75 research outputs found
Frequency-Domain Coherent Control of Femtosecond Two-Photon Absorption: Intermediate-Field vs. Weak-Field Regime
Coherent control of femtosecond two-photon absorption in the
intermediate-field regime is analyzed in detail in the powerful frequency
domain using an extended 4th-order perturbative description. The corresponding
absorption is coherently induced by the weak-field non-resonant two-photon
transitions as well as by four-photon transitions involving three absorbed
photons and one emitted photons. The interferences between these two groups of
transitions lead to a difference between the intermediate-field and weak-field
absorption dynamics. The corresponding interference nature (constructive or
destructive) strongly depends on the detuning direction of the pulse spectrum
from half the two-photon transition frequency. The model system of the study is
atomic sodium, for which both experimental and theoretical results are
obtained. The detailed understanding obtained here serves as a basis for
coherent control with rationally-shaped femtosecond pulses in a regime of
sizable absorption yields.Comment: 25 pages, 5 figure
Population redistribution in optically trapped polar molecules
We investigate the rovibrational population redistribution of polar molecules
in the electronic ground state induced by spontaneous emission and blackbody
radiation. As a model system we use optically trapped LiCs molecules formed by
photoassociation in an ultracold two-species gas. The population dynamics of
vibrational and rotational states is modeled using an ab-initio electric dipole
moment function and experimental potential energy curves. Comparison with the
evolution of the v"=3 electronic ground state yields good qualitative
agreement. The analysis provides important input to assess applications of
ultracold LiCs molecules in quantum simulation and ultracold chemistry.Comment: 6 pages, 5 figures, EPJD Topical issue on Cold Quantum Matter -
Achievements and Prospect
Experimental Implementation of the Deutsch-Jozsa Algorithm for Three-Qubit Functions using Pure Coherent Molecular Superpositions
The Deutsch-Jozsa algorithm is experimentally demonstrated for three-qubit
functions using pure coherent superpositions of Li rovibrational
eigenstates. The function's character, either constant or balanced, is
evaluated by first imprinting the function, using a phase-shaped femtosecond
pulse, on a coherent superposition of the molecular states, and then projecting
the superposition onto an ionic final state, using a second femtosecond pulse
at a specific time delay
Optimal control theory for unitary transformations
The dynamics of a quantum system driven by an external field is well
described by a unitary transformation generated by a time dependent
Hamiltonian. The inverse problem of finding the field that generates a specific
unitary transformation is the subject of study. The unitary transformation
which can represent an algorithm in a quantum computation is imposed on a
subset of quantum states embedded in a larger Hilbert space. Optimal control
theory (OCT) is used to solve the inversion problem irrespective of the initial
input state. A unified formalism, based on the Krotov method is developed
leading to a new scheme. The schemes are compared for the inversion of a
two-qubit Fourier transform using as registers the vibrational levels of the
electronic state of Na. Raman-like transitions through the
electronic state induce the transitions. Light fields are found
that are able to implement the Fourier transform within a picosecond time
scale. Such fields can be obtained by pulse-shaping techniques of a femtosecond
pulse. Out of the schemes studied the square modulus scheme converges fastest.
A study of the implementation of the qubit Fourier transform in the Na
molecule was carried out for up to 5 qubits. The classical computation effort
required to obtain the algorithm with a given fidelity is estimated to scale
exponentially with the number of levels. The observed moderate scaling of the
pulse intensity with the number of qubits in the transformation is
rationalized.Comment: 32 pages, 6 figure
Comparison of synthetic jet actuators based on sharp-edged and round-edged nozzles
Axisymmetric synthetic jet actuators based on a loudspeaker and on two types of flanged nozzles were tested and compared experimentally. The first type of the nozzle was a sharp-edged circular hole. The second one had a special design with fillets at inner and outer nozzle exit and with a small step in the middle of the nozzle. The function of the step was to prevent the flow reattachment during the extrusion stroke. The actuators with the two types of nozzles were operated at resonance and were compared first qualitatively using a simple phase locked flow visualization. Then the hot-wire anemometer was used to measure velocity distributions along nozzle axis and velocity profiles at the nozzle exit. Comparison of the nozzles was based on evaluation of the characteristic velocity and integral quantities (volumetric, momentum, and kinetic energy fluxes). It was found out that these quantities, which were evaluated at the nozzle exit, differ substantially for both nozzles. On the other hand the velocity flow field in farther distances from the nozzle exit area did not exhibit such prominent differences
Gameplay as a source of intrinsic motivation in a randomized controlled trial of auditory training
Background: Previous studies of frequency discrimination training (FDT) for tinnitus used repetitive task-based training programmes relying on extrinsic factors to motivate participation. Studies reported limited improvement in tinnitus symptoms.
Purpose: To evaluate FDT exploiting intrinsic motivations by integrating training with computer-gameplay.
Methods: Sixty participants were randomly assigned to train on a conventional taskbased training, or one of two interactive game-based training platforms over six weeks. Outcomes included assessment of motivation, tinnitus handicap, and performance on tests of attention.
Results: Participants reported greater intrinsic motivation to train on the interactive game-based platforms, yet compliance of all three groups was similar (~70%) and changes in self-reported tinnitus severity were not significant. There was no difference between groups in terms of change in tinnitus severity or performance on measures of attention.
Conclusion: FDT can be integrated within an intrinsically motivating game. Whilst this may improve participant experience, in this instance it did not translate to additional compliance or therapeutic benefit
Sensory theories of developmental dyslexia: three challenges for research.
Recent years have seen the publication of a range of new theories suggesting that the basis of dyslexia might be sensory dysfunction. In this Opinion article, the evidence for and against several prominent sensory theories of dyslexia is closely scrutinized. Contrary to the causal claims being made, my analysis suggests that many proposed sensory deficits might result from the effects of reduced reading experience on the dyslexic brain. I therefore suggest that longitudinal studies of sensory processing, beginning in infancy, are required to successfully identify the neural basis of developmental dyslexia. Such studies could have a powerful impact on remediation.This is the accepted manuscript. The final version is available from NPG at http://www.nature.com/nrn/journal/v16/n1/abs/nrn3836.html
Estratégias de enfrentamento em crianças em situação pré-cirúrgica: relação com idade, sexo, experiência com cirurgia e estresse
Curve crossing and branching ratios in the dissociative recombination of HD+
We present an experimental and theoretical study of the branching ratios in the dissociative recombination of HD+ with low energy electrons. The results give direct insight into the dynamics of the avoided curve crossing process between the dissociative state and the Rydberg series of the neutral molecule. Excellent agreement between the experimental results and the theory, based on a Landau-Zener formulation of the crossing process, is obtained
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