  A current due to a tunneling event that involves three times the charge of an
electron was observed in the current - voltage characteristics of a
superconducting single-electron tunneling transistor. In this tunnel event, a
Cooper pair tunnels through one tunnel barrier simultaneously with a
quasiparticle that tunnels through a second tunnel barrier which is about 0.5
microns distant from the first tunnel barrier. This current was observed in a
bias regime where current flow due to sequential quasiparticle tunneling is
forbidden due to the Coulomb blockade.
