Scanning tunneling microscopy, resonant tunneling, and counting electrons: A quantum standard of current

We present the theory for establishing a quantum standard of current. This is achieved by operating a scanning tunneling microscope at resonant tunneling through small metallic grains deposited on oxide layers, which exhibit Coulomb blockade. By adjusting the scanning period of the tip, a fixed number of electrons per oscillation can be made to tunnel. The current is directly related to the frequency and the electron charge.