40 research outputs found
Effects of atomic interactions on Quantum Accelerator Modes
We consider the influence of the inclusion of interatomic interactions on the
delta-kicked accelerator model. Our analysis concerns in particular quantum
accelerator modes, namely quantum ballistic transport near quantal resonances.
The atomic interaction is modelled by a Gross-Pitaevskii cubic nonlinearity,
and we address both attractive (focusing) and repulsive (defocusing) cases. The
most remarkable effect is enhancement or damping of the accelerator modes,
depending on the sign of the nonlinear parameter. We provide arguments showing
that the effect persists beyond mean-field description, and lies within the
experimentally accessible parameter range.Comment: 4 pages, 6 figure
Arnol'd Tongues and Quantum Accelerator Modes
The stable periodic orbits of an area-preserving map on the 2-torus, which is
formally a variant of the Standard Map, have been shown to explain the quantum
accelerator modes that were discovered in experiments with laser-cooled atoms.
We show that their parametric dependence exhibits Arnol'd-like tongues and
perform a perturbative analysis of such structures. We thus explain the
arithmetical organisation of the accelerator modes and discuss experimental
implications thereof.Comment: 20 pages, 6 encapsulated postscript figure
Delocalized and Resonant Quantum Transport in Nonlinear Generalizations of the Kicked Rotor Model
We analyze the effects of a nonlinear cubic perturbation on the delta-Kicked
Rotor. We consider two different models, in which the nonlinear term acts
either in the position or in the momentum representation. We numerically
investigate the modifications induced by the nonlinearity in the quantum
transport in both localized and resonant regimes and a comparison between the
results for the two models is presented. Analyzing the momentum distributions
and the increase of the mean square momentum, we find that the quantum
resonances asymptotically are very stable with respect to the nonlinear
perturbation of the rotor's phase evolution. For an intermittent time regime,
the nonlinearity even enhances the resonant quantum transport, leading to
superballistic motion.Comment: 8 pages, 10 figures; to appear in Phys. Rev.
Stable Quantum Resonances in Atom Optics
A theory for stabilization of quantum resonances by a mechanism similar to
one leading to classical resonances in nonlinear systems is presented. It
explains recent surprising experimental results, obtained for cold Cesium atoms
when driven in the presence of gravity, and leads to further predictions. The
theory makes use of invariance properties of the system, that are similar to
those of solids, allowing for separation into independent kicked rotor
problems. The analysis relies on a fictitious classical limit where the small
parameter is {\em not} Planck's constant, but rather the detuning from the
frequency that is resonant in absence of gravity.Comment: 5 pages, 3 figure
Mathematically-Based Algorithms for Film Digital Restoration
Since its invention, cinema has become an important media of popular culture, becoming part of our historical memory. Unfortunately, films are subject to a fast decay and aging, especially when the conservation conditions are not appropriate. The decay process is irreversible, and the digitalization is becoming the most diffuse and suitable way to conserve and restore films.
Classic restoration software involves a significant human intervention and a work of supervision by qualified operators. Indeed, they require a frame-by-frame control and a further phase of manual cleaning. This workflow makes the restoration process expensive in terms of time and money, so that it can\u2019t be afford by small audiovisual archives.
The idea of our work is to provide a software which reduce the human intervention and that could be easily used even by small archives, being open source. Here we present, DustRemover, a semi-automatic software for digital film restoration
Nonlinearity effects in the kicked oscillator
The quantum kicked oscillator is known to display a remarkable richness of
dynamical behaviour, from ballistic spreading to dynamical localization. Here
we investigate the effects of a Gross Pitaevskii nonlinearity on quantum
motion, and provide evidence that the qualitative features depend strongly on
the parameters of the system.Comment: 4 pages, 5 figure
Effects of a nonlinear perturbation on dynamical tunneling in cold atoms
We perform a numerical analysis of the effects of a nonlinear perturbation on
the quantum dynamics of two models describing non-interacting cold atoms in a
standing wave of light with a periodical modulated amplitude . One model
is the driven pendulum, considered in ref.\cite{raiz1}, and the other is a
variant of the well-known Kicked Rotator Model. In absence of the nonlinear
perturbation, the system is invariant under some discrete symmetries and
quantum dynamical tunnelling between symmetric classical islands is found. The
presence of nonlinearity destroys tunnelling, breaking the symmetries of the
system. Finally, further consequences of nonlinearity in the kicked rotator
case are considered.Comment: 10 pages, 15 figure
Nonlinear Lattice Waves in Random Potentials
Localization of waves by disorder is a fundamental physical problem
encompassing a diverse spectrum of theoretical, experimental and numerical
studies in the context of metal-insulator transition, quantum Hall effect,
light propagation in photonic crystals, and dynamics of ultra-cold atoms in
optical arrays. Large intensity light can induce nonlinear response, ultracold
atomic gases can be tuned into an interacting regime, which leads again to
nonlinear wave equations on a mean field level. The interplay between disorder
and nonlinearity, their localizing and delocalizing effects is currently an
intriguing and challenging issue in the field. We will discuss recent advances
in the dynamics of nonlinear lattice waves in random potentials. In the absence
of nonlinear terms in the wave equations, Anderson localization is leading to a
halt of wave packet spreading.
Nonlinearity couples localized eigenstates and, potentially, enables
spreading and destruction of Anderson localization due to nonintegrability,
chaos and decoherence. The spreading process is characterized by universal
subdiffusive laws due to nonlinear diffusion. We review extensive computational
studies for one- and two-dimensional systems with tunable nonlinearity power.
We also briefly discuss extensions to other cases where the linear wave
equation features localization: Aubry-Andre localization with quasiperiodic
potentials, Wannier-Stark localization with dc fields, and dynamical
localization in momentum space with kicked rotors.Comment: 45 pages, 19 figure