High contrast interferometry and finite temperature coherence of Bose-Einstein condensates

• Main Author: Carson, Christopher Hugh
• Published: University of Strathclyde 2015
• Subjects: 530
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.675245

This thesis outlines the main experimental results of a Bose-Einstein condensate(BEC) interferometer and the finite temperature coherence of elongated BECs. Cold atom and BEC-based interferometers take advantage of the wave nature of atoms as they are cooled and can be used for precision measurements of fundamental physics, fundamental constants, rotations and gravitational gradients. The coherence properties of atomic matter-waves are of great interest, in particular phase coherence, which has played an important role in fundamental research involving BEC interferometry. When subject to high cloud aspect ratios, BECs become elongated and exhibited phase fluctuations, which can have a dramatic affect on the performance of matter-wave interferometers. A brief overview of the history and basic theory of Bose-Einstein condensates is presented as well as introducing the various studies and applications in metrology involving BECs. The theory of the techniques used to create a BEC, such as laser cooling, trapping and evaporation, are discussed along with the dynamics of BECs, matter-wave interference and phase fluctuations. The experimental chapters describe the various concepts, techniques and mechanisms used to experimentally observe matter-wave interference fringes. The interference fringes are a result of realising two BECs from a double-well potential, which is created using a combination of magnetic and optical potentials, and allowing them to expand and overlap. The main interferometry results are then discussed, this includes the observation of single-shot interference fringes with contrast ≥95%, which has a strong dependence on the detuning of the imaging beam. Also, a strong dependence on fringe contrast with the focal location of the camera is observed, which can now be clearly attributed to the Talbot effect. This is the first reported observation of the spatial Talbot effect of light interacting with period BEC fringes, revealing the drastic effect it can have on the interference signal. The major results regarding phase fluctuations in elongated condensates are presented. These include the existence of large regular period phase fluctuations, which should normally be of random phase and size. By dynamically changing the aspect ratio of the condensate during an experimental sequence, a controlled generation and removal of phase fluctuations in a Bose-Einstein condensate is observed, indicating a phase revival. The thesis concludes by considering potential improvements and future experiments, which could be used towards the experimental implementation of a BEC interferometer.