Tunnel spectroscopy of a double superconducting island qubit

We have probed by tunnel spectroscopy a quantum system consisting of two superconducting islands strongly coupled by an ultrasmall Josephson junction. The Josephson junction allows Cooper pair delocalization between the two islands and gate voltages control the state of the system. Tunnel spectroscopy is made possible by means of two other junctions weakly coupling the islands to measurement leads. We have observed different transport regimes depending on the applied bias voltage.