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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 1,2 . Duality arguments for current and voltage 3-9 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 [10][11][12][13] . In this report, we embed an ultrasmall Josephson junction in a high impedance array of larger junctions to reveal dual Shapiro steps 8 . 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 14,15 , 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 16,17 and is an important step towards the long-sought closure of the quantum metrology electrical triangle [18][19][20] .

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