258 research outputs found
Local phase space control and interplay of classical and quantum effects in dissociation of a driven Morse oscillator
This work explores the possibility of controlling the dissociation of a
monochromatically driven one-dimensional Morse oscillator by recreating
barriers, in the form of invariant tori with irrational winding ratios, at
specific locations in the phase space. The control algorithm proposed by Huang
{\it et al.} (Phys. Rev. A {\bf 74}, 053408 (2006)) is used to obtain an
analytic expression for the control field. We show that the control term,
approximated as an additional weaker field, is efficient in recreating the
desired tori and suppresses the classical as well as the quantum dissociation.
However, in the case when the field frequency is tuned close to a two-photon
resonance the local barriers are not effective in suppressing the dissociation.
We establish that in the on-resonant case quantum dissociation primarily occurs
via resonance-assisted tunneling and controlling the quantum dynamics requires
a local perturbation of the specific nonlinear resonance in the underlying
phase space.Comment: 12 pages, 6 figures (reduced quality), submitted to Phys. Rev.
Kicked Bose-Hubbard systems and kicked tops -- destruction and stimulation of tunneling
In a two-mode approximation, Bose-Einstein condensates (BEC) in a double-well
potential can be described by a many particle Hamiltonian of Bose-Hubbard type.
We focus on such a BEC whose interatomic interaction strength is modulated
periodically by -kicks which represents a realization of a kicked top.
In the (classical) mean-field approximation it provides a rich mixed phase
space dynamics with regular and chaotic regions. By increasing the
kick-strength a bifurcation leads to the appearance of self-trapping states
localized on regular islands. This self-trapping is also found for the many
particle system, however in general suppressed by coherent many particle
tunneling oscillations. The tunneling time can be calculated from the
quasi-energy splitting of the corresponding Floquet states. By varying the
kick-strength these quasi-energy levels undergo both avoided and even actual
crossings. Therefore stimulation or complete destruction of tunneling can be
observed for this many particle system
Dynamical Localization: Hydrogen Atoms in Magnetic and Microwave fields
We show that dynamical localization for excited hydrogen atoms in magnetic
and microwave fields takes place at quite low microwave frequency much lower
than the Kepler frequency. The estimates of localization length are given for
different parameter regimes, showing that the quantum delocalization border
drops significantly as compared to the case of zero magnetic field. This opens
up broad possibilities for laboratory investigations.Comment: revtex, 11 pages, 3 figures, to appear in Phys. Rev. A, Feb (1997
Hundred photon microwave ionization of Rydberg atoms in a static electric field
We present analytical and numerical results for the microwave excitation of
nonhydrogenic atoms in a static electric field when up to 1000 photons are
required to ionize an atom. For small microwave fields, dynamical localization
in photon number leads to exponentially small ionization while above quantum
delocalization border ionization goes in a diffusive way. For alkali atoms in a
static field the ionization border is much lower than in hydrogen due to
internal chaos.Comment: revtex, 4 pages, 5 figure
Frequency Dependence of Quantum Localization in a Periodically Driven System
We study the quantum localization phenomena for a random matrix model
belonging to the Gaussian orthogonal ensemble (GOE). An oscillating external
field is applied on the system. After the transient time evolution, energy is
saturated to various values depending on the frequencies. We investigate the
frequency dependence of the saturated energy. This dependence cannot be
explained by a naive picture of successive independent Landau-Zener transitions
at avoided level crossing points. The effect of quantum interference is
essential. We define the number of Floquet states which have large overlap with
the initial state, and calculate its frequency dependence. The number of
Floquet states shows approximately linear dependence on the frequency, when the
frequency is small. Comparing the localization length in Floquet states and
that in energy states from the viewpoint of the Anderson localization, we
conclude that the Landau-Zener picture works for the local transition processes
between levels.Comment: 12 pages and 6 figure
Quantum Poincare Recurrences for Hydrogen Atom in a Microwave Field
We study the time dependence of the ionization probability of Rydberg atoms
driven by a microwave field, both in classical and in quantum mechanics. The
quantum survival probability follows the classical one up to the Heisenberg
time and then decays algebraically as P(t) ~ 1/t. This decay law derives from
the exponentially long times required to escape from some region of the phase
space, due to tunneling and localization effects. We also provide parameter
values which should allow to observe such decay in laboratory experiments.Comment: revtex, 4 pages, 4 figure
Diffusive Ionization of Relativistic Hydrogen-Like Atom
Stochastic ionization of highly excited relativistic hydrogenlike atom in the
monochromatic field is investigated. A theoretical analisis of chaotic dynamics
of the relativistic electron based on Chirikov criterion is given for the cases
of one- and three-dimensional atoms. Critical value of the external field is
evaluated analitically. The diffusion coefficient and ionization time are
calculated.Comment: 13 pages, latex, no figures, submitted to PR
Fully quantum-mechanical treatment of proton-hydrogen scattering
© Published under licence by IOP Publishing Ltd. A fully quantum-mechanical convergent close-coupling approach to proton collisions with atomic hydrogen has been developed. Cross sections for target ionisation and electron capture by the projectile have been calculated in the energy range from 20 keV to 1 MeV. Calculated electron capture cross sections are in good agreement with the experiment, however for ionisation discrepancies between theory and experiment at intermediate energies still remain
Stochastic ionization through noble tori: Renormalization results
We find that chaos in the stochastic ionization problem develops through the
break-up of a sequence of noble tori. In addition to being very accurate, our
method of choice, the renormalization map, is ideally suited for analyzing
properties at criticality. Our computations of chaos thresholds agree closely
with the widely used empirical Chirikov criterion
The Fermi accelerator in atom optics
We study the classical and quantum dynamics of a Fermi accelerator realized
by an atom bouncing off a modulated atomic mirror. We find that in a window of
the modulation amplitude dynamical localization occurs in both position and
momentum. A recent experiment [A. Steane, P. Szriftgiser, P. Desbiolles, and J.
Dalibard, Phys. Rev. Lett. {\bf 74}, 4972 (1995)] shows that this system can be
implemented experimentally.Comment: 5 pages, 5 figure
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