1435 research outputs found
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Quanto costano le lavorazioni SLS
No full textSUMMARY Reduction of stair stepping in selective laser sintering, based on the part curvature and process parameters
RIASSUNTO Il lavoro si propone di effettuare un'analisi preliminare del ruolo giocato dai parametri di processo nella stima dei costi delle lavorazioni SLS su sabbia rivestita e nylon.
Di conseguenza si valuterà quali saranno i parametri opportuni da modificare per ottimizzare la produzione dal punto di vista economico, inoltre si farà un'analisi qualitativa dei risultati delle modifiche
A Nonmonotone Proximal Bundle Method With (Potentially) Continuous Step Decisions
No full textWe discuss a numerical algorithm for minimization of a convex nondifferentiable function belonging to the family of proximal bundle methods. Unlike all of its brethren, the approach does not rely on measuring descent of the objective function at the so-called ``serious steps'', while ``null steps'' only serve at improving the descent direction in case of unsuccessful steps. Rather, a merit function is defined which is decreased at each iteration, leading to a (potentially) continuous choice of the stepsize between zero (the null step) and one (the serious step). By avoiding the discrete choice the convergence analysis is simplified, and we can more easily obtain efficiency estimates for the method. Simple choices for the step selection actually reproduce the dichotomic 0/1 behavior of standard proximal bundle methods, but shedding new light on the rationale behind the process, and ultimately with different rules. Yet, using nonlinear upper models of the function in the step selection process can lead to actual fractional steps
Kilohertz-driven Bose-Einstein condensates in optical lattices
No full textWe analyze time-of-flight absorption images obtained with dilute
Bose-Einstein con-densates released from shaken optical lattices, both
theoretically and experimentally. We argue that weakly interacting, ultracold
quantum gases in kilohertz-driven optical potentials constitute equilibrium
systems characterized by a steady-state distri-bution of Floquet-state
occupation numbers. Our experimental results consistently indicate that a
driven ultracold Bose gas tends to occupy a single Floquet state, just as it
occupies a single energy eigenstate when there is no forcing. When the driving
amplitude is sufficiently high, the Floquet state possessing the lowest mean
energy does not necessarily coincide with the Floquet state connected to the
ground state of the undriven system. We observe strongly driven Bose gases to
condense into the former state under such conditions, thus providing nontrivial
examples of dressed matter waves
Quantum dynamics and entanglement of a 1D Fermi gas released from a trap
No full textWe investigate the entanglement properties of the nonequilibrium dynamics of
one-dimensional noninteracting Fermi gases released from a trap. The gas of N
particles is initially in the ground state within hard-wall or harmonic traps,
then it expands after dropping the trap. We compute the time dependence of the
von Neumann and Renyi entanglement entropies and the particle fluctuations of
spatial intervals around the original trap, in the limit of a large number N of
particles. The results for these observables apply to one-dimensional gases of
impenetrable bosons as well.
We identify different dynamical regimes at small and large times, depending
also on the initial condition, whether it is that of a hard-wall or harmonic
trap. In particular, we analytically show that the expansion from hard-wall
traps is characterized by the asymptotic small-time behavior of the von Neumann entanglement entropy, and the relation
where V is the particle variance, which are analogous to
the equilibrium behaviors whose leading logarithms are essentially determined
by the corresponding conformal field theory with central charge . The time
dependence of the entanglement entropy of extended regions during the expansion
from harmonic traps shows the remarkable property that it can be expressed as a
global time-dependent rescaling of the space dependence of the initial
equilibrium entanglement entropy
FastFlow tutorial
No full textFastFlow is a structured parallel programming framework targeting shared memory multicore architectures. Its layered design and the optimized implementation of the communication mechanisms used to implement the FastFlow streaming networks provided to the application programmer as algorithmic skeletons support the development of efficient fine grain applications.FastFlow is available at http://sourceforge.net/projects/mc-fastflow/. This work introduces FastFlow programming techniques and points out the different ways used to parallelize existing C/C++ code using FastFlow as a software accelerator. In short: this is a kind of tutorial on FastFlow.<br /
Rovibrational cooling of molecules by optical pumping
No full textSUMMARY We demonstrate rotational and vibrational cooling of cesium dimers by optical
pumping techniques. We use two laser sources exciting all the populated
rovibrational states, except a target state that thus behaves like a dark state
where molecules pile up thanks to absorption-spontaneous emission cycles. We
are able to accumulate photoassociated cold Cs2 molecules in their absolute
ground state (v = 0, J = 0) with up to 40% efficiency. Given its simplicity,
the method could be extended to other molecules and molecular beams. It also
opens up general perspectives in laser cooling the external degrees of freedom
of molecules
Cooperative excitation and many-body interactions in a cold Rydberg gas
No full textThe dipole blockade of Rydberg excitations is a hallmark of the strong
interactions between atoms in these high-lying quantum states. One of the
consequences of the dipole blockade is the suppression of fluctuations in the
counting statistics of Rydberg excitations, of which some evidence has been
found in previous experiments. Here we present experimental results on the
dynamics and the counting statistics of Rydberg excitations of ultra-cold
Rubidium atoms both on and off resonance, which exhibit sub- and
super-Poissonian counting statistics, respectively. We compare our results with
numerical simulations using a novel theoretical model based on Dicke states of
Rydberg atoms including dipole-dipole interactions, finding good agreement
between experiment and theory
Descent and Penalization Techniques for Equilibrium Problems with Nonlinear Constraints
No full textThis paper deals with equilibrium problems with nonlinear constraints. Exploiting the gap function recently introduced by the authors, which rely on a polyhedral approximation of the feasible region, we propose two descent methods. They are both based on the minimization of a suitable exact penalty function, but they use different rules for updating the penalization parameter and they rely on different types of line search. The convergence of both algorithms is proved under standard assumptions
QCD simulations with staggered fermions on GPUs
No full textWe report on our implementation of the RHMC algorithm for the simulation of
lattice QCD with two staggered flavors on Graphics Processing Units, using the
NVIDIA CUDA programming language. The main feature of our code is that the GPU
is not used just as an accelerator, but instead the whole Molecular Dynamics
trajectory is performed on it. After pointing out the main bottlenecks and how
to circumvent them, we discuss the obtained performances. We present some
preliminary results regarding OpenCL and multiGPU extensions of our code and
discuss future perspectives