Hybrid systems at finite temperatures

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.