Since it occurs away from the surface, an atomic scale irregularity on the surface hardly affects the reflectivity, i.e., the surface-preparation is not required. In order to investigate the quantum reflection experimentally, the particle’s kinetic energy perpendicular to the surface must be sufficiently small. There are two ways to achieve this prerequisite: 1) using ultracold atoms to exploit their larger wavelength; 2) employing a grazing incidence condition.
Instead of generating ultracold atoms, we have studied the quantum reflection of thermal atoms at a grazing incidence angle through high angular resolution matter-wave optics apparatus [28-31]. In typical experimental conditions, the grazing incidence angle of a few mrads provides a sufficiently small vertical kinetic energy, perpendicular to the surface. Hence, when an incident matter-wave propagates almost parallel to a surface, even fast atoms can be coherently reflected from the surface. Furthermore, in such circumstances, distinct resolved diffraction peaks have been observed from micro-fabricated reflection-gratings [ref].