Crossover from single electron counting to Cooper pair counting

We present experimental studies of charge transport in a one-dimensional array of Josephson junctions using a single charge counting device based upon a radio-frequency single-electron transistor$^1$. We observe a crossover from time-correlated tunneling of single electrons to Cooper pairs as a function of an applied magnetic field. At relatively high magnetic field, single electron transport dominates and the frequency is given by f=I/e. As the magnetic field is lowered the frequency gradually shifts to f=I/2e, indicating tunneling of Cooper pairs. $^1$Jonas Bylander, Tim Duty and Per Delsing, Nature 434 (2005) 285.