Quantized current steps due to the a.c. coherent quantum phase-slip effect.

The a.c. Josephson effect predicted in 1962 1 and observed experimentally in 1963 2 as quantized '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 a.c. coherent quantum phase slip (CQPS), photon-assisted tunnelling of magnetic fluxes through a superconducting nanowire, is envisaged to reveal itself as quantized 'current steps' 3,4 . The basic physical significance of the a.c. CQPS is also complemented by practical importance in future current standards, a missing element for closing the quantum metrology triangle 5,6 . In 2012, the CQPS was demonstrated as superposition of magnetic flux quanta in superconducting nanowires 7 . However, the direct flat 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 set-up bandwidth. The current steps were theoretically predicted in small Josephson junctions 30 years ago 5 . However, unavoidable broadening in Josephson junctions prevents their direct experimental observation 8,9 . We solve this problem by placing a thin NbN nanowire in an inductive environment.