Classical-Wigner Phase Space Approximation to Cumulative Matrix Elements in Coherent Control

The classical limit of the Wigner-Weyl representation is used to approximate products of bound-continuum matrix elements that are fundamental to many coherent control computations. The range of utility of the method is quantified through an examination of model problems, single-channel Na_2 dissociation and multi-arrangement channel photodissociation of CH_2IBr. Very good agreement with the exact quantum results is found for a wide range of system parameters.Comment: 17 pages, 12 figures, to appear in Journal of Chemical Physic

Vortex crystals

Vortex crystals is one name in use for the subject of vortex patterns that move without change of shape or size. Most of what is known pertains to the case of arrays of parallel line vortices moving so as to produce an essentially two-dimensional flow. The possible patterns of points indicating the intersections of these vortices with a plane perpendicular to them have been studied for almost 150 years. Analog experiments have been devised, and experiments with vortices in a variety of fluids have been performed. Some of the states observed are understood analytically. Others have been found computationally to high precision. Our degree of understanding of these patterns varies considerably. Surprising connections to the zeros of 'special functions' arising in classical mathematical physics have been revealed. Vortex motion on two-dimensional manifolds, such as the sphere, the cylinder (periodic strip) and torus (periodic parallelogram) has also been studied, because of the potential applications, and some results are available regarding the problem of vortex crystals in such geometries. Although a large amount of material is available for review, some results are reported here for the first time. The subject seems pregnant with possibilities for further development.published or submitted for publicationis peer reviewe

Single integrated device for optical CDMA code processing in dual-code environment

We report on the design, fabrication and performance of a matching integrated optical CDMA encoder-decoder pair based on holographic Bragg reflector technology. Simultaneous encoding/decoding operation of two multiple wavelength-hopping time-spreading codes was successfully demonstrated and shown to support two error-free OCDMA links at OC-24. A double-pass scheme was employed in the devices to enable the use of longer code length

On semi-classical pion production in heavy ion collisions

In high energy heavy ion collisions the pion multiplicity is large, and one might expect that pions are radiated semi-classically. The axial symmetry of the collision and approximately zero isotopic spin of the colliding nuclei result then in peculiar isotopic spin -- azimuth correlations of the produced pions. These correlations are easy to test -- and should be tested.Comment: Saclay-T93/06

A new peat bog testate amoeba transfer function and quantitative palaeohydrological reconstructions from southern Patagonia

Testate amoebae have been extensively used as proxies for environmental change and palaeoclimate reconstructions in European and North American peatlands. The presence of these micro-organisms near the peat surface is generally significantly linked to the local water table depth (WTD) and therefore preservation of the amoeba shells downcore allows for water table reconstructions over millennia. In the last decades, attention for the palaeoecology of the southern Patagonian peat bogs has increased, partly because of the particular climatological setting under the influence of the southern westerlies. These atypical peat bogs are characterised by a wide range of water tables, from wet hollows to hummocks exceeding 100 cm above the water table, and a dominance of Sphagnum magellanicum on low lawns up to the highest hummocks. Here we present the first transfer function for this region that allows for reliable WTD reconstructions, along with 2k-year palaeorecords from local peat bogs.A modern dataset (155 samples) was sampled along transects from five bogs in 2012 and 2013. Measurements of WTD, pH and conductivity were taken for all samples. Transfer function model was based on the 2012 dataset while the 2013 samples served as an independent test set to validate the model. Besides the standard leave-one- out cross-validation we applied leave-one-site-out and leave-one transect-out cross-validation, which are effective means of verifying the degree of clustering in the dataset. To assure the environmental gradient had been evenly sampled we quantified the root-mean-squared error of prediction (RMSEP) individually for segments of this gradient.Ordinations showed a clear hydrological gradient in amoeba assemblages, with the dominant Assulina muscorum at the dry end and Amphitrema wrightianum and Difflugia globulosa at the wet end. Taxa as Nebela certesi and Nebela cockayni, possibly exclusive to the southern hemisphere, were identified and their optima and tolerances were determined. Canonical correspondence analysis showed that WTD was the most important environmental variable, accounting for 18% of the variance in amoeba assemblages. A weighted averaging-partial least squares model showed best performance in cross-validation and using the 2013 data as an independent test set. Any spatial autocorrelation was minimal although the model still appeared less effective in predicting WTD for sites not included in the training set. The segment-wise RMSEP showed that the WTD gradient was generally evenly sampled with RMSEP below 15 cm for most of the gradient, much lower than the standard deviation of the mean of all WTDs (26 cm).Preliminary results from peat cores sampled from the same peat bogs show surprisingly stable water tables over the last 2k years in Andorra bog but more variation in nearby Tierra Australis bog. Peat accumulation rates in Andorra bog are among the highest recorded in temperate bogs with around 4 m of peat accumulated during the last 2000 year

A non-linear duality-invariant conformal extension of Maxwell's equations

All nonlinear extensions of the source-free Maxwell equations preserving both SO(2) electromagnetic duality invariance and conformal invariance are found, and shown to be limits of a one-parameter generalisation of Born-Infeld electrodynamics. The strong-field limit is the same as that found by Bialynicki-Birula from Born-Infeld theory but the weak-field limit is a new one-parameter extension of Maxwell electrodynamics, which is interacting but admits exact light-velocity plane-wave solutions of arbitrary polarisation. Small-amplitude waves on a constant uniform electromagnetic background exhibit birefringence, but one polarisation mode remains lightlike.Comment: 5 pages. v2 includes many simplifications and additional references, and additional material on birefringenc