  We report the experimental observation of resonances in the early-time
momentum diffusion rates for the atom-optical delta-kicked rotor. In this work
a Bose-Einstein condensate provides a source of ultra-cold atoms with an
ultra-narow initial momentum distribution, which is then subjected to periodic
pulses (or "kicks") using an intense far-detuned optical standing wave. A
quantum resonance occurs when the momentum eigenstates accumulate the same
phase between kicks leading to ballistic energy growth. Conversely, an
anti-resonance is observed when the phase accumulated from successive kicks
cancels and the system returns to its initial state. Our experimental results
are compared with theoretical predictions.
