Delocalization of a disordered bosonic system by repulsive interactions

TL;DR

This study uses ultracold atomic Bose-Einstein condensates in a quasi-periodic lattice to experimentally explore how weak repulsive interactions influence localization and delocalization in disordered bosonic systems, revealing three distinct regimes.

Contribution

First experimental investigation of the interplay between disorder and repulsive interactions in a controlled ultracold atomic system, identifying regimes of localization and delocalization.

Findings

Identification of exponentially localized Anderson glass.

Observation of locally coherent fragments.

Detection of a coherent, extended state.

Abstract

Clarifying the interplay of interactions and disorder is fundamental to the understanding of many quantum systems, including superfluid helium in porous media, granular and thin-film superconductors, and light propagating in disordered media. One central aspect for bosonic systems is the competition between disorder, which tends to localize particles, and weak repulsive interactions, which instead have a delocalizing effect. Since the required degree of independent control of the disorder and of the interactions is not easily achievable in most available physical systems, a systematic experimental investigation of this competition has so far not been possible. Here we employ an ultracold atomic Bose-Einstein condensate with tunable repulsive interactions in a quasi-periodic lattice potential to study this interplay in detail. We characterize the entire delocalization crossover through the study of the average local shape of the wavefunction, the spatial correlations, and the phase coherence. Three different regimes are identified and compared with theoretical expectations: an exponentially localized Anderson glass, the formation of locally coherent fragments, as well as a coherent, extended state. Our results illuminate the role of weak repulsive interactions on disordered bosonic systems and show that the system and the techniques we employ are promising for further investigations of disordered systems with interactions, also in the strongly correlated regime.