Resonant Cooper pair tunneling through a double-island qubit.

We have measured transport in the Coulomb blockade regime through a quantum system consisting of two superconducting islands coupled by a Josephson junction. The double island is weakly connected to leads by small superconducting tunnel junctions. At low bias voltages, a new resonant transport process for Cooper pairs is observed. The results show that Cooper pair transport can be used as a probe of the macroscopic energy levels of the double island.