  A well-characterised sample of silicon tunable-barrier electron pump has been
operated at a frequency of 2 GHz using a custom drive waveform, generating a
pump current of 320 pA. Precision measurements of the current were made as a
function of pump control parameters, using a blind protocol, over a 7-week
campaign. The combined standard uncertainty for each 10 hour measurement was
0.1 parts per million. The pump current exhibits a plateau along the exit gate
voltage flat to approximately 0.1 parts per million, but offset from ef by 0.2
parts per million. This offset may be a sign of errors in the current
traceability chain, indicating a limit to the accuracy of small current scaling
using existing methods based on cryogenic current comparators.
