  Using standard microfabrication techniques it is now possible to construct
devices, which appear to reliably manipulate electrons one at a time. These
devices have potential use as building blocks in quantum computing devices, or
as a standard of electrical current derived only from a frequency and the
fundamental charge. To date the error rate in semiconductor 'tuneable-barrier'
pump devices, those which show most promise for high frequency operation, have
not been tested in detail. We present high accuracy measurements of the current
from an etched GaAs quantum dot pump, operated at zero source-drain bias
voltage with a single AC-modulated gate driving the pump cycle. By comparison
with a reference current derived from primary standards, we show that the
electron transfer accuracy is better than 15 parts per million. High-resolution
studies of the dependence of the pump current on the quantum dot tuning
parameters also reveal possible deviations from a model used to describe the
pumping cycle.
