Shapiro steps in an atomic Josephson junction

When a superconductive Josephson junction is driven by an oscillating current, it develops a series of evenly spaced voltage plateaus known as Shapiro steps, like climbing a quantum staircase. As the height of each step is directly determined by the frequency of the applied current, the Shapiro steps provide the definition of the voltage standard.

In this work, we report the observation of Shapiro steps in periodically driven Josephson junctions of strongly interacting Fermi superfluids of ultracold atoms. Similarly to the superconducting case, we observe quantized plateaus in the current–potential characteristics, which plays the role of the voltage in neutral atomic Josephon junctions. By measuring the junction relative phase, we also directly probe the current–phase relationship, showing that Shapiro steps originate from the synchronization of the junction relative phase with the applied drive. In the n-th Shapiro steps the junction phase undergoes to n phase slips event, which we directly visualize as the emission of n vortex-antivortex pairs. 

G. Del Pace et al.
Shapiro steps in strongly-interacting Fermi gases
Science 390, 1125-1129 (2025)