Metrological applications of single electron tunneling

Recent progress on Coulomb blockade devices allows charge to be determined by counting electrons. These devices eventually may give a new intrinsic standard of capacitance and a new measurement of the fine structure constant. The author explains the requirements of three important components of such an experiment: the charge pump, the electrometer, and the cryogenic fixed capacitor. The performance of this standard is limited by the ability to transfer electrons through the electron pump with very small errors. He discussses an experiment in which a 5-junction pump was operated with an error for transferring electrons of approximately 0.5 ppm. The error predicted from existing theory is several orders of magnitude smaller, thus implying that the usual understanding of the Coulomb blockade is incomplete. It is deduced that the errors arise from photon-assisted tunneling, where the photon energy is supplied from the environment.<>