2,996 research outputs found
Phase reconstruction of strong-field excited systems by transient-absorption spectroscopy
We study the evolution of a V-type three-level system, whose two resonances
are coherently excited and coupled by two ultrashort laser pump and probe
pulses, separated by a varying time delay. We relate the quantum dynamics of
the excited multi-level system to the absorption spectrum of the transmitted
probe pulse. In particular, by analyzing the quantum evolution of the system,
we interpret how atomic phases are differently encoded in the
time-delay-dependent spectral absorption profiles when the pump pulse either
precedes or follows the probe pulse. We experimentally apply this scheme to
atomic Rb, whose fine-structure-split 5s\,^2S_{1/2}\rightarrow 5p\,^2P_{1/2}
and 5s\,^2S_{1/2}\rightarrow 5p\,^2P_{3/2} transitions are driven by the
combined action of a pump pulse of variable intensity and a delayed probe
pulse. The provided understanding of the relationship between quantum phases
and absorption spectra represents an important step towards full time-dependent
phase reconstruction (quantum holography) of bound-state wave-packets in
strong-field light-matter interactions with atoms, molecules and solids.Comment: 5 pages, 4 figure
Numerical study of multilayer adsorption on fractal surfaces
We report a numerical study of van der Waals adsoprtion and capillary
condensation effects on self-similar fractal surfaces. An assembly of uncoupled
spherical pores with a power-law distributin of radii is used to model fractal
surfaces with adjustable dimensions. We find that the commonly used fractal
Frankel-Halsey-Hill equation systematically fails to give the correct dimension
due to crossover effects, consistent with the findings of recent experiments.
The effects of pore coupling and curvature dependent surface tension were also
studied.Comment: 11 pages, 3 figure
Chaotic exploration and learning of locomotion behaviours
We present a general and fully dynamic neural system, which exploits intrinsic chaotic dynamics, for the real-time goal-directed exploration and learning of the possible locomotion patterns of an articulated robot of an arbitrary morphology in an unknown environment. The controller is modeled as a network of neural oscillators that are initially coupled only through physical embodiment, and goal-directed exploration of coordinated motor patterns is achieved by chaotic search using adaptive bifurcation. The phase space of the indirectly coupled neural-body-environment system contains multiple transient or permanent self-organized dynamics, each of which is a candidate for a locomotion behavior. The adaptive bifurcation enables the system orbit to wander through various phase-coordinated states, using its intrinsic chaotic dynamics as a driving force, and stabilizes on to one of the states matching the given goal criteria. In order to improve the sustainability of useful transient patterns, sensory homeostasis has been introduced, which results in an increased diversity of motor outputs, thus achieving multiscale exploration. A rhythmic pattern discovered by this process is memorized and sustained by changing the wiring between initially disconnected oscillators using an adaptive synchronization method. Our results show that the novel neurorobotic system is able to create and learn multiple locomotion behaviors for a wide range of body configurations and physical environments and can readapt in realtime after sustaining damage
A Theory of Cheap Control in Embodied Systems
We present a framework for designing cheap control architectures for embodied
agents. Our derivation is guided by the classical problem of universal
approximation, whereby we explore the possibility of exploiting the agent's
embodiment for a new and more efficient universal approximation of behaviors
generated by sensorimotor control. This embodied universal approximation is
compared with the classical non-embodied universal approximation. To exemplify
our approach, we present a detailed quantitative case study for policy models
defined in terms of conditional restricted Boltzmann machines. In contrast to
non-embodied universal approximation, which requires an exponential number of
parameters, in the embodied setting we are able to generate all possible
behaviors with a drastically smaller model, thus obtaining cheap universal
approximation. We test and corroborate the theory experimentally with a
six-legged walking machine. The experiments show that the sufficient controller
complexity predicted by our theory is tight, which means that the theory has
direct practical implications. Keywords: cheap design, embodiment, sensorimotor
loop, universal approximation, conditional restricted Boltzmann machineComment: 27 pages, 10 figure
Ethics of Artificial Intelligence Demarcations
In this paper we present a set of key demarcations, particularly important
when discussing ethical and societal issues of current AI research and
applications. Properly distinguishing issues and concerns related to Artificial
General Intelligence and weak AI, between symbolic and connectionist AI, AI
methods, data and applications are prerequisites for an informed debate. Such
demarcations would not only facilitate much-needed discussions on ethics on
current AI technologies and research. In addition sufficiently establishing
such demarcations would also enhance knowledge-sharing and support rigor in
interdisciplinary research between technical and social sciences.Comment: Proceedings of the Norwegian AI Symposium 2019 (NAIS 2019),
Trondheim, Norwa
Impacts de l’utilisation des eaux polluées en agriculture urbaine sur la qualité de la nappe de Dakar (Sénégal)
L’agriculture urbaine de la région de Dakar est un secteur en plein essor. À cause de
la salinisation progressive des eaux de la nappe peu profonde (eaux de Céanes ), des eaux
usées brutes sont utilisées pour irriguer les champs. L’objectif de notre étude est
d’évaluer la qualité chimique et microbiologique des eaux de la nappe sous-jacentes aux
champs irrigués et d’identifier les sources de pollution. Notre travail a été effectué dans
les sites de Pikine et de Patte d’Oie.L’analyse de la qualité chimique des eaux d’arrosage a montré qu’à Pikine, la
conductivité des eaux de Céanes est plus élevée (4822±2411 µS cm-1) par rapport à celle des
eaux usées (3579±1242 µS cm-1 ; p<0.04). Par contre à Patte d’Oie, les eaux d’arrosage
sont moins salées (<3000 µS cm-1). La quantité d’azote total de tous les types d’eaux
d’arrosage est supérieure à la valeur guide de l’OMS (5-30 mg l-1).Salmonella spp. a été isolée dans 35 % des eaux d’arrosage. Un échantillon d’eaux
usées a été positif pour Vibrio cholerae.L’impact des eaux d’irrigation sur la qualité chimique et microbiologique de la nappe
d’eau souterraine est fortement influencé par la pluviométrie et est différent selon le site
considéré.Ce travail a montré que l’irrigation avec les eaux polluées et l’usage de fumiers
organiques peut altérer la qualité de la nappe et constituer des risques pour la
santé.In Dakar capital city of Senegal, the urban agriculture is in high expansion. Since
the progressive increase of salinity in the local groundwater (Céanes water), raw wastewater
is used to water the crops. The objective of this study is to assess chemical and
microbiological quality of the groundwater underlying the irrigated plots and to identify
the sources of pollution. This work was carried out in the sites of Pikine and Patte d’Oie
in Dakar Senegal.Chemical analysis of irrigation water showed that in Pikine, the conductivity of
Céanes water was higher (4822±2411 µS cm-1) than those of the wastewater (3579±1242 µS
cm-1 ; p<0.04), while at Patte d’Oie it was less salted (<3000 µS cm-1). The quantity
of total nitrogen of irrigation water in both sites was higher than the WHO’s threshold
(5-30 mg l-1).Salmonella spp. was isolated in 35 % of the irrigation water. One wastewater sample
was Vibrio cholerae positive.The impact of irrigation water on the chemical and microbiological quality of the
groundwater is strongly influenced by precipitations and is different according to the site
considered.This work showed that the use of polluted water and organic manures can spoil the
quality of the groundwater and constitute a health threat.Urban agricultur
miR-155 regulates differentiation of brown and beige adipocytes via a bistable circuit
Brown adipocytes are a primary site of energy expenditure and reside not only in classical brown adipose tissue but can also be found in white adipose tissue. Here we show that microRNA 155 is enriched in brown adipose tissue and is highly expressed in proliferating brown preadipocytes but declines after induction of differentiation. Interestingly, microRNA 155 and its target, the adipogenic transcription factor CCAAT/enhancer-binding protein beta, form a bistable feedback loop integrating hormonal signals that regulate proliferation or differentiation. Inhibition of microRNA 155 enhances brown adipocyte differentiation and induces a brown adipocyte-like phenotype ('browning') in white adipocytes. Consequently, microRNA 155-deficient mice exhibit increased brown adipose tissue function and 'browning' of white fat tissue. In contrast, transgenic overexpression of microRNA 155 in mice causes a reduction of brown adipose tissue mass and impairment of brown adipose tissue function. These data demonstrate that the bistable loop involving microRNA 155 and CCAAT/enhancer-binding protein b regulates brown lineage commitment, thereby, controlling the development of brown and beige fat cells
Two-Dimensional Spectral Interferometry using the Carrier-Envelope Phase
Two- and multi-dimensional spectroscopy is used in physics and chemistry to obtain structural and dynamical information that would otherwise be invisible by the projection into a one-dimensional data set such as a single emission or absorption spectrum. Here, we introduce a qualitatively new two-dimensional spectroscopy method by employing the carrier-envelope phase (CEP). Instead of measuring spectral vs. spectral information, the combined application of spectral interferometry and CEP control allows the measurement of otherwise inseparable temporal events on an attosecond time scale. As a specific example, we apply this general method to the case of attosecond pulse train generation, where it allows to separate contributions of three different sub-cycle electron quantum paths within one and the same laser pulse, resulting in a better physical understanding and quantification of the transition region between cutoff and plateau harmonics. The CEP-dependent separation in time between two full-cycle spaced attosecond pulses was determined to modulate by (54 +/- 16) attoseconds
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