Observation of two-dimensional Anderson localisation of ultracold atoms

Abstract

Anderson localisation -- the inhibition of wave propagation in disordered media -- is a surprising interference phenomenon which is particularly intriguing in two-dimensional (2D) systems. While an ideal, non-interacting 2D system of infinite size is always localised, the localisation length-scale may be too large to be unmistakably observed in an experiment. In this sense, 2D is a marginal dimension between one-dimension where all states are strongly localised, and three-dimensions where a well-defined localisation-delocalisation phase transition exists. Motivated by the goal of observing and closely studying the quantum interference leading to Anderson localisation in a 2D quantum system, we design a transmission experiment in which ultracold atoms propagate through a custom-shaped disordered channel connecting two reservoirs. This design overcomes many of the technical challenges that have hampered observation in previous works. Our precise control of disorder allows us to tune the localisation length to be shorter than the system size. We observe exponential localisation and demonstrate the presence of strong localisation in a 2D ultracold atom system.