Simultaneous Switching on Vertically Stacked Josephson Junctions with Very Thin Intermediate Electrode

We have demonstrated vertically stacked Josephson junctions with very thin superconductors for the intermediate electrode. The gap voltage of a thin film decreases as the thickness decreases, but the decrease of the energy gap does not worsen the quality of the junction. In the double junction, simultaneous switching between the lower and upper junctions is observed. Although simultaneous switching is also observed in the series-connected Josephson junction, we have found some differences between them and have investigated the condition of simultaneous switching in the double junction.