Precision measurement of an electron pump at 2 GHz; the frontier of small DC current metrology

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 ${\sim} 10$ h measurement was in the range $0.16-0.18$ parts per million (ppm). The pump current exhibits a plateau along the exit gate voltage flat to approximately 0.15 ppm, but offset from ef by 0.2 ppm. 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.