  The AC Josephson effect predicted in 1962 and observed experimentally in 1963
as quantised voltage steps (the Shapiro steps) from photon assisted tunnelling
of Cooper pairs is among the most fundamental phenomena of quantum mechanics
and is vital for metrological quantum voltage standards. The physically dual
effect, the AC coherent quantum phase slip (CQPS), photon assisted tunnelling
of magnetic fluxes through a superconducting nanowire, is envisaged to reveal
itself as quantised current steps. The basic physical significance of the AC
CQPS is also complemented by practical importance in future current standards;
a missing element for closing the Quantum Metrology Triangle. In 2012, the CQPS
was demonstrated as superposition of magnetic flux quanta in superconducting
nanowires. However the direct sharp current steps in superconductors; the only
unavailable basic effect of superconductivity to date, was unattainable due to
lack of appropriate materials and challenges in circuit engineering. Here we
report the direct observation of the dual Shapiro steps in a superconducting
nanowire. The sharp steps are clear up to 26 GHz frequency with current values
8.3 nA and limited by the present setup bandwidth. The current steps have been
theoretically predicted in small Josephson junctions (JJs) 30 years ago.
However, broadening unavoidable in JJs prevents their direct experimental
observation. We solve this problem by placing a thin NbN nanowire in an
inductive environment.
