Disorder Makes a Robust Matter-Wave Interferometer

Ultracold atoms can coherently scatter backwards from a disordered potential in a way similar to classical waves.

hen a coherent wave scatters from a disordered medium, it creates a random interference pattern known as “speckle.” We can easily observe speckle patterns in reflections of a laser pointer from a rough wall, for example. They are, however, completely washed out upon averaging over an ensemble of different arrangements of scattering objects. Coherent backscattering (CBS) is a unique interference effect that, on the contrary, builds up upon ensemble averaging—a peak in the direction opposite to the direction of the incident wave appears in the averaged signal, though it is not visible in a single arrangement of scatterers. This effect is a signature of wave coherence and a sensitive probe of the scattering medium because its shape depends on the statistical properties of the latter. It was shown to exist for many types of waves, from electrons in disordered solids [1] to light in colloidal suspensions [2, 3] and seismic waves [4]. In a recent paper in Physical Review Letters, Fred Jendrzejewski at the University of Paris-Sud, France, and co-workers [5] report on its observation for ultracold atoms in random optical potentials. In contrast to previous experiments, CBS has now been measured using a macroscopic quantum object—the Bose-Einstein condensate—and inside the region of space where the disordered potential exists. CBS may become a powerful tool to study fine features of matter-wave propagation in the presence of disorder. Such studies are likely to substantially improve our understanding of this particular problem, as well as shine new light on those of its aspects that are general for all waves.