30 research outputs found
Nonclassical excitation and quantum interference in a three level atom
Non-classical properties and quantum interference (QI) in two-photon excitation of
a three level atom (|1〉), |2〉, |3〉) in a ladder configuration, illuminated by multiple
fields in non-classical (squeezed) and/or classical (coherent) states, is studied. Fundamentally
new effects associated with quantum correlations in the squeezed fields
and QI due to multiple excitation pathways have been observed. Theoretical studies
and extrapolations of these findings have revealed possible applications which are
far beyond any current capabilities, including ultrafast nonlinear mixing, ultrafast
homodyne detection and frequency metrology. The atom used throughout the experiments
was Cesium, which was magneto-optically trapped in a vapor cell to produce
a Doppler-free sample. For the first part of the work the |1〉 → |2〉 → |3〉 transition
(corresponding to the 6S1/2F = 4 → 6P3/2F' = 5 → 6D5/2F" = 6 transition) was
excited by using the quantum-correlated signal (Ɛs) and idler (Ɛi) output fields of a
subthreshold non-degenerate optical parametric oscillator, which was tuned so that
the signal and idler fields were resonant with the |1〉 → |2〉 and |2〉 → |3〉 transitions,
respectively. In contrast to excitation with classical fields for which the excitation
rate as a function of intensity has always an exponent greater than or equal to two,
excitation with squeezed-fields has been theoretically predicted to have an exponent
that approaches unity for small enough intensities. This was verified experimentally
by probing the exponent down to a slope of 1.3, demonstrating for the first time a
purely non-classical effect associated with the interaction of squeezed fields and atoms.
In the second part excitation of the two-photon transition by three phase coherent
fields Ɛ1 , Ɛ2 and Ɛ0, resonant with the dipole |1〉 → |2〉 and |2〉 → |3〉 and quadrupole
|1〉 → |3〉 transitions, respectively, is studied. QI in the excited state population is
observed due to two alternative excitation pathways. This is equivalent to nonlinear
mixing of the three excitation fields by the atom. Realizing that in the experiment
the three fields are spaced in frequency over a range of 25 THz, and extending this
scheme to other energy triplets and atoms, leads to the discovery that ranges up to
100's of THz can be bridged in a single mixing step. Motivated by these results,
a master equation model has been developed for the system and its properties have
been extensively studied
A global database of ant species abundances
What forces structure ecological assemblages? A key limitation to general insights about assemblage structure is the availability of data that are collected at a small spatial grain (local assemblages) and a large spatial extent (global coverage). Here, we present published and unpublished data from 51, 388 ant abundance and occurrence records of more than 2,693 species and 7,953 morphospecies from local assemblages collected at 4,212 locations around the world. Ants were selected because they are diverse and abundant globally, comprise a large fraction of animal biomass in most terrestrial communities, and are key contributors to a range of ecosystem functions. Data were collected between 1949 and 2014, and include, for each geo-referenced sampling site, both the identity of the ants collected and details of sampling design, habitat type, and degree of disturbance. The aim of compiling this data set was to provide comprehensive species abundance data in order to test relationships between assemblage structure and environmental and biogeographic factors. Data were collected using a variety of standardized methods, such as pitfall and Winkler traps, and will be valuable for studies investigating large-scale forces structuring local assemblages. Understanding such relationships is particularly critical under current rates of global change. We encourage authors holding additional data on systematically collected ant assemblages, especially those in dry and cold, and remote areas, to contact us and contribute their data to this growing data set
Climate mediates the effects of disturbance on ant assemblage structure
Many studies have focused on the impacts of climate change on biological assemblages, yet little is known about howclimate interacts with other major anthropogenic influences on biodiversity, such as habitat disturbance. Using a unique global database of 1128 local ant assemblages, we examined whether climate mediates the effects of habitat disturbance on assemblage structure at a global scale. Species richness and evenness were associated positively with temperature, and negatively with disturbance. However, the interaction among temperature, precipitation and disturbance shaped species richness and evenness. The effectwas manifested through a failure of species richness to increase substantially with temperature in transformed habitats at low precipitation. At low precipitation levels, evenness increased with temperature in undisturbed sites, peaked at medium temperatures in disturbed sites and remained low in transformed sites. In warmer climates with lower rainfall, the effects of increasing disturbance on species richness and evenness were akin to decreases in temperature of up to 98C. Anthropogenic disturbance and ongoing climate change may interact in complicated ways to shape the structure of assemblages, with hot, arid environments likely to be at greatest risk. © 2015 The Author(s) Published by the Royal Society. All rights reserved
Studien über die Markzellengenese bei der Bildung des roten Marks der Röhrenknochen in anämischen Zuständen
Experimentelle Untersuchungen über die Veränderungen der Nieren bei akuter Sublimatvergiftung
Anatomische Befunde bei einem männlichen Scheinzwitter und die Steinachsche Hypothese über Hermaphroditismus
Climate mediates the effects of disturbance on ant assemblage structure
Many studies have focused on the impacts of climate change on biological assemblages, yet little is known about how climate interacts with other major anthropogenic influences on biodiversity, such as habitat disturbance. Using a unique global database of 1128 local ant assemblages, we examined whether climate mediates the effects of habitat disturbance on assemblage structure at a global scale. Species richness and evenness were associated positively with temperature, and negatively with disturbance. However, the interaction among temperature, precipitation and disturbance shaped species richness and evenness. The effect was manifested through a failure of species richness to increase substantially with temperature in transformed habitats at low precipitation. At low precipitation levels, evenness increased with temperature in undisturbed sites, peaked at medium temperatures in disturbed sites and remained low in transformed sites. In warmer climates with lower rainfall, the effects of increasing disturbance on species richness and evenness were akin to decreases in temperature of up to 9°C. Anthropogenic disturbance and ongoing climate change may interact in complicated ways to shape the structure of assemblages, with hot, arid environments likely to be at greatestPeer reviewe