  With the anticipated redefinition of the international system of units (SI)
the base units will be linked to fundamental constants of nature [1]. As for
the electrical base unit "Ampere", it will be linked to the elementary charge
e, requiring a corresponding quantum standard [2, 3]. Many concepts for such a
standard have been investigated [4-14] relying on controlling the
time-dependent tunnelling of electrons. However, the stochastic nature of
quantum mechanical tunnelling intrinsically evokes uncontrolled deviations from
the nominally quantized current. Alternatively, the counting of electrons [15,
16] has been explored but is severely limited in current amplitude and
uncertainty by the low detector bandwidth. The late M. Wulf proposed [17] that
this fundamental problem of electrical quantum metrology could be overcome by
combining serial single-electron pumps with charge detectors allowing the
generation of a quantized current and the in-situ detection of its stochastic
deviations. Here, we experimentally demonstrate such quantized-current
generation with in-situ detection of tunnelling errors at low frequencies and a
reduction of the total current uncertainty by more than one order of magnitude.
After frequency scaling this should enable a validated primary standard for the
redefined SI base unit Ampere.
