2,844 research outputs found
Simplified landscapes for optimization of shaken lattice interferometry
Motivated by recent results using shaken optical lattices to perform atom
interferometry, we explore splitting of an atom cloud trapped in a
phase-modulated ("shaken") optical lattice. Using a simple analytic model we
are able to show that we can obtain the simplest case of splitting via single-frequency shaking. This is confirmed both
via simulation and experiment. Furthermore, we are able to split with a
relative phase between the two split arms of or depending on
our shaking frequency. Addressing higher-order splitting, we determine that
splitting is sufficient to be able to accelerate the
atoms in counter-propagating lattices. Finally, we show that we can use a
genetic algorithm to optimize and splitting to within by restricting our
optimization to the resonance frequencies corresponding to single- and
two-photon transitions between Bloch bands
Star Formation in a Cosmological Simulation of Reionization
We study the luminosity functions of high-redshift galaxies in detailed
hydrodynamic simulations of cosmic reionization, which are designed to
reproduce the evolution of the Lyman-alpha forest between z=5 and z=6. We find
that the luminosity functions and total stellar mass densities are in agreement
with observations when plausible assumptions about reddenning at z=6 are made.
Our simulations support the conclusion that stars alone reionized the universe.Comment: Accepted for publication in Ap
Flow establishment in a generic scramjet combustor
The establishment of a quasi-steady flow in a generic scramjet combustor was studied for the case of a time varying inflow to the combustor. Such transient flow is characteristic of the reflected shock tunnel and expansion tube test facilities. Several numerical simulations of hypervelocity flow through a straight duct combustor with either a side wall step fuel injector or a centrally located strut injector are presented. Comparisons were made between impulsively started but otherwise constant flow conditions (typical of the expansion tube or tailored operations of the reflected shock tunnel) and the relaxing flow produced by the 'undertailored' operations of the reflected shock tunnel. Generally the inviscid flow features, such as the shock pattern and pressure distribution, were unaffected by the time varying inlet conditions and approached steady state in approx. the times indicated by experimental correlations. However, viscous features, such as heat transfer and skin friction, were altered by the relaxing inlet flow conditions
Escaping stars from young low-N clusters
With the use of N-body calculations the amount and properties of escaping
stars from low-N (N = 100 and 1000) young embedded star clusters prior to gas
expulsion are studied over the first 5 Myr of their existence. Besides the
number of stars also different initial radii and binary populations are
examined as well as virialised and collapsing clusters. It is found that these
clusters can loose substantial amounts (up to 20%) of stars within 5 Myr with
considerable velocities up to more than 100 km/s. Even with their mean
velocities between 2 and 8 km/s these stars will still be travelling between 2
and 30 pc during the 5 Myr. Therefore can large amounts of distributed stars in
star-forming regions not necessarily be counted as evidence for the isolated
formation of stars.Comment: 10 pages, 10 figures, accepted for publication by MNRA
Chemo-Archaeological Downsizing in a Hierarchical Universe: Impact of a Top Heavy IGIMF
We make use of a semi-analytical model of galaxy formation to investigate the
origin of the observed correlation between [a/Fe] abundance ratios and stellar
mass in elliptical galaxies. We implement a new galaxy-wide stellar initial
mass function (Top Heavy Integrated Galaxy Initial Mass Function, TH-IGIMF) in
the semi-analytic model SAG and evaluate its impact on the chemical evolution
of galaxies. The SFR-dependence of the slope of the TH-IGIMF is found to be key
to reproducing the correct [a/Fe]-stellar mass relation. Massive galaxies reach
higher [a/Fe] abundance ratios because they are characterized by more top-heavy
IMFs as a result of their higher SFR. As a consequence of our analysis, the
value of the minimum embedded star cluster mass and of the slope of the
embedded cluster mass function, which are free parameters involved in the
TH-IGIMF theory, are found to be as low as 5 solar masses and 2, respectively.
A mild downsizing trend is present for galaxies generated assuming either a
universal IMF or a variable TH-IGIMF. We find that, regardless of galaxy mass,
older galaxies (with formation redshifts > 2) are formed in shorter time-scales
(< 2 Gyr), thus achieving larger [a/Fe] values. Hence, the time-scale of galaxy
formation alone cannot explain the slope of the [a/Fe]-galaxy mass relation,
but is responsible for the big dispersion of [a/Fe] abundance ratios at fixed
stellar mass.We further test the hyphothesis of a TH-IGIMF in elliptical
galaxies by looking into mass-to-light ratios, and luminosity functions. Models
with a TH-IGIMF are also favoured by these constraints. In particular,
mass-to-light ratios agree with observed values for massive galaxies while
being overpredicted for less massive ones; this overprediction is present
regardless of the IMF considered.Comment: 24 pages, 15 figures, 2 tables. (Comments most welcome). Summited to
MNRA
Properties of hierarchically forming star clusters
We undertake a systematic analysis of the early (< 0.5 Myr) evolution of
clustering and the stellar initial mass function in turbulent fragmentation
simulations. These large scale simulations for the first time offer the
opportunity for a statistical analysis of IMF variations and correlations
between stellar properties and cluster richness. The typical evolutionary
scenario involves star formation in small-n clusters which then progressively
merge; the first stars to form are seeds of massive stars and achieve a
headstart in mass acquisition. These massive seeds end up in the cores of
clusters and a large fraction of new stars of lower mass is formed in the outer
parts of the clusters. The resulting clusters are therefore mass segregated at
an age of 0.5 Myr, although the signature of mass segregation is weakened
during mergers. We find that the resulting IMF has a smaller exponent
(alpha=1.8-2.2) than the Salpeter value (alpha=2.35). The IMFs in subclusters
are truncated at masses only somewhat larger than the most massive stars (which
depends on the richness of the cluster) and an universal upper mass limit of
150 Msun is ruled out. We also find that the simulations show signs of the
IGIMF effect proposed by Weidner & Kroupa, where the frequency of massive stars
is suppressed in the integrated IMF compared to the IMF in individual clusters.
We identify clusters through the use of a minimum spanning tree algorithm which
allows easy comparison between observational survey data and the predictions of
turbulent fragmentation models. In particular we present quantitative
predictions regarding properties such as cluster morphology, degree of mass
segregation, upper slope of the IMF and the relation between cluster richness
and maximum stellar mass. [abridged]Comment: 21 Pages, 25 Figure
Reinforcement Learning vs. Gradient-Based Optimisation for Robust Energy Landscape Control of Spin-1/2 Quantum Networks
We explore the use of policy gradient methods in reinforcement learning for
quantum control via energy landscape shaping of XX-Heisenberg spin chains in a
model agnostic fashion. Their performance is compared to finding controllers
using gradient-based L-BFGS optimisation with restarts, with full access to an
analytical model. Hamiltonian noise and coarse-graining of fidelity
measurements are considered. Reinforcement learning is able to tackle
challenging, noisy quantum control problems where L-BFGS optimization
algorithms struggle to perform well. Robustness analysis under different levels
of Hamiltonian noise indicates that controllers found by reinforcement learning
appear to be less affected by noise than those found with L-BFGS.Comment: 7 pages, 7 figure
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