Description
This thesis investigates quantum transport in the energy space of two paradigm
systems of quantum chaos theory. These are highly excited hydrogen atoms
subject to a microwave field, and kicked atoms which mimic
the delta-kicked rotor model. Both of these systems show a complex
dynamical evolution arising from the interaction with an external
time-periodic driving force. In particular two quantum phenomena,
which have no counterpart on the classical level, are studied:
the suppression of classical diffusion, known as
dynamical localisation, and quantum resonances as a regime of enhanced
transport for the delta-kicked rotor.
The first part of the thesis provides new
support for the quantitative analogy between energy transport
in strongly driven highly excited atoms and particle transport
in Anderson-localised solids. A comprehensive numerical analysis of the
atomic ionisation rates shows that they obey a universal power-law
distribution, in agreement with Anderson localisation theory.
This is demonstrated for a one-dimensional model as well as
for the real three-dimensional atom. We also discuss the
implications of the universal decay-rate distributions for the
asymptotic time-decay of the survival probability of the atoms.
The second part of the thesis clarifies the effect of
decoherence, induced by spontaneous emission, on the quantum resonances
which have been observed in a recent experiment with delta-kicked atoms.
Scaling laws are derived, based on a quasi-classical approximation of the
quantum evolution. These laws describe
the shape of the resonance peaks in the mean energy of
an experimental ensemble of kicked atoms. Our analytical results
match perfectly numerical computations and explain the initially
surprising experimental observations.
Furthermore, they open the door to the study of the competing
effects of decoherence and chaos on the stability of the time evolution
of kicked atoms.
This stability may be characterised by the overlap of two identical initial
states which are subject to different time evolutions. This overlap, called
fidelity, is investigated in an experimentally accessible situation.
Die moderne Astrophysik steht vor der Herausforderung, neueste
Beobachtungen mit den theoretischen und numerischen Modellen der
Galaxienentstehung und -entwicklung zu konfrontieren. So hofft man, die
wichtigsten physikalischen Prozesse und ihre Zeitskalen identifizieren zu
koennen.
In dieser...
Published 09/10/04
This work presents the results of a detailed study of the statistical and physical properties of binary ultracool dwarfs and brown dwarfs (spectral type later than M7).
As for the statistical properties, we found that the frequency of binaries among ultracool objects is significantly lower than...
Published 09/04/04