  The basis of synchronous manipulation of individual electrons in solid-state
devices was laid by the rise of single-electronics about two decades ago.
Ultra-small structures in a low temperature environment form an ideal domain of
addressing electrons one by one. A long-standing challenge in this field has
been the realization of a source of electric current that is accurately related
to the operation frequency $f$. There is an urgent call for a quantum standard
of electric current and for the so-called metrological triangle, where voltage
from Josephson effect and resistance from quantum Hall effect are tested
against current via Ohm's law for a consistency check of the fundamental
constants of Nature, $\hbar$ and $e$. Several attempts to create a metrological
current source that would comply with the demanding criteria of extreme
accuracy, high yield, and implementation with not too many control parameters
have been reported. However, no satisfactory solution exists as yet despite
many ingenious achievements that have been witnessed over the years. Here we
propose and prove the unexpected concept of a hybrid metal-superconductor
turnstile in the form of a one-island single-electron transistor with one gate,
which demonstrates robust current plateaus at multiple levels of $ef$ within
the uncertainty of our current measurement. Our theoretical estimates show that
the errors of the present system can be efficiently suppressed by further
optimizations of design and proper choice of the device parameters and
therefore we expect it to eventually meet the stringent specifications of
quantum metrology.
