Phase and micromotion of Bose-Einstein condensates in a time-averaged ring trap

TL;DR

This study investigates the phase and micromotion of Bose-Einstein condensates in a ring-shaped optical trap created by rapid beam scanning, revealing phase imprinting effects and methods to improve coherence for interferometry.

Contribution

It provides a combined theoretical and experimental analysis of phase profiles and density signatures in scanning ring traps, highlighting the impact of scanning direction on coherence.

Findings

Unidirectional scanning causes a measurable phase step in the condensate.

Time-of-flight imaging reveals in-trap phase steps through density signatures.

Bidirectional scanning eliminates phase gradients, improving coherence.

Abstract

Rapidly scanning magnetic and optical dipole traps have been widely utilised to form time-averaged potentials for ultracold quantum gas experiments. Here we theoretically and experimentally characterise the dynamic properties of Bose-Einstein condensates in ring-shaped potentials that are formed by scanning an optical dipole beam in a circular trajectory. We find that unidirectional scanning leads to a non-trivial phase profile of the condensate that can be approximated analytically using the concept of phase imprinting. While the phase profile is not accessible through in-trap imaging, time-of-flight expansion manifests clear density signatures of an in-trap phase step in the condensate, coincident with the instantaneous position of the scanning beam. The phase step remains significant even when scanning the beam at frequencies two orders of magnitude larger than the characteristic frequency of the trap. We map out the phase and density properties of the condensate in the scanning trap, both experimentally and using numerical simulations, and find excellent agreement. Furthermore, we demonstrate that bidirectional scanning eliminated the phase gradient, rendering the system more suitable for coherent matter wave interferometry.