494 research outputs found
Nuclear spin state narrowing via gate--controlled Rabi oscillations in a double quantum dot
We study spin dynamics for two electrons confined to a double quantum dot
under the influence of an oscillating exchange interaction. This leads to
driven Rabi oscillations between the --state and the
--state of the two--electron system. The width of the
Rabi resonance is proportional to the amplitude of the oscillating exchange. A
measurement of the Rabi resonance allows one to narrow the distribution of
nuclear spin states and thereby to prolong the spin decoherence time. Further,
we study decoherence of the two-electron states due to the hyperfine
interaction and give requirements on the parameters of the system in order to
initialize in the --state and to perform a
operation with unit fidelity.Comment: v1:9 pages, 1 figure; v2: 13 pages, 2 figures, added section on
measurement, to appear in Phys. Rev.
Entanglement transfer from electron spins to photons in spin light-emitting diodes containing quantum dots
We show that electron recombination using positively charged excitons in
single quantum dots provides an efficient method to transfer entanglement from
electron spins onto photon polarizations. We propose a scheme for the
production of entangled four-photon states of GHZ type. From the GHZ state, two
fully entangled photons can be obtained by a measurement of two photons in the
linear polarization basis, even for quantum dots with observable fine structure
splitting for neutral excitons and significant exciton spin decoherence.
Because of the interplay of quantum mechanical selection rules and
interference, maximally entangled electron pairs are converted into maximally
entangled photon pairs with unity fidelity for a continuous set of observation
directions. We describe the dynamics of the conversion process using a
master-equation approach and show that the implementation of our scheme is
feasible with current experimental techniques.Comment: 5 pages, 2 figures. v2: Extended scheme, revised version. v3: Minor
additions and extended title, published versio
Subjetividad y aprendizaje en los límites de la novela educativa. Una mirada sobre En busca del tiempo perdido , de Marcel Proust.
No es inusual que En busca del tiempo perdido (PROUST, 1990/1996) sea considerada, de una manera amplia, dentro de las "novelas de formación" o "educativas". Este tipo de caracterización no suele ser ingenua, ya que supone no sólo una eventual definición de género sino también una orientación de lectura o una preconcepción de análisis. En este trabajo voy a sostener que la obra mayor de Proust puede ser pensada como consumación y, a la vez, como quiebre de aquella forma de definirla. En consecuencia, se llevará adelante un pasaje del sentido más o menos clásico que suele atribuírsele dentro de las novelas educativas hacia las condiciones de su acabamiento o ruptura. Este desplazamiento permitirá pensar la cuestión de la constitución de la subjetividad desde un punto de vista diferente
Enseñar filosofía:: de la pregunta filosófica a la propuesta metodológica
En este trabajo me interesará abordar la cuestión de la enseñanza de la filosofía como un problema filosófico intentando superar la contraposición “producción-reproducción”, que condena a la didáctica filosófica a no ser más que un conjunto de técnicas facilitadoras de la comprensión de algunos contenidos filosóficos. Para ello será necesario volver sobre la estructura pedagógica que sostiene aquella contraposición, retomar algunas preguntas fundacionales (¿por qué filosofar?, ¿qué significa enseñar filosofía?) y, a partir de su reconceptualización, evaluar el lugar y la función que debemos atribuir a una metodología de la enseñanza filosófica
¿Para qué sirve la filosofía en la escuela?
La mayoría de las preguntas suelen resonar de manera muy diferente -lo sabemos- de acuerdo con quien las formula, cómo lo hace, a quien as dirige, o en qué momento o lugar las plantea. La pregunta "¿para qué sirve la filosofía?" es un buen ejemplo. No es extraño verla surgir, como inocente inquietud de un joven estudiante ante su profesor de filosofía, el primer día de clase, en una escuela secundaria cualquiera.
(Párrafo extraído del texto a modo de resumen)Departamento de Filosofí
Recent advances in exciton based quantum information processing in quantum dot nanostructures
Recent experimental developments in the field of semiconductor quantum dot
spectroscopy will be discussed. First we report about single quantum dot
exciton two-level systems and their coherent properties in terms of single
qubit manipulations. In the second part we report on coherent quantum coupling
in a prototype "two-qubit" system consisting of a vertically stacked pair of
quantum dots. The interaction can be tuned in such quantum dot molecule devices
using an applied voltage as external parameter.Comment: 37 pages, 15 figures, submitted to New Journal of Physics, focus
issue on Solid State Quantum Information, added reference
Spin-orbit interaction in three-dimensionally bounded semiconductor nanostructures
The structural inversion asymmetry-induced spin-orbit interaction of
conduction band electrons in zinc-blende and wurtzite semiconductor structures
is analysed allowing for a three-dimensional (3D) character of the external
electric field and variation of the chemical composition. The interaction,
taking into account all remote bands perturbatively, is presented with two
contributions: a heterointerface term and a term caused by the external
electric field. They have generally comparable strength and can be written in a
unified manner only for 2D systems, where they can partially cancel each other.
For quantum wires and dots composed of wurtzite semiconductors new terms
appear, absent in zinc-blende structures, which acquire the standard Rashba
form in 2D systems.Comment: 18 pages, 1 figur
Resonant tunnelling features in the transport spectroscopy of quantum dots
We present a review of features due to resonant tunnelling in transport
spectroscopy experiments on quantum dots and single donors. The review covers
features attributable to intrinsic properties of the dot as well as extrinsic
effects, with a focus on the most common operating conditions. We describe
several phenomena that can lead to apparently identical signatures in a bias
spectroscopy measurement, with the aim of providing experimental methods to
distinguish between their different physical origins. The correct
classification of the resonant tunnelling features is an essential requirement
to understand the details of the confining potential or predict the performance
of the dot for quantum information processing.Comment: 18 pages, 7 figures. Short review article submitted to
Nanotechnology, special issue on 'Quantum Science and Technology at the
Nanoscale
Age- and sex-related variations in platelet count in Italy: a proposal of reference ranges based on 40987 subjects' data
BACKGROUND AND OBJECTIVES: Although several studies demonstrated that platelet count is higher in women, decreases with age, and is influenced by genetic background, most clinical laboratories still use the reference interval 150-400×10(9) platelets/L for all subjects. The present study was to identify age- and sex-specific reference intervals for platelet count. METHODS: We analysed electronic records of subjects enrolled in three population-based studies that investigated inhabitants of seven Italian areas including six geographic isolates. After exclusion of patients with malignancies, liver diseases, or inherited thrombocytopenias, which could affect platelet count, reference intervals were estimated from 40,987 subjects with the non parametric method computing the 2.5° and 97.5° percentiles. RESULTS: Platelet count was similar in men and women until the age of 14, but subsequently women had steadily more platelets than men. The number of platelets decreases quickly in childhood, stabilizes in adulthood, and further decreases in oldness. The final result of this phenomenon is that platelet count in old age was reduced by 35% in men and by 25% in women compared with early infancy. Based on these findings, we estimated reference intervals for platelet count ×10(9)/L in children (176-452), adult men (141-362), adult women (156-405), old men (122-350) and, old women (140-379). Moreover, we calculated an extended reference interval that takes into account the differences in platelet count observed in different geographic areas. CONCLUSIONS: The age-, sex-, and origin-related variability of platelet count is very wide, and the patient-adapted reference intervals we propose change the thresholds for diagnosing both thrombocytopenia and thrombocytosis in Italy
Valley filter and valley valve in graphene
It is known that the lowest propagating mode in a narrow ballistic ribbon of
graphene may lack the twofold valley degeneracy of higher modes. Depending on
the crystallographic orientation of the ribbon axis, the lowest mode mixes both
valleys or lies predominantly in a single valley (chosen by the direction of
propagation). We show, using a tight-binding model calculation, that a
nonequilibrium valley polarization can be realized in a sheet of graphene, upon
injection of current through a ballistic point contact with zigzag edges. The
polarity can be inverted by local application of a gate voltage to the point
contact region. Two valley filters in series may function as an
electrostatically controlled ``valley valve'', representing a
zero-magnetic-field counterpart to the familiar spin valve.Comment: RevTeX, 4 pages, 5 figure
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