Ballistic peaks at quantum resonance

We report studies of the motion of cold atoms in a time-dependent optical p otential. The dynamics of our system are that of the quantum kicked rotor, and exhibit a wide variety of phenomena. One purely quantum effect is the q uantum resonance, which occurs for well-chosen initial conditions and speci fic values of the period between kicks. Distinctly nonclassical behavior, s uch as ballistic growth in momentum, is possible at a quantum resonance. Pr evious experimental studies have observed these resonances, but have not cl early resolved the expected ballistic motion. We now observe ballistic moti on at quantum resonances and compare our momentum distributions with theory and numerical simulations. (C) 2000 Elsevier Science B.V. All rights reser ved.