Anomalous diffusion and localization in a disorder-free atomic mixture

TL;DR

This study demonstrates anomalous diffusion and localization of light impurities in a disorder-free ultracold atomic mixture, revealing quantum interference effects that challenge traditional Fermi-liquid theory.

Contribution

It provides experimental evidence of disorder-free localization and subdiffusion in a minimal quantum system, advancing understanding of quantum interference in complex media.

Findings

Observation of crossover from normal diffusion to subdiffusion

Emergence of a localized fraction of light atoms

Incompatibility with conventional Fermi-liquid theory

Abstract

The concept of random walk, in which particles or waves undergo multiple collisions with the microscopic constituents of a surrounding medium, is central to understanding diffusive transport across many research areas. However, this paradigm may break down in complex systems, where quantum interference and memory effects render the particle propagation anomalous, often fostering localization. Here we report on the observation of such anomalous dynamics in a minimal setting: an ultracold mass-imbalanced mixture of two fermionic gases in three dimensions. We release light impurities into a gas of heavier atoms and follow their evolution across different collisional regimes. Under strong interspecies interactions, by lowering the temperature we unveil a crossover from normal diffusion to subdiffusion. Simultaneously, a localized fraction of the light gas emerges, displaying no discernible dynamics over hundreds of collisions. Our findings, incompatible with the conventional Fermi-liquid picture, are instead captured by a model of an atom propagating through a (quasi-)static disordered landscape of point-like scatterers. These results highlight the key role of quantum interference in our mixture, which emerges as a versatile platform for exploring disorder-free localization phenomena.