Excitation picture of an interacting Bose gas

TL;DR

This paper introduces an excitation picture for strongly interacting Bose gases, enabling a cluster-expansion approach that captures interaction effects and modifies number fluctuations even in nearly coherent BECs.

Contribution

A novel excitation picture transformation that simplifies the many-body kinetics of interacting Bose gases by focusing on atom clusters within the normal component.

Findings

The transformation is systematically studied and shown to be cluster-expansion friendly.

Interaction effects visibly modify BEC number fluctuations.

Even near perfect coherence, non-Poissonian fluctuations reveal interaction effects.

Abstract

Atomic Bose-Einstein condensates (BECs) can be viewed as macroscopic objects where atoms form correlated atom clusters to all orders. Therefore, the presence of a BEC makes the direct use of the cluster-expansion approach --- lucrative e.g. in semiconductor quantum optics --- inefficient when solving the many-body kinetics of a strongly interacting Bose. An excitation picture is introduced with a nonunitary transformation that exclusively describes the system in terms of atom clusters within the normal component alone. The nontrivial properties of this transformation are systematically studied, which yields a cluster-expansion friendly formalism for a strongly interacting Bose gas. Its connections and corrections to the standard Hartree-Fock Bogoliubov approach are discussed and the role of the order parameter and the Bogoliubov excitations are identified. The resulting interaction effects are shown to visibly modify number fluctuations of the BEC. Even when the BEC has a nearly perfect second-order coherence, the BEC number fluctuations can still resolve interaction-generated non-Poissonian fluctuations.