Synchronization of Bloch Oscillations in a Strongly Coupled Pair of Small Josephson Junctions: Evidence for a Shapiro-like Current Step

Bloch oscillations are a fundamental phenomenon linking the adiabatic transport of Cooper pairs to time. Here, we investigate synchronization of the Bloch oscillations in a strongly coupled system of sub-100 nm $\mathrm{Al}/{\mathrm{AlO}}_x/\mathrm{Al}$ Josephson junctions in a high-Ohmic environment composed of highly inductive meanders of granulated aluminum and high-Ohmic titanium microstrips. We observe a pronounced current mirror effect in the coupled junctions and demonstrate current plateaus, akin to the first dual Shapiro step in microwave experiments. These findings suggest that our circuit design holds promise for realizing protected Bloch oscillations and precise Shapiro steps of interest for current metrology.