Deflection of quantum particles by impenetrable boundary

The evolution of quantum packets in the presence of an impenetrable boundar y is studied. Due to an effective 'quantum repulsive force', the mean value of the momentum in the direction perpendicular to the boundary gradually c hanges in time at the expense of the energy of quantum fluctuations, and th e packet becomes significantly narrower than it would be in the free space under the same initial conditions. For zero initial mean transverse velocit y, the asymptotical value of the transverse momentum depends only on the in itial form of the packet, being independent of its initial position (provid ed this position is much greater than the initial transverse width of the p acket). As a consequence, narrow beams of ultracold particles (e.g., atoms with velocities about 1 m/s and the initial transverse uncertainty in posit ion about 0.01 mu m) directed along an impenetrable surface will be deflect ed to observable angles due to the quantum (wave) nature of particles. The possibilities of the experimental verification of the effect are discussed. The conclusion on the inelasticity of collisions between ultracold particl es due to the transformations of the shapes of their wave functions is made . (C) 2000 Elsevier Science B.V. All rights reserved.