3,957 research outputs found
Experimental polarization encoded quantum key distribution over optical fibres with real-time continuous birefringence compensation
In this paper we demonstrate an active polarization drift compensation scheme
for optical fibres employed in a quantum key distribution experiment with
polarization encoded qubits. The quantum signals are wavelength multiplexed in
one fibre along with two classical optical side channels that provide the
control information for the polarization compensation scheme. This set-up
allows us to continuously track any polarization change without the need to
interrupt the key exchange. The results obtained show that fast polarization
rotations of the order of 40*pi rad/s are effectively compensated for. We
demonstrate that our set-up allows continuous quantum key distribution even in
a fibre stressed by random polarization fluctuations. Our results pave the way
for Bell-state measurements using only linear optics with parties separated by
long-distance optical fibres
Avaliação de biossegurança de transgênico por DGGE: estudo de caso do feijão VMDF, evento M1-4.
O objetivo deste estudo foi avaliar possíveis efeitos da modificação genética, evento M1-4, sobre organismos não-alvos, estudando a comunidade bacteriana rizosférica
Constraining planet structure and composition from stellar chemistry: trends in different stellar populations
The chemical composition of stars that have orbiting planets provides
important clues about the frequency, architecture, and composition of exoplanet
systems. We explore the possibility that stars from different galactic
populations that have different intrinsic abundance ratios may produce planets
with a different overall composition. We compiled abundances for Fe, O, C, Mg,
and Si in a large sample of solar neighbourhood stars that belong to different
galactic populations. We then used a simple stoichiometric model to predict the
expected iron-to-silicate mass fraction and water mass fraction of the planet
building blocks, as well as the summed mass percentage of all heavy elements in
the disc. Assuming that overall the chemical composition of the planet building
blocks will be reflected in the composition of the formed planets, we show that
according to our model, discs around stars from different galactic populations,
as well as around stars from different regions in the Galaxy, are expected to
form rocky planets with significantly different iron-to-silicate mass
fractions. The available water mass fraction also changes significantly from
one galactic population to another. The results may be used to set constraints
for models of planet formation and chemical composition. Furthermore, the
results may have impact on our understanding of the frequency of planets in the
Galaxy, as well as on the existence of conditions for habitability.Comment: Accepted for publication in Astronomy & Astrophysic
Distinguishing the albedo of exoplanets from stellar activity
Light curves show the flux variation from the target star and its orbiting
planets as a function of time. In addition to the transit features created by
the planets, the flux also includes the reflected light component of each
planet, which depends on the planetary albedo. This signal is typically
referred to as phase curve and could be easily identified if there were no
additional noise. As well as instrumental noise, stellar activity, such as
spots, can create a modulation in the data, which may be very difficult to
distinguish from the planetary signal. We analyze the limitations imposed by
the stellar activity on the detection of the planetary albedo, considering the
limitations imposed by the predicted level of instrumental noise and the short
duration of the observations planned in the context of the CHEOPS mission. As
initial condition, we have assumed that each star is characterized by just one
orbiting planet. We built mock light curves that included a realistic stellar
activity pattern, the reflected light component of the planet and an
instrumental noise level, which we have chosen to be at the same level as
predicted for CHEOPS. We then fit these light curves to try to recover the
reflected light component, assuming the activity patterns can be modeled with a
Gaussian process.We estimate that at least one full stellar rotation is
necessary to obtain a reliable detection of the planetary albedo. This result
is independent of the level of noise, but it depends on the limitation of the
Gaussian process to describe the stellar activity when the light curve
time-span is shorter than the stellar rotation. Finally, in presence of typical
CHEOPS gaps in the simulations, we confirm that it is still possible to obtain
a reliable albedo.Comment: Accepted for publication in A&A, 14 pages, 12 figure
Abundance trend with condensation temperature for stars with different Galactic birth places
During the past decade, several studies reported a correlation between
chemical abundances of stars and condensation temperature (also known as Tc
trend). However, the real astrophysical nature of this correlation is still
debated. The main goal of this work is to explore the possible dependence of
the Tc trend on stellar Galactocentric distances, Rmean. We used high-quality
spectra of about 40 stars observed with the HARPS and UVES spectrographs to
derive precise stellar parameters, chemical abundances, and stellar ages. A
differential line-by-line analysis was applied to achieve the highest possible
precision in the chemical abundances. We confirm previous results that [X/Fe]
abundance ratios depend on stellar age and that for a given age, some elements
also show a dependence on Rmean. When using the whole sample of stars, we
observe a weak hint that the Tc trend depends on Rmean. The observed dependence
is very complex and disappears when only stars with similar ages are
considered. To conclude on the possible dependence of the Tc trend on the
formation place of stars, a larger sample of stars with very similar
atmospheric parameters and stellar ages observed at different Galactocentric
distances is neededComment: Accepted by A&
Modelling the Recoherence of Mesoscopic Superpositions in Dissipative Environments
A model is presented to describe the recently proposed experiment (J.
Raimond,
M. Brune and S. Haroche Phys. Rev. Lett {\bf 79}, 1964 (1997)) where a
mesoscopic superposition of radiation states is prepared in a high-Q cavity
which is coupled to a similar resonator. The dynamical coherence loss of such
state in the absence of dissipation is reversible and can in principle be
observed. We show how this picture is modified due to the presence of the
environmental couplings. Analytical expressions for the experimental
conditional probabilities and the linear entropy are given. We conclude that
the phenomenon can still be observed provided the ratio between the damping
constant and the inter-cavities coupling does not exceed about a few percent.
This observation is favored for superpositions of states with large overlap.Comment: 13 pages, 6 figure
Avaliação das silagens do sorgo BRS-610 em sete estádios de maturação pela técnica in vitro semiautomática de produção de gases.
Avaliou-se a qualidade da silagem do sorgo BRS-610 produzida em sete estádios de maturação dos grãos, pela técnica in vitro semi automática de produção de gases. O delineamento foi inteiramente ao acaso, com quatro repetições por tratamento, e as médias foram comparadas pelo teste Scott-knott. Os parâmetros potencial máximo de produção de gases, tempo de colonização, taxa de produção de gases e degradabilidade efetiva da matéria seca para as taxas de passagem de 2, 5 e 8%/h foram obtidos pelo modelo de France et al. O potencial máximo de produção de gases não diferiu entre as silagens e indicou média de 170,5mLmg de matéria seca. Entre leitoso/pastoso e pastoso houve menor tempo de colonização. As maiores taxas de fermentação foram observadas nas silagens produzida entre os estádios pastoso/farináceo a seco. As degradabilidades efetivas foram semelhantes entres as silagens, com média de 46,2%. O sorgo BRS-610 produziu silagem de qualidade satisfatória, com destaque para a silagem produzida com a planta no estádio pastoso/farináceo, por associar melhor taxa de fermentação e menor tempo de colonização
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