Dissertation Defense: Sam Nerenberg

    Monday, April 13, 2020 - 2:00pm

    Zoom link TBD 


    Dissertation Title: Methods for Generation and Detection of Vorticity in Atomic Bose-Einstein Condensates


    Dilute gas Bose-Einstein condensates (BECs) provide a unique and powerful experimental platform to study fluid turbulence. Aspects of their hydrodynamics are specific to quantum fluids such as quantized vortices and flexible trapping geometries. However, there are features of turbulence which are universal and simpler to study in such systems. BECs and quantum turbulence are also objects of interest in fundamental physics themselves and a rich synthesis between theory and experiment has yielded powerful numerical methods to simulate their dynamics. In this dissertation I present two studies, one experimental and one numerical, describing novel experiments concerned with the generation and detection of vorticity in Bose-Einstein condensates.

    Accurate models of BEC dynamics at zero temperature are provided by the Gross-Pitaevskii equation. Simple and fast numerical solutions of this equation have yielded a wealth of literature. However, in experimental reality the atomic ensemble exists at a finite temperature and consists of a BEC in equilibrium with a thermal cloud of non-condensed atoms. The interaction between the condensate and the thermal fraction yields rich and complicated physics which require stochastic models. We present an experiment demonstrating the relaxation dynamics of a BEC in a rotating TOP trap perturbed by a repulsive laser barrier. The data provided by this experiment is valuable to theoretical models as it presents insight into stochastic BEC processes.

     In the field of two-dimensional quantum turbulence determining the position and charge of vortices is an important and unrealized measurement. Onsager's point-vortex model of turbulence completely determines the kinetic energy of a fluid by these degrees of freedom. We present proof-of-principle simulations which describe a method of spatially sampling the velocity field of a two-dimensional BEC by using an optical lattice analogously to a Shack-Hartmann Wavefront Sensor. Extracting vortex information from the appropriately sampled velocity field can be accomplished either qualitatively or with machine-vision algorithms. This method requires minimal experimental infrastructure and is generally applicable across atomic species. The implications of measuring a condensate velocity field are broad and these initial results provide the first step towards realizing a valuable tool in BEC physics.