27,376 research outputs found
Climate Change in the High Andes:implications and adaptation strategies for small-scale farmers
Abstract: Global climate change represents a major threat to sustainable farming in the Andes. Farmers have used local ecological knowledge and intricate production systems to cope, adapt and reorganize to meet climate uncertainty and risk, which have always been a fact of life. Those traditional systems are generally highly resilient, but the predicted effects, rates and variability of climate change may push them beyond their range of adaptability. This article examines the extent of actual and potential impacts of climate variability and change on small-scale farmers in the highland Andes of Bolivia, Ecuador and Peru. It describes how climate change impacts agriculture through deglaciation, changes in hydrology, soil and pest and disease populations. The article highlights some promising adaptive strategies currently in use by or possible for producers, rural communities and local institutions to mitigate climate change effects while preserving the livelihoods and environmental and social sustainability of the regio
Perturbations with angular momentum of Robinson-Trautman spacetimes
We study the possible asymptotically flat perturbations of Robinson-Trautman
spacetimes. We differentiate between algebraically special perturbations and
general perturbations. The equations that determine physically realistic
spacetimes with angular momentum are presented.Comment: 23 pages, no figure
Properties of bright squeezed vacuum at increasing brightness
A bright squeezed vacuum (BSV) is a nonclassical macroscopic state of light, which is generated through high-gain parametric down-conversion or four-wave mixing. Although the BSV is an important tool in quantum optics and has a lot of applications, its theoretical description is still not complete. In particular, the existing description in terms of Schmidt modes with gain-independent shapes fails to explain the spectral broadening observed in the experiment as the mean number of photons increases. Meanwhile, the semiclassical description accounting for the broadening does not allow us to decouple the intermodal photon-number correlations. In this work, we present a new generalized theoretical approach to describe the spatial properties of a multimode BSV. In the multimode case, one has to take into account the complicated interplay between all involved modes: each plane-wave mode interacts with all other modes, which complicates the problem significantly. The developed approach is based on exchanging the (k, t ) and (ω, z) representations and solving a system of integrodifferential equations. Our approach predicts correctly the dynamics of the Schmidt modes and the broadening of the angular distribution with the increase in the BSV mean photon number due to a stronger pumping. Moreover, the model correctly describes various properties of a widely used experimental configuration with two crystals and an air gap between them, namely, an SU(1,1) interferometer. In particular, it predicts the narrowing of the intensity distribution, the reduction and shift of the side lobes, and the decline in the interference visibility as the mean photon number increases due to stronger pumping. The presented experimental results confirm the validity of the new approach. The model can be easily extended to the case of the frequency spectrum, frequency Schmidt modes, and other experimental configurations
Mid-infrared sources in the ELAIS Deep X-ray Survey
We present a cross‐correlation of the European Large Area Infrared Space Observatory (ISO) survey (ELAIS) with the ELAIS Deep X‐ray Survey of the N1 and N2 fields. There are seven Chandra point sources with matches in the ELAIS Final Analysis 15‐μm catalogue, out of a total of 28 extragalactic ISO sources present in the Chandra fields. Five of these are consistent with active galactic nuclei (AGN) giving an AGN fraction of ∼19 per cent in the 15‐μm flux range 0.8–6 mJy. We have co‐added the hard X‐ray fluxes of the individually undetected ISO sources and find a low significance detection consistent with star formation in the remaining population. We combine our point source cross‐correlation fraction with the XMM–Newton observations of the Lockman Hole and Chandra observations of the Hubble Deep Field North to constrain source count models of the mid‐infrared galaxy population. The low dust‐enshrouded AGN fraction in ELAIS implied by the number of cross‐identifications between the ELAIS mid‐infrared sample and the Chandra point sources is encouraging for the use of mid‐infrared surveys to constrain the cosmic star formation history, provided there are not further large undetected populations of Compton‐thick AGN
Information criteria for efficient quantum state estimation
Recently several more efficient versions of quantum state tomography have
been proposed, with the purpose of making tomography feasible even for
many-qubit states. The number of state parameters to be estimated is reduced by
tentatively introducing certain simplifying assumptions on the form of the
quantum state, and subsequently using the data to rigorously verify these
assumptions. The simplifying assumptions considered so far were (i) the state
can be well approximated to be of low rank, or (ii) the state can be well
approximated as a matrix product state. We add one more method in that same
spirit: we allow in principle any model for the state, using any (small) number
of parameters (which can, e.g., be chosen to have a clear physical meaning),
and the data are used to verify the model. The proof that this method is valid
cannot be as strict as in above-mentioned cases, but is based on
well-established statistical methods that go under the name of "information
criteria." We exploit here, in particular, the Akaike Information Criterion
(AIC). We illustrate the method by simulating experiments on (noisy) Dicke
states
The ELAIS Deep X-ray Survey
We present initial follow-up results of the ELAIS Deep X-ray Survey which is
being undertaken with the Chandra and XMM-Newton Observatories. 235 X-ray
sources are detected in our two 75 ks ACIS-I observations in the well-studied
ELAIS N1 and N2 areas. 90% of the X-ray sources are identified optically to
R=26 with a median magnitude of R=24. We show that objects which are unresolved
optically (i.e. quasars) follow a correlation between their optical and X-ray
fluxes, whereas galaxies do not. We also find that the quasars with fainter
optical counterparts have harder X-ray spectra, consistent with absorption at
both wavebands. Initial spectroscopic follow-up has revealed a large fraction
of high-luminosity Type 2 quasars. The prospects for studying the evolution of
the host galaxies of X-ray selected Type 2 AGN are considered.Comment: 9 pages, 5 figures, To appear in Proceedings of XXI Moriond
Conference: "Galaxy Clusters and the High Redshift Universe Observed in
X-rays", edited by D. Neumann, F.Durret, & J. Tran Thanh Va
The ELAIS deep X-ray survey - I. Chandra source catalogue and first results
We present an analysis of two deep (75 ks) Chandra observations of the European Large Area Infrared Space Observatory (ISO) Survey (ELAIS) fields N1 and N2 as the first results from the ELAIS deep X-ray survey. This survey is being conducted in well-studied regions with extensive multiwavelength coverage. Here we present the Chandra source catalogues along with an analysis of source counts, hardness ratios and optical classifications. A total of 233 X-ray point sources are detected in addition to two soft extended sources, which are found to be associated with galaxy clusters. An overdensity of sources is found in N1 with 30 per cent more sources than N2, which we attribute to large-scale structure. A similar variance is seen between other deep Chandra surveys. The source count statistics reveal an increasing fraction of hard sources at fainter fluxes. The number of galaxy-like counterparts also increases dramatically towards fainter fluxes, consistent with the emergence of a large population of obscured sources
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