7,524 research outputs found
Entangling two defects via a surrounding crystal
We theoretically show how two impurity defects in a crystalline structure can
be entangled through coupling with the crystal. We demonstrate this with a
harmonic chain of trapped ions in which two ions of a different species are
embedded. Entanglement is found for sufficiently cold chains and for a certain
class of initial, separable states of the defects. It results from the
interplay between localized modes which involve the defects and the interposed
ions, it is independent of the chain size, and decays slowly with the distance
between the impurities. These dynamics can be observed in systems exhibiting
spatial order, viable realizations are optical lattices, optomechanical
systems, or cavity arrays in circuit QED.Comment: 5 pages, 5 figure
Role for astroglia-derived BDNF and MSK1 in homeostatic synaptic plasticity
Homeostatic scaling of synaptic strength in response to environmental stimuli may underlie the beneficial effects of an active lifestyle on brain function. Our previous results highlighted a key role for brain-derived neurotrophic factor (BDNF) and mitogen- and stress-activated protein kinase 1 (MSK1) in experience-related homeostatic synaptic plasticity. Astroglia have recently been shown to serve as an important source of BDNF. To elucidate a role for astroglia-derived BDNF, we explored homeostatic synaptic plasticity in transgenic mice with an impairment in the BDNF/MSK1 pathway (MSK1 kinase dead knock-in (KD) mice) and impairment of glial exocytosis (dnSNARE mice). We observed that prolonged tonic activation of astrocytes caused BDNF-dependent upregulation of excitatory synaptic currents accompanied by enlargement of synaptic boutons. We found that exposure to environmental enrichment (EE) and caloric restriction (CR) strongly upregulated excitatory but downregulated inhibitory synaptic currents in old wild-type mice, thus counterbalancing the impact of ageing on synaptic transmission. In parallel, EE and CR enhanced astrocytic Ca2+-signalling. Importantly, we observed a significant deficit in the effects of EE and CR on synaptic transmission in the MSK1 KD and dnSNARE mice. Combined, our results strongly support the importance of astrocytic exocytosis of BDNF for the beneficial effects of EE and CR on synaptic transmission and plasticity in the ageing brain
Extremely reducing conditions reached during basaltic intrusion in organic matter-bearing sediments
International audienceRedox conditions in magma are widely interpreted as internally buffered and closely related to that of their mantle source regions. We use thermodynamic calculations to show that high-temperature interaction between magma and organic matter can lead to a dramatic reduction of the magma redox state, and significant departure from that of the original source. Field studies provide direct evidence of the process that we describe, with reported occurrences of graphite and native iron in igneous mafic rocks, implying very reducing conditions that are almost unknown in average terrestrial magmas. We calculate that the addition of 0.6 wt% organic matter (in the form of CH or CH2) to a standard basalt triggers graphite and native iron crystallisation at depths of few hundred meters. Interaction with organic matter also profoundly affects the abundance and the redox state of the gases in equilibrium with the magma, which are CO-dominated with H2 as the second most abundant species on a molar basis, H2O and CO2 being minor constituents. The assimilation of only 0.1 wt% organic matter by a basalt causes a decrease in its oxygen fugacity of 2-orders of magnitude. The assimilation of 0.6 wt% organic matter at depths < 500 m implies minimum CO content in the magma of 1 wt%, other gas components being less than 0.1 wt%. In the light of our calculations, we suggest that the production of native iron-bearing lava flows and associated intrusions was most likely accompanied by degassing of CO-rich gases, whose fluxes depended on the magma production rates
Titan Science with the James Webb Space Telescope (JWST)
The James Webb Space Telescope (JWST), scheduled for launch in 2018, is the
successor to the Hubble Space Telescope (HST) but with a significantly larger
aperture (6.5 m) and advanced instrumentation focusing on infrared science
(0.6-28.0 m ). In this paper we examine the potential for scientific
investigation of Titan using JWST, primarily with three of the four
instruments: NIRSpec, NIRCam and MIRI, noting that science with NIRISS will be
complementary. Five core scientific themes are identified: (i) surface (ii)
tropospheric clouds (iii) tropospheric gases (iv) stratospheric composition and
(v) stratospheric hazes. We discuss each theme in depth, including the
scientific purpose, capabilities and limitations of the instrument suite, and
suggested observing schemes. We pay particular attention to saturation, which
is a problem for all three instruments, but may be alleviated for NIRCam
through use of selecting small sub-arrays of the detectors - sufficient to
encompass Titan, but with significantly faster read-out times. We find that
JWST has very significant potential for advancing Titan science, with a
spectral resolution exceeding the Cassini instrument suite at near-infrared
wavelengths, and a spatial resolution exceeding HST at the same wavelengths. In
particular, JWST will be valuable for time-domain monitoring of Titan, given a
five to ten year expected lifetime for the observatory, for example monitoring
the seasonal appearance of clouds. JWST observations in the post-Cassini period
will complement those of other large facilities such as HST, ALMA, SOFIA and
next-generation ground-based telescopes (TMT, GMT, EELT).Comment: 50 pages, including 22 figures and 2 table
PIM: Video Coding using Perceptual Importance Maps
Human perception is at the core of lossy video compression, with numerous
approaches developed for perceptual quality assessment and improvement over the
past two decades. In the determination of perceptual quality, different
spatio-temporal regions of the video differ in their relative importance to the
human viewer. However, since it is challenging to infer or even collect such
fine-grained information, it is often not used during compression beyond
low-level heuristics. We present a framework which facilitates research into
fine-grained subjective importance in compressed videos, which we then utilize
to improve the rate-distortion performance of an existing video codec (x264).
The contributions of this work are threefold: (1) we introduce a web-tool which
allows scalable collection of fine-grained perceptual importance, by having
users interactively paint spatio-temporal maps over encoded videos; (2) we use
this tool to collect a dataset with 178 videos with a total of 14443 frames of
human annotated spatio-temporal importance maps over the videos; and (3) we use
our curated dataset to train a lightweight machine learning model which can
predict these spatio-temporal importance regions. We demonstrate via a
subjective study that encoding the videos in our dataset while taking into
account the importance maps leads to higher perceptual quality at the same
bitrate, with the videos encoded with importance maps preferred
over the baseline videos. Similarly, we show that for the 18 videos in test
set, the importance maps predicted by our model lead to higher perceptual
quality videos, preferred over the baseline at the same bitrate
Deformed Boost Transformations That Saturate at the Planck Scale
We derive finite boost transformations based on the Lorentz sector of the
bicross-product-basis -Poincare' Hopf albegra. We emphasize the role of
these boost transformations in a recently-proposed new relativistic theory. We
find that when the (dimensionful) deformation parameter is identified with the
Planck length, which together with the speed-of-light constant has the status
of observer-independent scale in the new relativistic theory, the deformed
boosts saturate at the value of momentum that corresponds to the inverse of the
Planck length.Comment: 6 pages, LaTex (revtex
Interplay between curvature and Planck-scale effects in astrophysics and cosmology
Several recent studies have considered the implications for astrophysics and
cosmology of some possible nonclassical properties of spacetime at the Planck
scale. The new effects, such as a Planck-scale-modified energy-momentum
(dispersion) relation, are often inferred from the analysis of some quantum
versions of Minkowski spacetime, and therefore the relevant estimates depend
heavily on the assumption that there could not be significant interplay between
Planck-scale and curvature effects. We here scrutinize this assumption, using
as guidance a quantum version of de Sitter spacetime with known Inonu-Wigner
contraction to a quantum Minkowski spacetime. And we show that, contrary to
common (but unsupported) beliefs, the interplay between Planck-scale and
curvature effects can be significant. Within our illustrative example, in the
Minkowski limit the quantum-geometry deformation parameter is indeed given by
the Planck scale, while in the de Sitter picture the parameter of quantization
of geometry depends both on the Planck scale and the curvature scalar. For the
much-studied case of Planck-scale effects that intervene in the observation of
gamma-ray bursts we can estimate the implications of "quantum spacetime
curvature" within robust simplifying assumptions. For cosmology at the present
stage of the development of the relevant mathematics one cannot go beyond
semiheuristic reasoning, and we here propose a candidate approximate
description of a quantum FRW geometry, obtained by patching together pieces
(with different spacetime curvature) of our quantum de Sitter. This
semiheuristic picture, in spite of its limitations, provides rather robust
evidence that in the early Universe the interplay between Planck-scale and
curvature effects could have been particularly significant.Comment: 26 pages
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