3,061 research outputs found
A sum-product theorem in function fields
Let be a finite subset of \ffield, the field of Laurent series in
over a finite field . We show that for any there
exists a constant dependent only on and such that
. In particular such a result is
obtained for the rational function field . Identical results
are also obtained for finite subsets of the -adic field for
any prime .Comment: Simplification of argument and note that methods also work for the
p-adic
Improved bounds on the set A(A+1)
For a subset A of a field F, write A(A + 1) for the set {a(b + 1):a,b\in A}.
We establish new estimates on the size of A(A+1) in the case where F is either
a finite field of prime order, or the real line.
In the finite field case we show that A(A+1) is of cardinality at least
C|A|^{57/56-o(1)} for some absolute constant C, so long as |A| < p^{1/2}. In
the real case we show that the cardinality is at least C|A|^{24/19-o(1)}. These
improve on the previously best-known exponents of 106/105-o(1) and 5/4
respectively
Quantum Process Tomography of the Quantum Fourier Transform
The results of quantum process tomography on a three-qubit nuclear magnetic
resonance quantum information processor are presented, and shown to be
consistent with a detailed model of the system-plus-apparatus used for the
experiments. The quantum operation studied was the quantum Fourier transform,
which is important in several quantum algorithms and poses a rigorous test for
the precision of our recently-developed strongly modulating control fields. The
results were analyzed in an attempt to decompose the implementation errors into
coherent (overall systematic), incoherent (microscopically deterministic), and
decoherent (microscopically random) components. This analysis yielded a
superoperator consisting of a unitary part that was strongly correlated with
the theoretically expected unitary superoperator of the quantum Fourier
transform, an overall attenuation consistent with decoherence, and a residual
portion that was not completely positive - although complete positivity is
required for any quantum operation. By comparison with the results of computer
simulations, the lack of complete positivity was shown to be largely a
consequence of the incoherent errors during the quantum process tomography
procedure. These simulations further showed that coherent, incoherent, and
decoherent errors can often be identified by their distinctive effects on the
spectrum of the overall superoperator. The gate fidelity of the experimentally
determined superoperator was 0.64, while the correlation coefficient between
experimentally determined superoperator and the simulated superoperator was
0.79; most of the discrepancies with the simulations could be explained by the
cummulative effect of small errors in the single qubit gates.Comment: 26 pages, 17 figures, four tables; in press, Journal of Chemical
Physic
Systematic Reviews Published in Emergency Medicine Journals Do Not Routinely Search Clinical Trials Registries: A Cross-Sectional Analysis
Publication bias compromises the validity of systematic reviews. This problem can be addressed in part through searching clinical trials registries to identify unpublished studies. This study aims to determine how often systematic reviews published in emergency medicine journals include clinical trials registry searches
Three-Dimensional Spectral Classification of Low-Metallicity Stars Using Artificial Neural Networks
We explore the application of artificial neural networks (ANNs) for the
estimation of atmospheric parameters (Teff, logg, and [Fe/H]) for Galactic F-
and G-type stars. The ANNs are fed with medium-resolution (~ 1-2 A) non
flux-calibrated spectroscopic observations. From a sample of 279 stars with
previous high-resolution determinations of metallicity, and a set of (external)
estimates of temperature and surface gravity, our ANNs are able to predict Teff
with an accuracy of ~ 135-150 K over the range 4250 <= Teff <= 6500 K, logg
with an accuracy of ~ 0.25-0.30 dex over the range 1.0 <= logg <= 5.0 dex, and
[Fe/H] with an accuracy ~ 0.15-0.20 dex over the range -4.0 <= [Fe/H] <= +0.3.
Such accuracies are competitive with the results obtained by fine analysis of
high-resolution spectra. It is noteworthy that the ANNs are able to obtain
these results without consideration of photometric information for these stars.
We have also explored the impact of the signal-to-noise ratio (S/N) on the
behavior of ANNs, and conclude that, when analyzed with ANNs trained on spectra
of commensurate S/N, it is possible to extract physical parameter estimates of
similar accuracy with stellar spectra having S/N as low as 13. Taken together,
these results indicate that the ANN approach should be of primary importance
for use in present and future large-scale spectroscopic surveys.Comment: 51 pages, 11 eps figures, uses aastex; to appear in Ap
Hadamard Products of Product Operators and the Design of Gradient-Diffusion Experiments for Simulating Decoherence by NMR Spectroscopy
An extension of the product operator formalism of NMR is introduced, which
uses the Hadamard matrix product to describe many simple spin 1/2 relaxation
processes. The utility of this formalism is illustrated by deriving NMR
gradient-diffusion experiments to simulate several decoherence models of
interest in quantum information processing, along with their Lindblad and Kraus
representations. Gradient-diffusion experiments are also described for several
more complex forms of decoherence, including the well-known collective
isotropic model. Finally, it is shown that the Hadamard formalism gives a
concise representation of decoherence with arbitrary correlations among the
fluctuating fields at the different spins involved, and that this can be
applied to both decoherence (T2) as well as nonadiabatic relaxation (T1)
processes.Comment: RevTeX, 11 page single-spaced preprint, no figures. Version two has
new title, abstract, introduction & conclusions, while the main body of the
text remains substantially the sam
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