3,009 research outputs found
Generation and dynamics of optical beams with polarization singularities
We present a convenient method to generate vector beams of light having
polarization singularities on their axis, via partial spin-to-orbital angular
momentum conversion in a suitably patterned liquid crystal cell. The resulting
polarization patterns exhibit a C-point on the beam axis and an L-line loop
around it, and may have different geometrical structures such as \qo{lemon},
\qo{star}, and \qo{spiral}. Our generation method allows us to control the
radius of L-line loop around the central C-point. Moreover, we investigate the
free-air propagation of these fields across a Rayleigh range.Comment: 6 pages, 4 figures, appears on Optics Express
Violation of Leggett-type inequalities in the spin-orbit degrees of freedom of a single photon
We report the experimental violation of Leggett-type inequalities for a
hybrid entangled state of spin and orbital angular momentum of a single photon.
These inequalities give a physical criterion to verify the possible validity of
a class of hidden-variable theories, originally named "crypto non-local", that
are not excluded by the violation of Bell-type inequalities. In our case, the
tested theories assume the existence of hidden variables associated with
independent degrees of freedom of the same particle, while admitting the
possibility of an influence between the two measurements, i.e. the so-called
contextuality of observables. We observe a violation the Leggett inequalities
for a range of experimental inputs, with a maximum violation of seven standard
deviations, thus ruling out this class of hidden variable models with a high
confidence.Comment: 5 pages, 4 figure
Inherent electronic trap states in TiO2 nanocrystals: effect of saturation and sintering
We report a quantum mechanical investigation on the nature of electronic trap states in realistic models of
individual and sintered anatase TiO2
nanocrystals (NCs) of ca. 3 nm diameter. We find unoccupied
electronic states of lowest energy to be localized within the central part of the NCs, and to originate
from under-coordinated surface Ti atoms lying mainly at the edges between the (100) and (101) facets.
These localized states are found at about 0.3–0.4 eV below the fully delocalized conduction band states,
in good agreement with both electrochemical and spectro-electrochemical results. The overall DensityOf-States (DOS) below the conduction band (CB) can be accurately fitted to an exponential distribution
of states, in agreement with capacitance data. Water molecules adsorbed on the NC surface raise the
energy and reduce the number of localized states, thus modifying the DOS. As a possible origin of
additional trap states, we further investigated the oriented attachment of two TiO2
NCs at various
possible interfaces. For the considered models, we found only minor differences between the DOS of
two interacting NCs and those of the individual constituent NCs. Our results point at the presence of
inherent trap states even in perfectly stoichiometric and crystalline TiO2
NCs due to the unavoidable
presence of under-coordinated surface Ti(IV) ions at the (100) facets
Uncertainties and Perspectives on Forest Height Estimates by Sentinel-1 Interferometry
Forest height is a key parameter in forestry. SAR interferometry (InSAR) techniques have been extensively adopted to retrieve digital elevation models (DEM) to give a representation of the continuous variation of the Earth’s topography, including forests. Unfortunately, InSAR has been proven to fail over vegetation due to low coherence values; therefore, all phase unwrapping algorithms tend to avoid these areas, making InSAR-derived DEM over vegetation unreliable. In this work, a sensitivity analysis was performed with the aim of properly initializing the relevant operational parameters (baseline and multilooking factor) to maximize the theoretical accuracy of the height difference between the forest and reference point. Some scenarios were proposed to test the resulting “optimal values”, as estimated at the previous step. A simple model was additionally proposed and calibrated, aimed at predicting the optimal baseline value (and therefore image pair selection) for height uncertainty minimization. All our analyses were conducted using free available data from the Copernicus Sentinel-1 mission to support the operational transfer into the forest sector. Finally, the potential uncertainty affecting resulting height measures was quantified, showing that a value lower than 5 m can be expected once all user-dependent parameters (i.e., baseline, multilooking factor, temporal baseline) are properly tuned
Hardy's paradox tested in the spin-orbit Hilbert space of single photons
We test experimentally the quantum ``paradox'' proposed by Lucien Hardy in
1993 [Phys. Rev. Lett. 71, 1665 (1993)] by using single photons instead of
photon pairs. This is achieved by addressing two compatible degrees of freedom
of the same particle, namely its spin angular momentum, determined by the
photon polarization, and its orbital angular momentum, a property related to
the optical transverse mode. Because our experiment involves a single particle,
we cannot use locality to logically enforce non-contextuality, which must
therefore be assumed based only on the observables' compatibility. On the other
hand, our single-particle experiment can be implemented more simply and allows
larger detection efficiencies than typical two-particle ones, with a potential
future advantage in terms of closing the detection loopholes.Comment: 7 pages, 5 figures and 1 tabl
Dynamical moments reveal a topological quantum transition in a photonic quantum walk
Many phenomena in solid-state physics can be understood in terms of their
topological properties. Recently, controlled protocols of quantum walks are
proving to be effective simulators of such phenomena. Here we report the
realization of a photonic quantum walk showing both the trivial and the
non-trivial topologies associated with chiral symmetry in one-dimensional
periodic systems, as in the Su-Schrieffer-Heeger model of polyacetylene. We
find that the probability distribution moments of the walker position after
many steps behave differently in the two topological phases and can be used as
direct indicators of the quantum transition: while varying a control parameter,
these moments exhibit a slope discontinuity at the transition point, and remain
constant in the non-trivial phase. Extending this approach to higher
dimensions, different topological classes, and other typologies of quantum
phases may offer new general instruments for investigating quantum transitions
in such complex systems
A Possible Role of Copernicus Sentinel-2 Data to Support Common Agricultural Policy Controls in Agriculture
Farmers that intend to access Common Agricultural Policy (CAP) contributions must submit an application to the territorially competent Paying Agencies (PA). Agencies are called to verify consistency of CAP contributions requirements through ground campaigns. Recently, EU regulation (N. 746/2018) proposed an alternative methodology to control CAP applications based on Earth Observation data. Accordingly, this work was aimed at designing and implementing a prototype of service based on Copernicus Sentinel-2 (S2) data for the classification of soybean, corn, wheat, rice, and meadow crops. The approach relies on the classification of S2 NDVI time-series (TS) by “user-friendly” supervised classification algorithms: Minimum Distance (MD) and Random Forest (RF). The study area was located in the Vercelli province (NW Italy), which represents a strategic agricultural area in the Piemonte region. Crop classes separability proved to be a key factor during the classification process. Confusion matrices were generated with respect to ground checks (GCs); they showed a high Overall Accuracy (>80%) for both MD and RF approaches. With respect to MD and RF, a new raster layer was generated (hereinafter called Controls Map layer), mapping four levels of classification occurrences, useful for administrative procedures required by PA. The Control Map layer highlighted that only the eight percent of CAP 2019 applications appeared to be critical in terms of consistency between farmers’ declarations and classification results. Only for these ones, a GC was warmly suggested, while the 12% must be desirable and the 80% was not required. This information alone suggested that the proposed methodology is able to optimize GCs, making possible to focus ground checks on a limited number of fields, thus determining an economic saving for PA and/or a more effective strategy of controls
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