Kilohertz-driven Bose-Einstein condensates in optical lattices

TL;DR

This paper investigates the behavior of dilute Bose-Einstein condensates in shaken optical lattices driven at kilohertz frequencies, revealing their steady-state Floquet distributions and the conditions under which they condense into specific Floquet states.

Contribution

It provides both theoretical and experimental evidence that driven ultracold gases tend to occupy a single Floquet state, including cases where the lowest energy Floquet state differs from the undriven ground state.

Findings

Driven Bose gases tend to occupy a single Floquet state.

High driving amplitudes can lead to condensation into non-ground Floquet states.

Experimental results confirm theoretical predictions about Floquet state occupation.

Abstract

We analyze time-of-flight absorption images obtained with dilute Bose-Einstein con-densates released from shaken optical lattices, both theoretically and experimentally. We argue that weakly interacting, ultracold quantum gases in kilohertz-driven optical potentials constitute equilibrium systems characterized by a steady-state distri-bution of Floquet-state occupation numbers. Our experimental results consistently indicate that a driven ultracold Bose gas tends to occupy a single Floquet state, just as it occupies a single energy eigenstate when there is no forcing. When the driving amplitude is sufficiently high, the Floquet state possessing the lowest mean energy does not necessarily coincide with the Floquet state connected to the ground state of the undriven system. We observe strongly driven Bose gases to condense into the former state under such conditions, thus providing nontrivial examples of dressed matter waves.