Collective Effects in Ultra-Cold Dipolar Atom Systems
In this thesis we investigate theoretically two distinct systems of dipole-coupled atoms. In Part I we investigate light scattering from a system of N ultracold atoms, with laser induced electric dipole interactions. We derive the equations of motion in the Heisenberg picture for the atomic operators, for an arbitrary dipole transition. Then we take the specific case of a jl = 0 → ju = 1 transition and derive the second and third-order inter-atomic correlations. We solve the resulting set of linear equations for two and three atoms exactly, with particular focus on the exact two-atom system. We find a steady-state analytic solution for the two atoms in a specific geometric configuration, and use this to derive explicit expressions for the spatial and coherence properties of the far-field scattered light intensity, valid for the full range of system parameters. A comprehensive survey of the scattering behaviour is given, with key features precisely characterised, including subradiant scattering. We examine in detail a decorrelation approximation that has potential applications for larger systems of atoms that are intractable in a full quantum treatment. We introduce the concept of an effective driving field and show that it can provide a direct and intuitive physical interpretation of key aspects of the system behaviour, including subradiant scattering. Finally, we show that this effective field interpretation, and the decorrelation approximation, is readily extended to a three atom system.In Part II of this thesis we present our investigation of the quantum many-body dynamics of a large ensemble of bosonic S = 3 chromium atoms in a three-dimensional lattice. We implement an extended Bose-Hubbard model which includes the tunnelling, onsite spinor in- teractions and dipole-dipole interactions (giving interactions between atoms on neighbouring sites). We study the dynamics of the population of the different Zeeman levels as a function of lattice depth following a sudden spin rotation, and provide comparison to experimental res- ults. We are able to identify two distinct regimes: At low lattice depths, where atoms are in the superfluid regime, we observe the spin dynamics are strongly determined by the competi- tion between particle transport, onsite interactions and external magnetic field gradients. On the contrary, at high lattice depths, transport is largely frozen out and the spin populations are mainly driven by the long range dipolar interactions and quantum fluctuations.
Advisor: Ballagh, Rob; Blakie, Blair
Degree Name: Doctor of Philosophy
Degree Discipline: Physics
Publisher: University of Otago
Keywords: dipolar; light scattering; atom-light interactions; dipole-dipole interactions; optical lattices; spinor gases
Research Type: Thesis