5,847 research outputs found
Solovay-Kitaev Decomposition Strategy for Single-Qubit Channels
Inspired by the Solovay-Kitaev decomposition for approximating unitary
operations as a sequence of operations selected from a universal quantum
computing gate set, we introduce a method for approximating any single-qubit
channel using single-qubit gates and the controlled-NOT (CNOT). Our approach
uses the decomposition of the single-qubit channel into a convex combination of
"quasiextreme" channels. Previous techniques for simulating general
single-qubit channels would require as many as 20 CNOT gates, whereas ours only
needs one, bringing it within the range of current experiments
Neurobiological successor features for spatial navigation
The hippocampus has long been observed to encode a representation of an animal's position in space. Recent evidence suggests that the nature of this representation is somewhat predictive and can be modeled by learning a successor representation (SR) between distinct positions in an environment. However, this discretization of space is subjective making it difficult to formulate predictions about how some environmental manipulations should impact the hippocampal representation. Here, we present a model of place and grid cell firing as a consequence of learning a SR from a basis set of known neurobiological featuresâboundary vector cells (BVCs). The model describes place cell firing as the successor features of the SR, with grid cells forming a lowâdimensional representation of these successor features. We show that the place and grid cells generated using the BVCâSR model provide a good account of biological data for a variety of environmental manipulations, including dimensional stretches, barrier insertions, and the influence of environmental geometry on the hippocampal representation of space
CoreTSAR: Task Scheduling for Accelerator-aware Runtimes
Heterogeneous supercomputers that incorporate computational accelerators
such as GPUs are increasingly popular due to their high
peak performance, energy efficiency and comparatively low cost.
Unfortunately, the programming models and frameworks designed
to extract performance from all computational units still lack the
flexibility of their CPU-only counterparts. Accelerated OpenMP
improves this situation by supporting natural migration of OpenMP
code from CPUs to a GPU. However, these implementations currently
lose one of OpenMPâs best features, its flexibility: typical
OpenMP applications can run on any number of CPUs. GPU implementations
do not transparently employ multiple GPUs on a node
or a mix of GPUs and CPUs. To address these shortcomings, we
present CoreTSAR, our runtime library for dynamically scheduling
tasks across heterogeneous resources, and propose straightforward
extensions that incorporate this functionality into Accelerated
OpenMP. We show that our approach can provide nearly linear
speedup to four GPUs over only using CPUs or one GPU while
increasing the overall flexibility of Accelerated OpenMP
The Star Formation Law in Nearby Galaxies on Sub-Kpc Scales
(Abridged) We present a comprehensive analysis of the relationship between
star formation rate surface density (SFR SD) and gas surface density (gas SD)
at sub-kpc resolution in a sample of 18 nearby galaxies. We use high resolution
HI data from THINGS, CO data from HERACLES and BIMA SONG, 24 micron data from
the Spitzer Space Telescope, and UV data from GALEX. We target 7 spiral
galaxies and 11 late-type/dwarf galaxies and investigate how the star formation
law differs between the H2-dominated centers of spiral galaxies, their
HI-dominated outskirts and the HI-rich late-type/dwarf galaxies.
We find that a Schmidt-type power law with index N=1.0+-0.2 relates the SFR
SD and the H2 SD across our sample of spiral galaxies, i.e., that H2 forms
stars at a constant efficiency in spirals. The average molecular gas depletion
time is ~2*10^9 yrs. We interpret the linear relation and constant depletion
time as evidence that stars are forming in GMCs with approximately uniform
properties and that the H2 SD may be more a measure of the filling fraction of
giant molecular clouds than changing conditions in the molecular gas.
The relationship between total gas SD and SFR SD varies dramatically among
and within spiral galaxies. Most galaxies show little or no correlation between
the HI SD and the SFR SD. As a result, the star formation efficiency (SFE = SFR
SD / gas SD) varies strongly across our sample and within individual galaxies.
We show that in spirals the SFE is a clear function of radius, while the dwarf
galaxies in our sample display SFEs similar to those found in the outer optical
disks of the spirals. Another general feature of our sample is a sharp
saturation of the HI SD at ~9 M_sol/pc^2 in both the spiral and dwarf galaxies.Comment: Accepted for publication in the AJ special THINGS issue. For a
high-resolution version visit: http://www.mpia.de/THINGS/Publications.htm
Analysis of the Brinkman-Forchheimer equations with slip boundary conditions
In this work, we study the Brinkman-Forchheimer equations driven under slip
boundary conditions of friction type. We prove the existence and uniqueness of
weak solutions by means of regularization combined with the Faedo-Galerkin
approach. Next we discuss the continuity of the solution with respect to
Brinkman's and Forchheimer's coefficients. Finally, we show that the weak
solution of the corresponding stationary problem is stable
The unusual hydrocarbon emission from the early carbon star HD 100764: The connection between aromatics and aliphatics
We have used the Infrared Spectrograph (IRS) on the Spitzer Space Telescope
to obtain spectra of HD 100764, an apparently single carbon star with a
circumstellar disk. The spectrum shows emission features from polycyclic
aromatic hydrocarbons (PAHs) that are shifted to longer wavelengths than
normally seen, as characteristic of ``class C'' systems in the classification
scheme of Peeters et al. All seven of the known class C PAH sources are
illuminated by radiation fields that are cooler than those which typically
excite PAH emission features. The observed wavelength shifts are consistent
with hydrocarbon mixtures containing both aromatic and aliphatic bonds. We
propose that the class C PAH spectra are distinctive because the carbonaceous
material has not been subjected to a strong ultraviolet radiation field,
allowing relatively fragile aliphatic materials to survive.Comment: 11 pages (in emulateapj), 5 tables, 7 figures. Accepted for
publication in Ap
Money, Finance and Demography: The Consequences of Ageing
A significant ageing trend can be observed in Europe and in other parts of the world. Fertility is decreasing and life expectancy increasing. The impact of migration is growing. The book deals with the implications for financial markets of these demographic trends. Leading economists and financial experts from Europe and the United States evaluate the challenges to public pension systems and the private pension industry. Based on long-term projections of productivity and employment they look at potential growth in GDP per capita and implications for savings and wealth. Pension fund portfolio management is discussed together with the ability of capital markets to serve retirement-financing purposes. Fiscal as well as financial sustainability are analysed in depth. The roles of global imbalances and international capital movements are included. Most chapters also discuss policy implications - in particular with regard to how pension saving incentives and rules and incentives for retirement should be in order to ensure fiscal and financial sustainability. All contributions in the book are based on presentations at the 26th SUERF Colloquium on "Money, Finance and Demography - the Consequences of Ageing" held on 12-14 October, 2006 in Lisbon sponsored by Banco de Portugal and Millennium bcp and in cooperation with the Universidade Nova de Lisboa.
GALEX UV Color Relations for Nearby Early-Type Galaxies
We use GALEX/optical photometry to construct color-color relationships for
early-type galaxies sorted by morphological type. We have matched objects in
the GALEX GR1 public release and the first IR1.1 internal release, with the RC3
early-type galaxies having a morphological type -5.5<T<-1.5 with mean error in
T<1.5, and mean error on (B-V)T<0.05. After visual inspection of each match, we
are left with 130 galaxies with a reliable GALEX pipeline photometry in the
far-UV and near-UV bands. This sample is divided into Ellipticals (-5.5<T<-3.5)
and Lenticulars (-3.5<T<-1.5). After correction for the Galactic extinction,
the color-color diagrams FUV-NUV vs. (B-V)_{Tc} are plotted for the two
subsamples. We find a tight anti-correlation between the FUV-NUV and (B-V)_{Tc}
colors for Ellipticals, the UV color getting bluer when the (B-V)_{Tc} get
redder. This relationship very likely is an extension of the color-metallicity
relationship into the GALEX NUV band. We suspect that the main source of the
correlation is metal line blanketing in the NUV band. The FUV-NUV vs B-V
correlation has larger scatter for lenticular galaxies; we speculate this
reflects the presence of low level star formation. If the latter objects (i.e.
those that are blue both in FUV-NUV and B-V) are interpreted as harboring
recent star formation activity, this would be the case for a few percent (~4%)
of Ellipticals and ~15% of Lenticulars; this would make about 10% of early-type
galaxies with residual star formation in our full sample of 130 early-type
galaxies. We also plot FUV-NUV vs. the Mg_2 index and central velocity
dispersion. We find a tight anti-correlation between FUV-NUV and the Mg_2
index(...).Comment: 25 pages, 5 figures, accepted for publication in ApJS (abstract
abridged), typos corrected in section 2.
Mutual Influence Between Macrospin Reversal Order and Spin-Wave Dynamics in Isolated Artificial Spin-Ice Vertices
We theoretically and experimentally investigate magnetization reversal and associated spin-wave dynamics of isolated threefold vertices that constitute a Kagome lattice. The three permalloy macrospins making up the vertex have an elliptical cross section and a uniform thickness. We study the dc magnetization curve and the frequency versus field curves (dispersions) of those spin-wave modes that produce the largest response. We also investigate each macrospin reversal from a dynamic perspective, by performing micromagnetic simulations of the reversal processes, and revealing their relationships to the soft-mode profile calculated at the equilibrium state immediately before reversal. The theoretical results are compared with the measured magnetization curves and ferromagnetic resonance spectra. The agreement achieved suggests that a much deeper understanding of magnetization reversal and accompanying hysteresis can be achieved by combining theoretical calculations with static and dynamic magnetization experiments
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