Accuracy verification of single-electron pumps with 0.2 ppm uncertainty

A current source based on clocked single-electron transfer is the natural candidate for the realization of the base unit ampere following a future redefinition by a fixed value of the elementary charge. However, the accuracy depends on microscopic processes and has to be verified for each device. Building on the recent advances on instrumentation we verify the accuracy of the generated current with an uncertainty of 0.16 ppm for one day of averaging.