Kinematic reversibility in a low Reynolds number cold atom fluid

TL;DR

This study demonstrates that a cold atom system modeled as a low Reynolds number fluid exhibits kinematic reversibility under certain conditions, even with interparticle interactions, but can also show deviations due to hysteresis.

Contribution

We experimentally show that kinematic reversibility persists in a cold atom fluid with interactions, expanding understanding of overdamped dynamics in such systems.

Findings

Reversibility observed when external forces are reversed in the MOT system.

Interparticle interactions do not destroy kinematic reversibility.

Hysteresis effects can cause deviations from reversibility under specific conditions.

Abstract

We have investigated kinematic reversibility in a cold atom system under strongly overdamped conditions. In such systems, inertia is negligible, and for noninteracting rigid particles, inverting the external force causes a perfect reversal of individual particle trajectories. We used a magneto-optical trap (MOT) as a model low Reynolds number fluid and show the kinematic reversibility survives in the presence of interparticle interactions. In our experiment, we applied controlled external forces via a linearly ramped magnetic bias field and monitored the resulting cloud dynamics. Despite the complex three-dimensional rearrangement induced by the forces, the system exhibits precise reversibility when the force is reversed, consistent with Purcell's framework for kinematic reversibility in low Reynolds number hydrodynamics. Reversibility was not universal,however-- under certain MOT alignment conditions we have also observed clear deviations associated with system hysteresis. Our work shows that strongly dissipative cold atom fluids are a versatile and rich platform for exploring overdamped dynamics.