Abstract:
A growing number of hybrid experiments consisting of cold atoms
combined with nano devices or single trapped ions, has created
the need for a theoretical tool to describe these systems. Because
heating or cooling of the cloud has been frequently observed in
this context, finite temperatures therefore play an important role.
Although there are many models describing finite temperatures in
cold gases in isolation, none of them has been applied to hybrid
systems so far.
This thesis outlines how the Zaremba-Nikuni-Griffin (ZNG)
model can be used to numerically simulate different hybrid systems.
ZNG is a method, which in addition to a mean-field description
of a Bose condensate, also gives a full dynamical description of
thermal excitations. The thesis presents a parallel implementation
of this method, which uses adaptive square collision cells to calculate
collision integrals. It therefore allows for the simulation of
arbitrary trap geometries on high-performance computers. With
the help of this implementation a cloud in front of a solid surface
is simulated and atom-loss as well as condensate-growth curves are
presented. Furthermore, simulation results of a single trapped ion
in a thermal gas, modeled by a quantum Boltzmann equation, are
shown. Where possible the simulation results are compared with
experimental data for both systems to confirm the applicability
of the models. In addition, effects of an oscillating nanotube on
the coherence of a cold cloud are examined. Beside simulations
with the ZNG model, a system with a pure condensate, which is
modeled using the Gross-Pitaevskii equation, is also investigated.
The remaining condensate fraction is determined by the Penrose-
Onsager criterion. Both methods reveal resonance frequencies that
are much smaller than the typical thermal oscillation frequencies
of a nanotube. Such oscillations are therefore unlikely to alter the
coherence of a condensate.
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