Loading Ultracold Gases in Topological Floquet Bands: Current and Center-of-Mass Responses

TL;DR

This paper investigates how to optimize loading sequences of ultracold gases into topological Floquet bands, demonstrating that center-of-mass responses provide a robust way to detect band topology despite complex micro-motion effects.

Contribution

It analyzes loading protocols for Floquet topological bands and shows that center-of-mass measurements are more reliable than current responses for probing topology.

Findings

Center-of-mass responses are robust against micro-motion effects.

Current responses exhibit irregularities due to drive micro-motion.

Optimized loading sequences improve topological band detection.

Abstract

Topological band structures can be designed by subjecting lattice systems to time-periodic modulations, as was recently demonstrated in cold atoms and photonic crystals. However, changing the topological nature of Floquet Bloch bands from trivial to non-trivial, by progressively launching the time-modulation, is necessarily accompanied with gap-closing processes: this has important consequences for the loading of particles into a target Floquet band with non-trivial topology, and hence, on the subsequent measurements. In this work, we analyse how such loading sequences can be optimized in view of probing the topology of Floquet bands through transport measurements. In particular, we demonstrate the robustness of center-of-mass responses, as compared to current responses, which present important irregularities due to an interplay between the micro-motion of the drive and inter-band interference effects. The results presented in this work illustrate how probing the center-of-mass displacement of atomic clouds offers a reliable method to detect the topology of Floquet bands, after realistic loading sequences.