4,788 research outputs found
Atmospheric studies of habitability in the Gliese 581 system
The M-type star Gliese 581 is orbited by at least one terrestrial planet
candidate in the habitable zone, i.e. GL 581 d. Orbital simulations have shown
that additional planets inside the habitable zone of GL 581 would be
dynamically stable. Recently, two further planet candidates have been claimed,
one of them in the habitable zone.
In view of the ongoing search for planets around M stars which is expected to
result in numerous detections of potentially habitable Super-Earths, we take
the GL 581 system as an example to investigate such planets. In contrast to
previous studies of habitability in the GL 581 system, we use a consistent
atmospheric model to assess surface conditions and habitability. Furthermore,
we perform detailed atmospheric simulations for a much larger subset of
potential planetary and atmospheric scenarios than previously considered.
A 1D radiative-convective atmosphere model is used to calculate temperature
and pressure profiles of model atmospheres, which we assumed to be composed of
molecular nitrogen, water, and carbon dioxide. In these calculations, key
parameters such as surface pressure and CO2 concentration as well as orbital
distance and planetary mass are varied.
Results imply that surface temperatures above freezing could be obtained,
independent of the here considered atmospheric scenarios, at an orbital
distance of 0.117 AU. For an orbital distance of 0.146 AU, CO2 concentrations
as low as 10 times the present Earth's value are sufficient to warm the surface
above the freezing point of water. At 0.175 AU, only scenarios with CO2
concentrations of 5% and 95% were found to be habitable. Hence, an additional
Super-Earth planet in the GL 581 system in the previously determined dynamical
stability range would be considered a potentially habitable planet.Comment: 5 pages, 4 figures, accepted in Astronomy&Astrophysic
Stationary entanglement in N-atom subradiant degenerate cascade systems
We address ultracold -atom degenerate cascade systems and show that
stationary subradiant states, already observed in the semiclassical regime,
also exist in a fully quantum regime and for a small number of atoms. We
explicitly evaluate the amount of stationary entanglement for the two-atom
configuration and show full inseparability for the three-atom case. We also
show that a continuous variable description of the systems is not suitable to
detect entanglement due to the nonGaussianity of subradiant states.Comment: 4 figure
Two quantum Simpson's paradoxes
The so-called Simpson's "paradox", or Yule-Simpson (YS) effect, occurs in
classical statistics when the correlations that are present among different
sets of samples are reversed if the sets are combined together, thus ignoring
one or more lurking variables. Here we illustrate the occurrence of two
analogue effects in quantum measurements. The first, which we term
quantum-classical YS effect, may occur with quantum limited measurements and
with lurking variables coming from the mixing of states, whereas the second,
here referred to as quantum-quantum YS effect, may take place when coherent
superpositions of quantum states are allowed. By analyzing quantum measurements
on low dimensional systems (qubits and qutrits), we show that the two effects
may occur independently, and that the quantum-quantum YS effect is more likely
to occur than the corresponding quantum-classical one. We also found that there
exist classes of superposition states for which the quantum-classical YS effect
cannot occur for any measurement and, at the same time, the quantum-quantum YS
effect takes place in a consistent fraction of the possible measurement
settings. The occurrence of the effect in the presence of partial coherence is
discussed as well as its possible implications for quantum hypothesis testing.Comment: published versio
Robust generation of entanglement in Bose-Einstein condensates by collective atomic recoil
We address the dynamics induced by collective atomic recoil in a
Bose-Einstein condensate in presence of radiation losses and atomic
decoherence. In particular, we focus on the linear regime of the lasing
mechanism, and analyze the effects of losses and decoherence on the generation
of entanglement. The dynamics is that of three bosons, two atomic modes
interacting with a single-mode radiation field, coupled with a bath of
oscillators. The resulting three-mode dissipative Master equation is solved
analytically in terms of the Wigner function. We examine in details the two
complementary limits of {\em high-Q cavity} and {\em bad-cavity}, the latter
corresponding to the so-called superradiant regime, both in the quasi-classical
and quantum regimes. We found that three-mode entanglement as well as two-mode
atom-atom and atom-radiation entanglement is generally robust against losses
and decoherence,thus making the present system a good candidate for the
experimental observation of entanglement in condensate systems. In particular,
steady-state entanglement may be obtained both between atoms with opposite
momenta and between atoms and photons
The extrasolar planet Gliese 581 d: a potentially habitable planet? (Corrigendum to arXiv:1009.5814)
We report here that the equation for H2O Rayleigh scattering was incorrectly
stated in the original paper [arXiv:1009.5814]. Instead of a quadratic
dependence on refractivity r, we accidentally quoted an r^4 dependence. Since
the correct form of the equation was implemented into the model, scientific
results are not affected.Comment: accepted to Astronomy&Astrophysic
Sub-shot-noise photon-number correlation in mesoscopic twin-beam of light
We demonstrate sub-shot-noise photon-number correlations in a (temporal)
multimode mesoscopic ( detected photons) twin-beam produced by
ps-pulsed spontaneous non-degenerate parametric downconversion. We have
separately detected the signal and idler distributions of photons collected in
twin coherence areas and found that the variance of the photon-count difference
goes below the shot-noise limit by 3.25 dB. The number of temporal modes
contained in the twin-beam, as well as the size of the twin coherence areas,
depends on the pump intensity. Our scheme is based on spontaneous
downconversion and thus does not suffer from limitations due to the finite gain
of the parametric process. Twin-beams are also used to demonstrate the
conditional preparation of a nonclassical (sub-Poissonian) state.Comment: 5 pages, 5 (low-res) figures, to appear on PR
The Counterpart Principle of Analogical Support by Structural Similarity
We propose and investigate an Analogy Principle in the context of Unary Inductive Logic based on a notion of support by structural similarity which is often employed to motivate scientific conjectures
Estimating precipitation on early Mars using a radiative-convective model of the atmosphere and comparison with inferred runoff from geomorphology
We compare estimates of atmospheric precipitation during the Martian
Noachian-Hesperian boundary 3.8 Gyr ago as calculated in a radiative-convective
column model of the atmosphere with runoff values estimated from a
geomorphological analysis of dendritic valley network discharge rates. In the
atmospheric model, we assume CO2-H2O-N2 atmospheres with surface pressures
varying from 20 mb to 3 bar with input solar luminosity reduced to 75% the
modern value.
Results from the valley network analysis are of the order of a few mm d-1
liquid water precipitation (1.5-10.6 mm d-1, with a median of 3.1 mm d-1).
Atmospheric model results are much lower, from about 0.001-1 mm d-1 of snowfall
(depending on CO2 partial pressure). Hence, the atmospheric model predicts a
significantly lower amount of precipitated water than estimated from the
geomorphological analysis. Furthermore, global mean surface temperatures are
below freezing, i.e. runoff is most likely not directly linked to
precipitation. Therefore, our results strongly favor a cold early Mars with
episodic snowmelt as a source for runoff.
Our approach is challenged by mostly unconstrained parameters, e.g.
greenhouse gas abundance, global meteorology (for example, clouds) and
planetary parameters such as obliquity- which affect the atmospheric result -
as as well as by inherent problems in estimating discharge and runoff on
ancient Mars, such as a lack of knowledge on infiltration and evaporation rates
and on flooding timescales, which affect the geomorphological data.
Nevertheless, our work represents a first step in combining and interpreting
quantitative tools applied in early Mars atmospheric and geomorphological
studies.Comment: accepted in Planetary and Space Science, 37 pages, 14 figures, 2
table
Clouds in the atmospheres of extrasolar planets. II. Thermal emission spectra of Earth-like planets influenced by low and high-level clouds
We study the impact of multi-layered clouds (low-level water and high-level
ice clouds) on the thermal emission spectra of Earth-like planets orbiting
different types of stars. Clouds have an important influence on such planetary
emission spectra due to their wavelength dependent absorption and scattering
properties. We also investigate the influence of clouds on the ability to
derive information about planetary surface temperatures from low-resolution
spectra.Comment: accepted for publication in A&
Remote state preparation and teleportation in phase space
Continuous variable remote state preparation and teleportation are analyzed
using Wigner functions in phase space. We suggest a remote squeezed state
preparation scheme between two parties sharing an entangled twin beam, where
homodyne detection on one beam is used as a conditional source of squeezing for
the other beam. The scheme works also with noisy measurements, and provide
squeezing if the homodyne quantum efficiency is larger than 50%. Phase space
approach is shown to provide a convenient framework to describe teleportation
as a generalized conditional measurement, and to evaluate relevant degrading
effects, such the finite amount of entanglement, the losses along the line, and
the nonunit quantum efficiency at the sender location.Comment: 2 figures, revised version to appear in J.Opt.
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