2,176 research outputs found
Amplification and generation of ultra-intense twisted laser pulses via stimulated Raman scattering
Twisted Laguerre-Gaussian lasers, with orbital angular momentum and
characterised by doughnut shaped intensity profiles, provide a transformative
set of tools and research directions in a growing range of fields and
applications, from super-resolution microcopy and ultra-fast optical
communications to quantum computing and astrophysics. The impact of twisted
light is widening as recent numerical calculations provided solutions to
long-standing challenges in plasma-based acceleration by allowing for high
gradient positron acceleration. The production of ultrahigh intensity twisted
laser pulses could then also have a broad influence on relativistic
laser-matter interactions. Here we show theoretically and with ab-initio
three-dimensional particle-in-cell simulations, that stimulated Raman
backscattering can generate and amplify twisted lasers to Petawatt intensities
in plasmas. This work may open new research directions in non-linear optics and
high energy density science, compact plasma based accelerators and light
sources.Comment: 18 pages, 4 figures, 1 tabl
Optical spectrum of bottom-up graphene nanoribbons: towards efficient atom-thick excitonic solar cells
Recently, atomically well-defined cove-shaped graphene nanoribbons have been
obtained using bottom-up synthesis. These nanoribbons have an optical gap in
the visible range of the spectrum which make them candidates for donor
materials in photovoltaic devices. From the atomistic point of view, their
electronic and optical properties are not clearly understood. Therefore, in
this work we carry out ab-initio density functional theory calculations combine
with many-body perturbation formalism to study their electronic and optical
properties. Through the comparison with experimental measurements, we show that
an accurate description of the nanoribbon's optical properties requires the
inclusion of electron-hole correlation effects. The energy, binding energy and
the corresponding excitonic transitions involved are analyzed. We found that in
contrast to zigzag graphene nanoribbons, the excitonic peaks in the absorption
spectrum are a consequence of a group of transitions involving the first and
second conduction and valence bands. Finally, we estimate some relevant optical
properties that strengthen the potential of these nanoribbons for acting as a
donor materials in photovoltaic
Entanglement versus Quantum Discord in Two Coupled Double Quantum Dots
We study the dynamics of quantum correlations of two coupled double quantum
dots containing two excess electrons. The dissipation is included through the
contact with an oscillator bath. We solve the Redfield master equation in order
to determine the dynamics of the quantum discord and the entanglement of
formation. Based on our results, we find that the quantum discord is more
resistant to dissipation than the entanglement of formation for such a system.
We observe that this characteristic is related to whether the oscillator bath
is common to both qubits or not and to the form of the interaction Hamiltonian.
Moreover, our results show that the quantum discord might be finite even for
higher temperatures in the asymptotic limit.Comment: 14 pages, 8 figures (new version is the final version to appear in
NJP
Using continuous measurement to protect a universal set of quantum gates within a perturbed decoherence-free subspace
We consider a universal set of quantum gates encoded within a perturbed
decoherence-free subspace of four physical qubits. Using second-order
perturbation theory and a measuring device modeled by an infinite set of
harmonic oscillators, simply coupled to the system, we show that continuous
observation of the coupling agent induces inhibition of the decoherence due to
spurious perturbations. We thus advance the idea of protecting or even creating
a decoherence-free subspace for processing quantum information.Comment: 7 pages, 1 figure. To be published in Journal of Physics A:
Mathematical and Genera
Pressure and chemical substitution effects in the local atomic structure of BaFe2As2
The effects of K and Co substitutions and quasi-hydrostatic applied pressure
(P<9 GPa) in the local atomic structure of BaFe2As2, Ba(Fe{0.937}Co{0.063})2As2
and Ba{0.85}K{0.15}Fe2As2 superconductors were investigated by extended x-ray
absorption fine structure (EXAFS) measurements in the As K absorption edge. The
As-Fe bond length is found to be slightly reduced (<~ 0.01 Angstroms) by both
Co and K substitutions, without any observable increment in the corresponding
Debye Waller factor. Also, this bond is shown to be compressible (k =
3.3(3)x10^{-3} GPa^{-1}). The observed contractions of As-Fe bond under
pressure and chemical substitutions are likely related with a reduction of the
local Fe magnetic moments, and should be an important tuning parameter in the
phase diagrams of the Fe-based superconductors.Comment: 7 pages, 6 figure
Evaluation of Microencapsulation of The UFV-AREG1 Bacteriophage in Alginate-Ca Microcapsules using Microfluidic Devices
The indiscriminate use of antibiotics and the emergence of resistant
microorganisms have become a major challenge for the food industry. The purpose
of this work was to microencapsulate the bacteriophage UFV-AREG1 in a calcium
alginate matrix using microfluidic devices and to study the viability and
efficiency of retention. The microcapsules were added to gel of propylene
glycol for use as an antimicrobial in the food industry. The technique showed
the number of the phage encapsulation, yielding drops with an average 100-250
m of diameter, 82.1 2% retention efficiency and stability in the gel
matrix for 21 days. The gel added to the microencapsulated phage showed
efficiency (not detectable on the surface) in reducing bacterial contamination
on the surface at a similar level to antimicrobial chemicals (alcohol 70%).
Therefore, it was possible to microencapsulate bacteriophages in alginate-Ca
and apply the microcapsules in gels for use as sanitizers in the food industry.Comment: 8 pages, 5 figure
Plantas daninhas consumidas por bovinos em pastagem estabelecida em área de vegetação secundária no nordeste paraense.
Resumos do 24º Congresso Brasileiro da Ciência das Plantas Daninhas, 2004, São Pedro, PS
Avaliação de espécies leguminosas na formação de cafezais no segmento da agricultura familiar no Acre.
Leguminosas quando consorciadas com o café e usadas como adubação verde podem contribuir fornecendo nitrogênio e proteção ao solo pela adição de matéria orgânica. O trabalho foi conduzido com o objetivo de avaliar o efeito do uso de leguminosas no sistema de produção de café, no segmento de agricultura familiar, visando promover a implantação e a manutenção dos cafezais de forma técnica e economicamente sustentável. O experimento foi conduzido no perÃodo de novembro de 2000 a abril de 2003, em delineamento experimental de blocos ao acaso em esquema de parcelas subdivididas, com quatro repetições. Os tratamentos utilizados nas parcelas foram as leguminosas (Stizolobium atterrimum, Cajanus cajan, Canavalia endiformis e Flemingia congesta) e a testemunha (sem leguminosa) e, nas subparcelas, duas doses de N (0 e 22 g de N por cova). A Flemingia congesta e a Mucuna aterrima foram as leguminosas que mais influenciaram positivamente a produtividade dos cafeeiros, independente da adubação nitrogenada. Em relação à testemunha, o aumento em produção foi de 109% quando utilizou-se a Flemingia congesta e 52% com a Mucuna aterrima. A Flemingia congesta foi também a leguminosa que melhor controlou as invasoras, dado o volume de fitomassa produzida e a possibilidade de 2 cortes durante um perÃodo de doze meses, evidenciando o potencial desta leguminosa na formação de novos cafezais no Acre. Por outro lado, a Canavalia ensiformis, leguminosa que é normalmente utilizada nas entrelinhas dos cafeeiros pelos cafeicultores, do Acre e de outras regiões produtoras, neste trabalho influenciou negativamente a altura das plantas, diâmetro da copa e crescimento dos cafeeiros
Quantum Control of a Single Qubit
Measurements in quantum mechanics cannot perfectly distinguish all states and
necessarily disturb the measured system. We present and analyse a proposal to
demonstrate fundamental limits on quantum control of a single qubit arising
from these properties of quantum measurements. We consider a qubit prepared in
one of two non-orthogonal states and subsequently subjected to dephasing noise.
The task is to use measurement and feedback control to attempt to correct the
state of the qubit. We demonstrate that projective measurements are not optimal
for this task, and that there exists a non-projective measurement with an
optimum measurement strength which achieves the best trade-off between gaining
information about the system and disturbing it through measurement back-action.
We study the performance of a quantum control scheme that makes use of this
weak measurement followed by feedback control, and demonstrate that it realises
the optimal recovery from noise for this system. We contrast this approach with
various classically inspired control schemes.Comment: 12 pages, 7 figures, v2 includes new references and minor change
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