fine resonance
structures could be observed. Theoretical results were obtained from a
Dirac-Coulomb R-matrix approach. Photoionization cross section calculations
were performed for singly ionized atomic tungsten ions in their , =1/2, ground level and the associated
excited metastable levels with =3/2, 5/2, 7/2 and 9/2. Since the ion beams
used in the experiments must be expected to contain long-lived excited states
also from excited configurations, additional cross-section calculations were
performed for the second-lowest term, 5d^5 \; ^6{\rm S}_{J}, =5/2, and for
the F term, 5d^3 6s^2 \; ^4{\rm F}_{J}, with = 3/2, 5/2, 7/2 and 9/2.
Given the complexity of the electronic structure of W the calculations
reproduce the main features of the experimental cross section quite well.Comment: 23 pages, 7 figures, 1 table: Accepted for publication in J. Phys. B:
At. Mol. & Opt. Phy
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Timing is everything: Drivers of interannual variability in blue whale migration.
Blue whales need to time their migration from their breeding grounds to their feeding grounds to avoid missing peak prey abundances, but the cues they use for this are unknown. We examine migration timing (inferred from the local onset and cessation of blue whale calls recorded on seafloor-mounted hydrophones), environmental conditions (e.g., sea surface temperature anomalies and chlorophyll a), and prey (spring krill biomass from annual net tow surveys) during a 10 year period (2008-2017) in waters of the Southern California Region where blue whales feed in the summer. Colder sea surface temperature anomalies the previous season were correlated with greater krill biomass the following year, and earlier arrival by blue whales. Our results demonstrate a plastic response of blue whales to interannual variability and the importance of krill as a driving force behind migration timing. A decadal-scale increase in temperature due to climate change has led to blue whales extending their overall time in Southern California. By the end of our 10-year study, whales were arriving at the feeding grounds more than one month earlier, while their departure date did not change. Conservation strategies will need to account for increased anthropogenic threats resulting from longer times at the feeding grounds
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