Combined Josephson and charging behavior of the supercurrent in the superconducting single-electron transistor

Measurements of two ultrasmall Josephson junctions connected in series show a supercurrent with magnitude much greater than previously reported for similar devices. The dependence of the supercurrent on gate charge shows agreement with predictions made by Averin and Likharev that are based on a conjugate relationship between the junction phase and charge variables. The transport of Cooper pairs through the two junctions makes the devices behave effectively as a single junction whose critical current is modulated by the gate charge.