377 research outputs found
Tripartite entanglement in parametric down-conversion with spatially-structured pump
Most investigations of multipartite entanglement have been concerned with
temporal modes of the electromagnetic field, and have neglected its spatial
structure. We present a simple model which allows to generate tripartite
entanglement between spatial modes by parametric down-conversion with two
symmetrically-tilted plane waves serving as a pump. The characteristics of this
entanglement are investigated. We also discuss the generalization of our scheme
to 2N+1-partite entanglement using 2N symmetrically-tilted plane pump waves.
Another interesting feature is the possibility of entanglement localization in
just two spatial modes.Comment: 6 pages, 2 figure
Experience in the exploitation of a large control system
Experience of a four-year exploitation of the large control system of the CERN PS accelerator complex is presented with special emphasis on the parameters which are very sensitive to the exploitation team productivity. The software tools suite used in this daily maintenance is described and a particular analysis of the power and benefits of advance software technology used for the architecture of this suite is explained. The integration of this suite in the Control System is presented, as well as its use in the Control System development phase. Some considerations of the potential benefit of an Object Oriented equipment access are outlined
Properties of Stationary Nonequilibrium States in the Thermostatted Lorentz Gas I: the One Particle System
We study numerically and analytically the properties of the stationary state
of a particle moving under the influence of an electric field \bE in a two
dimensional periodic Lorentz gas with the energy kept constant by a Gaussian
thermostat. Numerically the current appears to be a continuous function of
\bE whose derivative varies very irregularly, possibly in a discontinuous
manner. We argue for the non differentibility of the current as a function of
\bE utilizing a symbolic description of the dynamics based on the
discontinuities of the collision map. The decay of correlations and the
behavior of the diffusion constant are also investigated
Properties of Stationary Nonequilibrium States in the Thermostatted Periodic Lorentz Gas II: The many point particles system
We study the stationary nonequilibrium states of N point particles moving
under the influence of an electric field E among fixed obstacles (discs) in a
two dimensional torus. The total kinetic energy of the system is kept constant
through a Gaussian thermostat which produces a velocity dependent mean field
interaction between the particles. The current and the particle distribution
functions are obtained numerically and compared for small E with analytic
solutions of a Boltzmann type equation obtained by treating the collisions with
the obstacles as random independent scatterings. The agreement is surprisingly
good for both small and large N. The latter system in turn agrees with a self
consistent one particle evolution expected to hold in the limit of N going to
infinity.Comment: 14 pages, 9 figure
Representation and usage of knowledge for initialization of accelerator control equipment
A knowledge based application, called SETUP, to initialize and diagnose the CERN/PS accelerators' control equipment is described. The object model and the general features of control algorithms are presented, together with their relation to the knowledge description of the setting up of the system. The different ways of the integration of the SETUP in the control system are outlined
Doped ordered mesoporous carbons as novel, selective electrocatalysts for the reduction of nitrobenzene to aniline
Ordered mesoporous carbons (OMCs) doped with nitrogen, phosphorus or boron were synthesised through a two-step nanocasting method and studied as electrocatalysts for the reduction of nitrobenzene to aniline in a half-cell setup. The nature of the dopant played a crucial role in the electrocatalytic performance of the doped OMCs, which was monitored by LSV with a rotating disk electrode setup. The incorporation of boron generated the electrocatalysts with the highest kinetic current density, whereas the incorporation of phosphorus led to the lowest overpotential. Doping with nitrogen led to intermediate behaviour in terms of onset potential and kinetic current density, but provided the highest selectivity towards aniline, thus resulting in the most promising electrocatalyst developed in this study. Density functional theory calculations allowed explaining the observed difference in the onset potentials between the various doped OMCs, and indicated that both graphiticN and pyrdinic N can generate active sites in the N-doped electrocatalyst. A chronoamperometric experiment over N-doped OMC performed at -0.75 V vs. Fc/Fc(+) in an acidic environment, resulted in a conversion of 61% with an overall selectivity of 87% to aniline. These are the highest activity and selectivity ever reported for an electrocatalyst for the reduction of nitrobenzene to aniline, making N-doped OMC a promising candidate for the electrochemical cogeneration of this industrially relevant product and electricity in a fuel cell setup
Pulse-driven quantum dynamics beyond the impulsive regime
We review various unitary time-dependent perturbation theories and compare
them formally and numerically. We show that the Kolmogorov-Arnold-Moser
technique performs better owing to both the superexponential character of
correction terms and the possibility to optimize the accuracy of a given level
of approximation which is explored in details here. As an illustration, we
consider a two-level system driven by short pulses beyond the sudden limit.Comment: 15 pages, 5 color figure
Non-intersecting squared Bessel paths and multiple orthogonal polynomials for modified Bessel weights
We study a model of non-intersecting squared Bessel processes in the
confluent case: all paths start at time at the same positive value , remain positive, and are conditioned to end at time at . In
the limit , after appropriate rescaling, the paths fill out a
region in the -plane that we describe explicitly. In particular, the paths
initially stay away from the hard edge at , but at a certain critical
time the smallest paths hit the hard edge and from then on are stuck to
it. For we obtain the usual scaling limits from random matrix
theory, namely the sine, Airy, and Bessel kernels. A key fact is that the
positions of the paths at any time constitute a multiple orthogonal
polynomial ensemble, corresponding to a system of two modified Bessel-type
weights. As a consequence, there is a matrix valued
Riemann-Hilbert problem characterizing this model, that we analyze in the large
limit using the Deift-Zhou steepest descent method. There are some novel
ingredients in the Riemann-Hilbert analysis that are of independent interest.Comment: 59 pages, 11 figure
Influence of the Composition and Preparation of the Rotating Disk Electrode on the Performance of Mesoporous Electrocatalysts in the Alkaline Oxygen Reduction Reaction
We report a systematic study of the influence of the composition and preparation method of the electrocatalyst layer deposited on the rotating (ring) disk electrodes (RDE/RRDE) employed in the alkaline oxygen reduction reaction (ORR). To investigate and rationalize the generally underestimated role of these factors on the ORR performance of mesoporous electrocatalysts, we studied the activity and selectivity of nitrogen-doped ordered mesoporous carbon as a function of the loading of electrocatalyst and of binder, of the type of binder, and of the addition order of the components onto the electrode. The use of an anion-exchange polymer (Fumion FAA-3 (R)) as the binder instead of the commonly employed Nafion (R) increased the selectivity towards H2O2 and led to a lower kinetic current density. In addition, higher selectivity towards H2O was observed when increasing the loading of the catalyst and of the binder, although the latter resulted in a decreased kinetic current density. These results prove the crucial effect of the composition and preparation method of the layer deposited on the electrode on the ORR performance of the mesoporous electrocatalyst and can provide useful guidelines in view of the translation of the results of RDE studies to an alkaline fuel-cell setup
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