Coherent Cooper pair tunneling in systems of Josephson junctions: effects of quasiparticle tunneling and of the electromagnetic environment

Small capacitance tunnel junctions show single electron effects and, in the superconducting state, the coherent tunneling of Cooper pairs. We study these effects in a system of two Josephson junctions, driven by a voltage source with a finite impedance. Novel features show up in theI–V characteristics, in particular pronounced structures at subgap voltages. These are due to Cooper pair tunneling, combined with tunneling of quasiparticles or with excitation of the electromagnetic environment