Single-electron pumps based on tunable-barrier quantum dots are the most
promising candidates for a direct realization of the unit ampere in the
recently revised SI: they are simple to operate and show high precision at high
operation frequencies. The current understanding of the residual transfer
errors at low temperature is based on the evaluation of backtunneling effects
in the decay cascade model. This model predicts a strong dependence on the
ratio of the time dependent changes in the quantum dot energy and the tunneling
barrier transparency. Here we employ a two-gate operation scheme to verify this
prediction and to demonstrate control of the backtunneling error. We derive and
experimentally verify a quantitative prediction for the error suppression,
thereby confirming the basic assumptions of the backtunneling (decay cascade)
model. Furthermore, we demonstrate a controlled transition from the
backtunneling dominated regime into the thermal (sudden decoupling) error
regime. The suppression of transfer errors by several orders of magnitude at
zero magnetic field was additionally verified by a sub-ppm precision
measurement