4,755 research outputs found
Shark Declines in the Mediterranean Sea
Summarizes a study of population and biomass trends of large sharks in the Mediterranean, and highlights the risk of some species becoming extinct as a result of unintended capture in fishing gear, targeted shark fishing, and human population pressure
Quantum teleportation of electrons in quantum wires with surface acoustic waves
We propose and numerically simulate a semiconductor device based on coupled
quantum wires, suitable for deterministic quantum teleportation of electrons
trapped in the minima of surface acoustic waves.We exploit a network of
interacting semiconductor quantum wires able to provide the universal set of
gates for quantum information processing, with the qubit defined by the
localization of a single electron in one of two coupled channels.The numerical
approach is based on a time-dependent solution of the three-particle
Schr\"odinger equation. First, a maximally entangled pair of electrons is
obtained via Coulomb interaction between carriers in different channels. Then,
a complete Bell-state measurement involving one electron from this pair and a
third electron is performed. Finally, the teleported state is reconstructed by
means of local one-qubit operations. The large estimated fidelity explicitely
suggests that an efficient teleportation process could be reached in an
experimental setup.Comment: 7 pages,4 figures, 1 tabl
Linear entropy as an entanglement measure in two-fermion systems
We describe an efficient theoretical criterion, suitable for
indistinguishable particles to quantify the quantum correlations of any pure
two-fermion state, based on the Slater rank concept. It represents the natural
generalization of the linear entropy used to treat quantum entanglement in
systems of non-identical particles. Such a criterion is here applied to an
electron-electron scattering in a two-dimensional system in order to perform a
quantitative evaluation of the entanglement dynamics for various spin
configurations and to compare the linear entropy with alternative approaches.
Our numerical results show the dependence of the entanglement evolution upon
the initial state of the system and its spin components. The differences with
previous analyses accomplished by using the von Neumann entropy are discussed.
The evaluation of the entanglement dynamics in terms of the linear entropy
results to be much less demanding from the computational point of view, not
requiring the diagonalization of the density matrix.Comment: 16 pages. Added references in section 1 Corrected typo
Recovering the unsigned photospheric magnetic field from Ca II K observations
We reassess the relationship between the photospheric magnetic field strength
and the Ca II K intensity for a variety of surface features as a function of
the position on the disc and the solar activity level. This relationship can be
used to recover the unsigned photospheric magnetic field from images recorded
in the core of Ca II K line. We have analysed 131 pairs of high-quality,
full-disc, near-co-temporal observations from SDO/HMI and Rome/PSPT spanning
half a solar cycle. To analytically describe the observationally-determined
relation, we considered three different functions: a power law with an offset,
a logarithmic function, and a power law function of the logarithm of the
magnetic flux density. We used the obtained relations to reconstruct maps of
the line-of-sight component of the unsigned magnetic field (unsigned
magnetograms) from Ca II K observations, which were then compared to the
original magnetograms. We find that both power-law functions represent the data
well, while the logarithmic function is good only for quiet periods. We see no
significant variation over the solar cycle or over the disc in the derived fit
parameters, independently of the function used. We find that errors in the
independent variable, usually not accounted for, introduce attenuation bias. To
address this, we binned the data with respect to the magnetic field strength
and Ca II K contrast separately and derived the relation for the bisector of
the two binned curves. The reconstructed unsigned magnetograms show good
agreement with the original ones. RMS differences are less than 90 G. The
results were unaffected by the stray-light correction of the SDO/HMI and
Rome/PSPT data. Our results imply that Ca~II~K observations, accurately
processed and calibrated, can be used to reconstruct unsigned magnetograms by
using the relations derived in our study.Comment: 18 pages, 22 figures, accepted in A&
Role of the impurity-potential range in disordered d-wave superconductors
We analyze how the range of disorder affects the localization properties of
quasiparticles in a two-dimensional d-wave superconductor within the standard
non-linear sigma-model approach to disordered systems. We show that for purely
long-range disorder, which only induces intra-node scattering processes, the
approach is free from the ambiguities which often beset the disordered
Dirac-fermion theories, and gives rise to a Wess-Zumino-Novikov-Witten action
leading to vanishing density of states and finite conductivities. We also study
the crossover induced by internode scattering due to a short range component of
the disorder, thus providing a coherent non-linear sigma-model description in
agreement with all the various findings of different approaches.Comment: 38 pages, 1 figur
Commercializing the laboratory: Faculty patenting and the open science environment
This paper investigates the relationship between patenting and publication of research results by university faculty members.
Our study adds to the limited evidence on this topic with an empirical investigation based on a panel data set for a broad sample of university researchers. Results suggest that publication and patenting are complementary, not substitute, activities for faculty members. This is not consistent with recent concerns regarding deleterious effects of patenting on the research output of faculty members. Average citations to publications, however, appear to decline for repeat patenters, suggesting either a decrease in quality or restrictions on use associated in patent protection
Junctions of one-dimensional quantum wires - correlation effects in transport
We investigate transport of spinless fermions through a single site dot
junction of M one-dimensional quantum wires. The semi-infinite wires are
described by a tight-binding model. Each wire consists of two parts: the
non-interacting leads and a region of finite extent in which the fermions
interact via a nearest-neighbor interaction. The functional renormalization
group method is used to determine the flow of the linear conductance as a
function of a low-energy cutoff for a wide range of parameters. Several fixed
points are identified and their stability is analyzed. We determine the scaling
exponents governing the low-energy physics close to the fixed points. Some of
our results can already be derived using the non-self-consistent Hartree-Fock
approximation.Comment: version accepted for publication in Phys. Rev. B, 14 pages, 7 figures
include
Thermodynamic identities and particle number fluctuations in weakly interacting Bose--Einstein condensates
We derive exact thermodynamic identities relating the average number of
condensed atoms and the root-mean-square fluctuations determined in different
statistical ensembles for the weakly interacting Bose gas confined in a box.
This is achieved by introducing the concept of {\it auxiliary partition
functions} for model Hamiltonians that do conserve the total number of
particles. Exploiting such thermodynamic identities, we provide the first,
completely analytical prediction of the microcanonical particle number
fluctuations in the weakly interacting Bose gas. Such fluctuations, as a
function of the volume V of the box are found to behave normally, at variance
with the anomalous scaling behavior V^{4/3} of the fluctuations in the ideal
Bose gas.Comment: 5 pages, 1 figur
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