24,372 research outputs found
Photoassociation adiabatic passage of ultracold Rb atoms to form ultracold Rb_2 molecules
We theoretically explore photoassociation by Adiabatic Passage of two
colliding cold ^{85}Rb atoms in an atomic trap to form an ultracold Rb_2
molecule. We consider the incoherent thermal nature of the scattering process
in a trap and show that coherent manipulations of the atomic ensemble, such as
adiabatic passage, are feasible if performed within the coherence time window
dictated by the temperature, which is relatively long for cold atoms. We show
that a sequence of ~2*10^7 pulses of moderate intensities, each lasting ~750
ns, can photoassociate a large fraction of the atomic ensemble at temperature
of 100 microkelvin and density of 10^{11} atoms/cm^3. Use of multiple pulse
sequences makes it possible to populate the ground vibrational state. Employing
spontaneous decay from a selected excited state, one can accumulate the
molecules in a narrow distribution of vibrational states in the ground
electronic potential. Alternatively, by removing the created molecules from the
beam path between pulse sets, one can create a low-density ensemble of
molecules in their ground ro-vibrational state.Comment: RevTex, 23 pages, 9 figure
Improved sampling of the pareto-front in multiobjective genetic optimizations by steady-state evolution: a Pareto converging genetic algorithm
Previous work on multiobjective genetic algorithms has been focused on preventing genetic drift and the issue of convergence has been given little attention. In this paper, we present a simple steady-state strategy, Pareto Converging Genetic Algorithm (PCGA), which naturally samples the solution space and ensures population advancement towards the Pareto-front. PCGA eliminates the need for sharing/niching and thus minimizes heuristically chosen parameters and procedures. A systematic approach based on histograms of rank is introduced for assessing convergence to the Pareto-front, which, by definition, is unknown in most real search problems.
We argue that there is always a certain inheritance of genetic material belonging to a population, and there is unlikely to be any significant gain beyond some point; a stopping criterion where terminating the computation is suggested. For further encouraging diversity and competition, a nonmigrating island model may optionally be used; this approach is particularly suited to many difficult (real-world) problems, which have a tendency to get stuck at (unknown) local minima. Results on three benchmark problems are presented and compared with those of earlier approaches. PCGA is found to produce diverse sampling of the Pareto-front without niching and with significantly less computational effort
Incomplete Photonic Bandgap as Inferred from the Speckle Pattern of Scattered Light Waves
Motivated by recent experiments on intensity correlations of the waves
transmitted through disordered media, we demonstrate that the speckle pattern
from disordered photonic crystal with incomplete band-gap represents a
sensitive tool for determination the stop-band width. We establish the
quantitative relation between this width and the {\em angualar anisotropy} of
the intensity correlation function.Comment: 6 pages, 3 figure
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Modelling the polymer migration phenomena in DNA-laden flows
This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Cross-stream migration of macromolecules transported in a fluid flow is typically encountered in microfluidic applications. This experimentally observed phenomenon leads to a decrease of the nearwall macromolecule concentration which can be detrimental in applications relying on a high intensity of polymer reactions in the near-wall zone, such as DNA-based bio-sensors. Despite a significant body of
experimental, theoretical and numerical research, there is no consensus regarding the nature of this phenomenon. In this paper a meta-modelling approach for macromolecule motion in the flow is presented. It is demonstrated that the hydrodynamic interaction resulting from the incorporation of Saffman lift force, together with Faxen correction to Stokes drag causes migration of DNA molecules towards the middle of a
pressure driven micro-flow, which is in agreement with experimental observations. The results suggest that the migration can occur due to macromolecule-flow rather than macromolecule-wall interaction.This work has been supported in part by the European Commission under the 6th Framework Program (Project: DINAMICS, NMP4-CT-2007-026804
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