Resonant Tunneling through a Macroscopic Charge State in a Superconducting Single Electron Transistor

We predict theoretically and observe in experiment that the differential conductance of a superconducting single electron transistor exhibits a peak which is a complete analog, in a macroscopic system, of a standard resonant tunneling peak associated with tunneling through a single quantum state. In particular, in a symmetric transistor, the peak height is universal and equal to e²/2πħ. Away from the resonance we clearly observe the cotunneling current which, in contrast to the normal-metal transistor, varies linearly with the bias voltage.