Controlled transfer of single charge carriers

Two devices that allow the coherent transfer of single electrons are described. They are based on the electrostatic control of tunnel processes across ultrasmall normal tunnel junctions. When appropriate AC signals are applied to gates a current I=e*f is forced to flow through the devices. There are, however, fundamental differences in their principles which make the pump the only suitable candidate for the controlled transfer of cooper pairs. Although in these experiments the I=e*f relation was verified with only a 1% accuracy, more elaborate versions of these devices could be promising candidates for a current standard.