  We report on high-accuracy measurements of quantized current, sourced by a
tunable-barrier single-electron pump at frequencies $f$ up to $1$ GHz. The
measurements were performed with a new picoammeter instrument, traceable to the
Josephson and quantum Hall effects. Current quantization according to $I=ef$
with $e$ the elementary charge was confirmed at $f=545$ MHz with a total
relative uncertainty of 0.2 ppm, improving the state of the art by about a
factor of 5. For the first time, the accuracy of a possible future quantum
current standard based on single-electron transport was experimentally
validated to be better than the best realization of the ampere within the
present SI.
