Ultracold gases far from equilibrium

TL;DR

This paper reviews advanced quantum field theoretical methods for understanding the complex dynamics of ultracold atomic gases driven far from equilibrium, highlighting their analytical and numerical capabilities beyond mean-field approximations.

Contribution

It introduces functional quantum field theory approaches, including two-particle irreducible effective action and renormalisation-group theory, for studying far-from-equilibrium quantum many-body systems.

Findings

Demonstrates the application of methods to ultracold Bose gases in one dimension.

Shows the emergence of quantum versus classical fluctuations from the functional-integral framework.

Highlights the relation between far-from-equilibrium dynamics and near-equilibrium kinetic theory.

Abstract

Ultracold atomic quantum gases belong to the most exciting challenges of modern physics. Their theoretical description has drawn much from classical field equations. These mean-field approximations are in general reliable for dilute gases in which the atoms collide only rarely with each other, and for situations where the gas is not too far from thermal equilibrium. With present-day technology it is, however, possible to drive and observe a system far away from equilibrium. Functional quantum field theory provides powerful tools to achieve both, analytical understanding and numerical computability, also in higher dimensions, of far-from-equilibrium quantum many-body dynamics. In the article, an outline of these approaches is given, including methods based on the two-particle irreducible effective action as well as on renormalisation-group theory. Their relation to near-equilibrium kinetic theory is discussed, and the distinction between quantum and classical statistical fluctuations is shown to naturally emerge from the functional-integral description. Example applications to the evolution of an ultracold atomic Bose gas in one spatial dimension underline the power of the methods. The article is compiled from the notes for lectures held at 46. Internationale Universitaetswochen fuer Theoretische Physik 2008 in Schladming, Austria.