Counting errors in a voltage-biased electron pump

We have measured the counting errors of a seven-junction electron pump when charge is pumped against a voltage difference. At finite bias voltages, we find that the errors increase exponentially with both pump voltage and temperature, in agreement with theoretical predictions. To compare experiment and simulation, all pump parameters were determined by independent electron-box experiments. Although we assume temperatures somewhat higher than those measured, simulations based on the ground-capacitance model yield excellent quantitative agreement with experiment and indicate that errors in the high-voltage regime are due to thermally activated tunneling. In addition, a surprising asymmetry between positive and negative voltages is explained by an asymmetry in the junction capacitances.