Analysis of an atom laser based on the spatial control of the scattering length

In this paper we analyze atom lasers based on the spatial modulation of the scattering length of a Bose-Einstein Condensate. We demonstrate, through numerical simulations and approximate analytical methods, the controllable emission of matter-wave bursts and study the dependence of the process on the spatial dependence of the scattering length along the axis of emission. We also study the role of an additional modulation of the scattering length in time.Comment: Submitted to Phys. Rev.

Laser tweezers for atomic solitons

We describe a controllable and precise laser tweezers for Bose-Einstein condensates of ultracold atomic gases. In our configuration, a laser beam is used to locally modify the sign of the scattering length in the vicinity of a trapped BEC. The induced attractive interactions between atoms allow to extract and transport a controllable number of atoms. We analyze, through numerical simulations, the number of emitted atoms as a function of the width and intensity of the outcoupling beam. We also study different configurations of our system, as the use of moving beams. The main advantage of using the control laser beam to modify the nonlinear interactions in comparison to the usual way of inducing optical forces, i.e. through linear trapping potentials, is to improve the controllability of the outcoupled solitary wave-packet, which opens new possibilities for engineering macroscopic quantum states.Comment: 6 pages, 7 figure

Switching and instabilities of optical vortices in nonlinear dual-core photonic crystal fibre couplers

We study switching of an optical vortex launched into one core of a dual-core waveguide coupler in a photonic crystal fibre with self-focusing nonlinearity. We analyse how the beam power and the angular momentum associated with the vortex mode transfer to the second core of the coupler in both linear and nonlinear regimes. We describe three major scenarios of the vortex dynamics and reveal novel symmetry-breaking instabilities associated with the vortex nonzero angular momentum

Stabilized vortex solitons in layered Kerr media

In this letter we demonstrate the possibility of stabilizing beams with angular momentum propagating in Kerr media. Large propagation distances without filamentation can be achieved in layered media with alternating focusing and defocusing nonlinearities. Stronger stabilization can be obtained with the addition of an incoherent beam.Comment: 4 pages, 3 figures. We have removed the sentence "Thus, they erroneously point out to the existence of fully stabilized vortex solitons" in page 2, column 2, line 7-8, because it might be confusin

The influence of age, sex and visual dysfunction on reading ability in school-children

t. Binocular and accommodative dysfunctions affect the efficiency and coordination of the eyes, and this may impair reading performance. Reading can be evaluated using accuracy and speed measures. In this study, we searched for evidence that dysfunctions of accommodation (i.e. accommodation insufficiency, IA) and binocular vision (i.e. convergence insufficiency, CI) and also hyperopia have impact on reading ability. A sample of 292 children read a text following vision screening. The sample was composed of control children and children with the conditions: AI; CI and hyperopia. Multilinear regression analysis was used to evaluate the influence of variables sex, age and visual condition (control, AI, CI and hyperopia) on reading accuracy and speed. The results showed that ~7% of the variation in reading accuracy was explained by a model where age (p=0.001), and presence of AI (p<0.001) and hyperopia (p=0.019) were statistically significant. On the other hand, ~38% of the variation in the index of speed was explained by age alone (p<0.001). These results confirm the importance of age on children’s ability to read, and suggests that AI and hyperopia, but not CI may affect reading accuracy. Reading speed does not appear to be affected by the presence of binocular or accommodative dysfunction.This work was supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UID/FIS/04650/2013

Precision tests of QED and non-standard models by searching photon-photon scattering in vacuum with high power lasers

We study how to search for photon-photon scattering in vacuum at present petawatt laser facilities such as HERCULES, and test Quantum Electrodynamics and non-standard models like Born-Infeld theory or scenarios involving minicharged particles or axion-like bosons. First, we compute the phase shift that is produced when an ultra-intense laser beam crosses a low power beam, in the case of arbitrary polarisations. This result is then used in order to design a complete test of all the parameters appearing in the low energy effective photonic Lagrangian. In fact, we propose a set of experiments that can be performed at HERCULES, eventually allowing either to detect photon-photon scattering as due to new physics, or to set new limits on the relevant parameters, improving by several orders of magnitude the current constraints obtained recently by PVLAS collaboration. We also describe a multi-cross optical mechanism that can further enhance the sensitivity, enabling HERCULES to detect photon-photon scattering even at a rate as small as that predicted by QED. Finally, we discuss how these results can be improved at future exawatt facilities such as ELI, thus providing a new class of precision tests of the Standard Model and beyond.Comment: 15 pages, 2 figures. Corrected few mistakes in section and 4. Results unchanged. Added a referenc