  We demonstrate controlled pumping of Cooper pairs down to the level of a
single pair per cycle, using an rf-driven Cooper-pair sluice. We also
investigate the breakdown of the adiabatic dynamics in two different ways. By
transferring many Cooper pairs at a time, we observe a crossover between pure
Cooper-pair and mixed Cooper-pair-quasiparticle transport. By tuning the
Josephson coupling that governs Cooper-pair tunneling, we characterize
Landau-Zener transitions in our device. Our data are quantitatively accounted
for by a simple model including decoherence effects.
