From single-particle excitations to sound waves in a box-trapped atomic Bose-Einstein condensate

TL;DR

This study explores how the lowest axial excitation in a box-trapped Bose-Einstein condensate transitions from a single-particle mode to a sound wave as density increases, confirming theoretical predictions and revealing nonlinear decay behaviors.

Contribution

It provides experimental and theoretical insights into the crossover of excitation modes in a BEC and links mode evolution to condensate shape changes and interactions.

Findings

Mode frequency matches Bogoliubov theory across densities

Transition from single-particle to sound wave observed

Nonlinear decay of large-amplitude excitations detected

Abstract

We experimentally and theoretically investigate the lowest-lying axial excitation of an atomic Bose-Einstein condensate in a cylindrical box trap. By tuning the atomic density, we observe how the nature of the mode changes from a single-particle excitation (in the low-density limit) to a sound wave (in the high-density limit). Throughout this crossover the measured mode frequency agrees with Bogoliubov theory. Using approximate low-energy models we show that the evolution of the mode frequency is directly related to the interaction-induced shape changes of the condensate and the excitation. Finally, if we create a large-amplitude excitation, and then let the system evolve freely, we observe that the mode amplitude decays non-exponentially in time; this nonlinear behaviour is indicative of interactions between the elementary excitations, but remains to be quantitatively understood.