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A classical field treatment of colliding Bose-Einstein condensates

In this thesis we develop a fully quantum-mechanical treatment of collisions between distinct atomic Bose-Einstein condensate wavepackets, with particular emphasis on the incoherently scattered atoms that form s-wave haloes around the condensate wavepackets. Previous theoretical treatments of these systems have been unable to account simultaneously for both the evolution of the halo and the depletion of the condensates, and were therefore restricted to the small scattering limit. Our approach uses the truncated Wigner method, a particular example of the classical field methods familiar from quantum optics. The atomic field is restricted to a low-energy subspace of single-particle states, and the method is applicable even to highly-scattered systems.
We present a comprehensive derivation of the truncated Wigner method for ultracold bosonic fields, and discuss in detail the validity regime of the Wigner truncation for inhomogeneous multimode systems. The method gives rise to a set of coupled stochastic differential equations that describe the evolution of a single realisation of the atomic field, and have a form similar to that of the well known Gross-Pitaevskii equation, but with the important difference that the stochastic differential equations include well prescribed quantum fluctuations. To propagate our systems we develop algorithms that allow for highly efficient numerical evolution of realistic experimental collisions.
By investigating individual trajectories of the colliding system, we find that the scattering halo is composed of many distinct highly-populated phase grains separated by large numbers of vortices, a behaviour we label quantum turbulence. We develop a spatial averaging method for approximately calculating quantum correlation functions from a single trajectory, and calculate various properties of the halo. Based on these results, we propose a mechanism to explain the observed features of scattering halo formation. We find by using an appropriately extended truncated Wigner approach that three-body recombination events have negligible effect on the collisions.
Using an ensemble of trajectories we calculate correlation functions of a particular collisional system to give a rigorous characterisation of the quantum statistics of the field, and obtain results that are remarkably similar to those obtained using single trajectory spatial averaging. For global field quantities, such as the total coherent population, we find that accurate estimates can be achieved using just two trajectories, a result we use to efficiently explore the dependence of the system on key physical parameters.
Finally, we apply the truncated Wigner method to collisions between condensates in differing hyperfine states, whose (single-trajectory and ensemble) behaviour we find is qualitatively similar to that of single-component collisions.
Date January 2005
CreatorsNorrie, Adam Anson, n/a
PublisherUniversity of Otago. Department of Physics
Source SetsAustraliasian Digital Theses Program
Detected LanguageEnglish
Rights, Copyright Adam Anson Norrie

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