Enhancement of Josephson Quasiparticle Current in Coupled Superconducting Single-electron Transistors

The Josephson quasiparticle (JQP) cycle in a voltage-biased superconducting single-electron transistor (SSET) combines coherent Cooper pair tunneling with incoherent quasiparticle decay. We have measured a striking modification of the JQP process in a system of two SSET's having a large mutual capacitance. We find, among other effects, that the JQP current in one SSET may be enhanced by the presence of a quasiparticle current through the other SSET. A simplified model of the coupled-SSET system is presented which reproduces the enhancement effect.