Non-linear Interactions in Single and Dual Component Bose-Einstein Condensates

Abstract

Non-linear systems give rise to a rich array of phenomena including chaos, turbulence, solitons, and dynamical instabilities. The great degree of experimental control over Bose Einstein condensates (BECs) makes them a unique tool to study these ubiquitous processes. Although single isotope BECs offer an ideal system for the study of simple non-linear physics, dual-component condensates allow the study of even richer coupled non-linear systems.

This thesis presents experimental results utilising a newly built dual-species 85Rb/87Rb BEC system where the precise manipulation of non-linear interactions is achieved through a Feshbach resonance. An overview of the experimental apparatus is given, with a focus on particular improvements and new features in comparison to previous designs. The BEC machine is capable of creating 2x10^5 and 2x10^6 condensates of 85Rb and 87Rb, respectively, as well as isotope mixtures, with a duty cycle of 13s. The minimalist design of the apparatus, focusing on increased optical access combined with accurate control over the interaction parameters of 85Rb, will open many avenues for future work.

Utilising the 85Rb system, the behaviour of single component BECs in an optical waveguide is investigated. Focusing on the propagation of BECs with attractive scattering lengths, we observe the formation of soliton trains. Using the nonpolynomial Schrodinger equation, it is shown that the formation of these trains can be understood as a manifestation of a modulational instability (MI). A non destructive imaging system, called shadowgraph imaging, allows multiple images of the stochastically forming soliton trains in a single run. Subsequently, we make the first real time observation of MI in a BEC.

Considering the full dual component 85Rb and 87Rb system, a wider variety of coupled non-linear phenomena can be investigated. Crucial to accessing applicable regimes is the reliable preparation of mixed groundstates in different configurations. We present a detailed experimental study of phase separated groundstates for the 85Rb/87Rb BEC system. It is shown that sensitively tuning the energy scales of the system allows the creation of a wide variety of immiscible (non-mixed) groundstates to be created. We demonstrate control over the shape of the interface between immiscible condensates for a wide range of interaction parameters and isotopes ratios. This work lays a foundation for future dual-component instability studies.