  The modern primary voltage standard is based on the AC Josephson effect and
the ensuing Shapiro steps, where a microwave tone applied to a Josephson
junction yields a constant voltage $hf/2e$ ($h$ is Planck's constant and $e$
the electron charge) determined by only the microwave frequency $f$ and
fundamental constants. Duality arguments for current and voltage have long
suggested the possibility of dual Shapiro steps -- that a Josephson junction
device could produce current steps with heights determined only on the applied
frequency. In this report, we embed an ultrasmall Josephson junction in a high
impedance array of larger junctions to reveal dual Shapiro steps. For multiple
frequencies, we detect that the AC response of the circuit is synchronised with
the microwave tone at frequency $f$, and the corresponding emergence of flat
steps in the DC response with current $2ef$, equal to the tunnelling of a
Cooper pair per tone period. This work sheds new light on phase-charge duality,
omnipresent in condensed matter physics, and extends it to Josephson circuits.
Looking forward, it opens a broad range of possibilities for new experiments in
the field of circuit quantum electrodynamics and is an important step towards
the long-sought closure of the quantum metrology electrical triangle.
