672 research outputs found
Cell-wall polysaccharides play an important role in decay resistance of Sphagnum and actively depressed decomposition in vitro
Sphagnum-dominated peatlands head the list of ecosystems with the largest known reservoirs of organic carbon (C). The bulk of this C is stored in decomposition-resistant litter of one bryophyte genus: Sphagnum. Understanding how Sphagnum litter chemistry controls C mineralization is essential for understanding potential interactions between environmental changes and C mineralization in peatlands. We aimed to separate the effects of phenolics from structural polysaccharides on decay of Sphagnum. Wemeasured aerobic microbial respiration of different moss litter types in a lab. We used chemical treatments to step-wise remove the chemical compounds thought to be important in decay-resistance in three taxonomically distant moss genera. We also focused on the effect of Sphagnum-specific cell-wall pectin-like polysaccharides (sphagnan) on C and N mineralization. Removing polymeric lignin-like phenolics had only negligible effects on C mineralization of Sphagnum litter, but increased mineralization of two other bryophyte genera, suggesting a minor role of these phenolics in decay resistance of Sphagnum but a major role of cell-wall polysaccharides. Carboxyl groups of pectin-like polysaccharides represented a C-source in non-Sphagnum litters but resisted decay in Sphagnum. Finally, isolated sphagnan did not serve as C-source but inhibited C and N mineralization instead, reminiscent of the effects reported for phenolics in other ecosystems. Our results emphasize the role of polysaccharides in resistance to, and active inhibition of, microbial mineralization in Sphagnum-dominated litter. As the polysaccharides displayed decay-inhibiting properties hitherto associated with phenolics (lignin, polyphenols), it raises the question if polysaccharide- dominated litter also shares similar environmental controls on decomposition, such as temperature or nutrient and water availabilit
Non-Equilibrium Modeling of the Fe XVII 3C/3D ratio for an Intense X-ray Free Electron Laser
We present a review of two methods used to model recent LCLS experimental
results for the 3C/3D line intensity ratio of Fe XVII (Bernitt et al. 2012),
the time-dependent collisional-radiative method and the density-matrix
approach. These are described and applied to a two-level atomic system excited
by an X-ray free electron laser. A range of pulse parameters is explored and
the effects on the predicted Fe XVII 3C and 3D line intensity ratio are
calculated. In order to investigate the behavior of the predicted line
intensity ratio, a particular pair of A-values for the 3C and 3D transitions
was chosen (2.22 10 s and 6.02 10
s for the 3C and 3D, respectively), but our conclusions are independent
of the precise values. We also reaffirm the conclusions from Oreshkina et
al.(2014, 2015): the non-linear effects in the density matrix are important and
the reduction in the Fe XVII 3C/3D line intensity ratio is sensitive to the
laser pulse parameters, namely pulse duration, pulse intensity, and laser
bandwidth. It is also shown that for both models the lowering of the 3C/3D line
intensity ratio below the expected time-independent oscillator strength ratio
has a significant contribution due to the emission from the plasma after the
laser pulse has left the plasma volume. Laser intensities above W/cm are required for a reduction in the 3C/3D line intensity
ratio below the expected time independent oscillator strength ratio
High-rate, high-fidelity entanglement of qubits across an elementary quantum network
We demonstrate remote entanglement of trapped-ion qubits via a
quantum-optical fiber link with fidelity and rate approaching those of local
operations. Two Sr qubits are entangled via the polarization
degree of freedom of two photons which are coupled by high-numerical-aperture
lenses into single-mode optical fibers and interfere on a beamsplitter. A novel
geometry allows high-efficiency photon collection while maintaining unit
fidelity for ion-photon entanglement. We generate remote Bell pairs with
fidelity at an average rate (success
probability ).Comment: v2 updated to include responses to reviewers, as published in PR
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
A Quantitative Comparison of Opacities Calculated Using the Distorted- Wave and -Matrix Methods
The present debate on the reliability of astrophysical opacities has reached
a new climax with the recent measurements of Fe opacities on the Z-machine at
the Sandia National Laboratory \citep{Bailey2015}. To understand the
differences between theoretical results, on the one hand, and experiments on
the other, as well as the differences among the various theoretical results,
detailed comparisons are needed. Many ingredients are involved in the
calculation of opacities; deconstructing the whole process and comparing the
differences at each step are necessary to quantify their importance and impact
on the final results. We present here such a comparison using the two main
approaches to calculate the required atomic data, the -Matrix and
distorted-wave methods, as well as sets of configurations and coupling schemes
to quantify the effects on the opacities for the and ions.Comment: 10 pages, 2 figure
Multi-qubit gate with trapped ions for microwave and laser-based implementation
A proposal for a phase gate and a Mølmer–Sørensen gate in the dressed state basis is presented. In order to perform the multi-qubit interaction, a strong magnetic field gradient is required to couple the phonon-bus to the qubit states. The gate is performed using resonant microwave driving fields together with either a radio-frequency (RF) driving field, or additional detuned microwave driving fields. The gate is robust to ambient magnetic field fluctuations due to an applied resonant microwave driving field. Furthermore, the gate is robust to fluctuations in the microwave Rabi frequency and is decoupled from phonon dephasing due to a resonant RF or a detuned microwave driving field. This makes this new gate an attractive candidate for the implementation of high-fidelity microwave based multi-qubit gates. The proposal can also be realized in laser-based set-ups
A large-scale R-matrix calculation for electron-impact excitation of the Ne O-like ion
The five J levels within a or ground state complex provide
an excellent testing ground for the comparison of theoretical line ratios with
astrophysically observed values, in addition to providing valuable electron
temperature and density diagnostics. The low temperature nature of the line
ratios ensure that the theoretically derived values are sensitive to the
underlying atomic structure and electron-impact excitation rates. Previous
R-matrix calculations for the Ne O-like ion exhibit large spurious
structure in the cross sections at higher electron energies, which may affect
Maxwellian averaged rates even at low temperatures. Furthermore, there is an
absence of comprehensive excitation data between the excited states that may
provide newer diagnostics to compliment the more established lines discussed in
this paper. To resolve these issues, we present both a small scale 56-level
Breit-Pauli (BP) calculation and a large-scale 554 levels R-matrix Intermediate
Coupling Frame Transformation (ICFT) calculation that extends the scope and
validity of earlier JAJOM calculations both in terms of the atomic structure
and scattering cross sections. Our results provide a comprehensive
electron-impact excitation data set for all transitions to higher shells.
The fundamental atomic data for this O-like ion is subsequently used within a
collisional radiative framework to provide the line ratios across a range of
electron temperatures and densities of interest in astrophysical observations.Comment: 17 pages, 8 figure
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