Title: When Anderson localization makes quantum particles move backward,
Abstract: We unveil a novel and unexpected manifestation of Anderson localization of
matter wave packets that carry a finite average velocity: after an initial
ballistic motion, the packet center-of-mass experiences a retroreflection and
slowly returns to its initial position. We describe this effect both
numerically and analytically in dimension 1, and show that it is destroyed by
weak particle interactions which act as a decoherence process. The
retroreflection is also present in higher dimensions, provided the dynamics is
Anderson localized.