98 research outputs found
Sub-quadratic time for Riemann-Roch spaces. The case of smooth divisors over nodal plane projective curves
International audienceWe revisit the seminal Brill-Noether algorithm in the rather generic situation of smooth divisors over a nodal plane projective curve. Our approach takes advantage of fast algorithms for polynomials and structured matrices. We reach sub-quadratic time for computing a basis of a Riemann-Roch space. This improves upon previously known complexity bounds
The ‘Blueprint’ framework for career management skills: a critical exploration
This article examines the Blueprint framework for career management skills as it has been revealed across sequential implementations in the USA, Canada and Australia. It is argued that despite its lack of an empirical basis, the framework forms a useful and innovative means through which career theory, practice and policy can be connected. The framework comprises both core elements (learning areas, learning model and levels) and contextual elements (resources, community of practice, service delivery approach and policy connection). Each of these elements is explored
Hacia una web social libre y federada: el caso de Lorea
This paper tackles the limitations of web 2.0, the keys to overcome them and the recent efforts towards a free and federated social web. Between these we will focus on the case of Lorea from our direct experience with it.Este artículo aborda las limitaciones de la web 2.0, las claves para superarlas y las recientes iniciativas de web social libre y federada. De entre ellas nos centraremos en el caso de Lorea desde nuestra experiencia directa con ella
Optically induced coherent intra-band dynamics in disordered semiconductors
On the basis of a tight-binding model for a strongly disordered semiconductor
with correlated conduction- and valence band disorder a new coherent dynamical
intra-band effect is analyzed. For systems that are excited by two, specially
designed ultrashort light-pulse sequences delayed by tau relatively to each
other echo-like phenomena are predicted to occur. In addition to the inter-band
photon echo which shows up at exactly t=2*tau relative to the first pulse, the
system responds with two spontaneous intra-band current pulses preceding and
following the appearance of the photon echo. The temporal splitting depends on
the electron-hole mass ratio. Calculating the population relaxation rate due to
Coulomb scattering, it is concluded that the predicted new dynamical effect
should be experimentally observable in an interacting and strongly disordered
system, such as the Quantum-Coulomb-Glass.Comment: to be published in Physical Review B15 February 200
Directed current due to broken time-space symmetry
We consider the classical dynamics of a particle in a one-dimensional
space-periodic potential U(X) = U(X+2\pi) under the influence of a
time-periodic space-homogeneous external field E(t)=E(t+T). If E(t) is neither
symmetric function of t nor antisymmetric under time shifts , an ensemble of trajectories with zero current at t=0 yields a nonzero
finite current as . We explain this effect using symmetry
considerations and perturbation theory. Finally we add dissipation (friction)
and demonstrate that the resulting set of attractors keeps the broken symmetry
property in the basins of attraction and leads to directed currents as well.Comment: 2 figure
Theory of Fast Quantum Control of Exciton Dynamics in Semiconductor Quantum Dots
Optical techniques for the quantum control of the dynamics of multiexciton
states in a semiconductor quantum dot are explored in theory. Composite
bichromatic phase-locked pulses are shown to reduce the time of elementary
quantum operations on excitons and biexcitons by an order of magnitude or more.
Analytic and numerical methods of designing the pulse sequences are
investigated. Fidelity of the operation is used to gauge its quality. A
modified Quantum Fourier Transform algorithm is constructed with only Rabi
rotations and is shown to reduce the number of operations. Application of the
designed pulses to the algorithm is tested by a numerical simulation.Comment: 11 pages,5 figure
On Multi-Index Filtrations Associated to Weierstraß Semigroups
This paper is a survey on the main techniques involved in the computation of the Weierstraß semigroup at several points of curves defined over perfect fields, with special emphasis on the case of two points. Some hints about the usage of some packages of the computer algebra software Singular are also given; these are however only valid for curves defined over Fp with p a prime number
Coherent control using adaptive learning algorithms
We have constructed an automated learning apparatus to control quantum
systems. By directing intense shaped ultrafast laser pulses into a variety of
samples and using a measurement of the system as a feedback signal, we are able
to reshape the laser pulses to direct the system into a desired state. The
feedback signal is the input to an adaptive learning algorithm. This algorithm
programs a computer-controlled, acousto-optic modulator pulse shaper. The
learning algorithm generates new shaped laser pulses based on the success of
previous pulses in achieving a predetermined goal.Comment: 19 pages (including 14 figures), REVTeX 3.1, updated conten
Identification of competing ultrafast all-optical switching mechanisms in Si woodpile photonic crystals
We present a systematic study of ultrafast all-optical switching of Si
photonic band gap woodpile crystals using broadband tunable nondegenerate
pump-probe spectroscopy. At pump-probe coincidence, we investigate the behavior
the differential reflectivity at the blue edge of the stopband for a wide range
of pump- and probe frequencies. Both dispersive and absorptive features are
observed from the probe spectra at coincidence. As the pump frequency is tuned
through half the electronic bandgap of Si, the magnitude of both these features
increases. For the first time we unambiguously identify this dispersive effect
with the electronic Kerr effect in photonic crystals, and attribute the the
absorptive features to nondegenerate two photon absorption. The dispersive and
absorptive nonlinear coefficients are extracted, and are found to agree well
with literature. Finally, we propose a nondegenerate figure of merit (NFOM),
which defines the quality of switching for all nondegenerate optical switching
processes.Comment: Accepted by JOSA
Spintronics: Fundamentals and applications
Spintronics, or spin electronics, involves the study of active control and
manipulation of spin degrees of freedom in solid-state systems. This article
reviews the current status of this subject, including both recent advances and
well-established results. The primary focus is on the basic physical principles
underlying the generation of carrier spin polarization, spin dynamics, and
spin-polarized transport in semiconductors and metals. Spin transport differs
from charge transport in that spin is a nonconserved quantity in solids due to
spin-orbit and hyperfine coupling. The authors discuss in detail spin
decoherence mechanisms in metals and semiconductors. Various theories of spin
injection and spin-polarized transport are applied to hybrid structures
relevant to spin-based devices and fundamental studies of materials properties.
Experimental work is reviewed with the emphasis on projected applications, in
which external electric and magnetic fields and illumination by light will be
used to control spin and charge dynamics to create new functionalities not
feasible or ineffective with conventional electronics.Comment: invited review, 36 figures, 900+ references; minor stylistic changes
from the published versio
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