Hyperbolic Bloch equations: atom-cluster kinetics of an interacting Bose gas

Abstract

Experiments with ultracold Bose gases can already produce so strong atom--atom interactions that one can observe intriguing many-body dynamics between the Bose-Einstein condensate (BEC) and the normal component. The excitation picture is applied to uniquely express the many-body state uniquely in terms of correlated atom clusters within the normal component alone. Implicit notation formalism is developed to {\it explicitly} derive the quantum kinetics of {\it all} atom clusters. The clusters are shown to build up sequentially, from smaller to larger ones, which is utilized to nonperturbatively describe the interacting BEC with as few clusters as possible. This yields the hyperbolic Bloch equations (HBEs) that not only generalize the Hartree-Fock Bogoliubov approach but also are analogous to the semiconductor Bloch equations (SBEs). This connection is utilized to apply sophisticated many-body techniques of semiconductor quantum optics to BEC investigations. Here, the HBEs are implemented to determine how a strongly interacting Bose gas reacts to a fast switching from weak to strong interactions, often referred to as unitarity. The computations for $^{35}$Rb demonstrate that molecular states (dimers) depend on atom density, and that the many-body interactions create coherent transients on a 100$\mu$s time scale converting BEC into normal state via quantum depletion.