Fluctuation-damping of isolated, oscillating Bose-Einstein condensates

TL;DR

This paper investigates how isolated Bose-Einstein condensates exhibit damping of Josephson oscillations due to resonant excitation of fluctuations, which cannot be explained by traditional mean-field models.

Contribution

The study introduces a multi-mode BEC model coupled to fluctuations using the Keldysh formalism, explaining damping phenomena beyond Gross-Pitaevskii theory.

Findings

Resonance between Josephson frequency and fluctuation energy causes damping.

Model quantitatively matches experimental observations of damping and non-damping.

Fluctuation excitation is key to understanding dissipation in isolated BECs.

Abstract

Experiments on the nonequilibrium dynamics of an isolated Bose-Einstein condensate (BEC) in a magnetic double-well trap exhibit a puzzling divergence: While some show dissipation-free Josephson oscillations, others find strong damping. Such damping in isolated BECs cannot be understood on the level of the coherent Gross-Pitaevskii dynamics. Using the Keldysh functional-integral formalism, we describe the time-dependent system dynamics by means of a multi-mode BEC coupled to fluctuations (single-particle excitations) beyond the Gross-Pitaevskii saddle point. We find that the Josephson oscillations excite an excess of fluctuations when the effective Josephson frequency, $\tilde{\omega}_J$, is in resonance with the effective fluctuation energy, $\tilde{\varepsilon}_m$, where both, $\tilde{\omega}_J$ and $\tilde{\varepsilon}_m$, are strongly renormalized with respect to their noninteracting values. Evaluating and using the model parameters for the respective experiments describes quantitatively the presence or absence of damping.