Coulomb blockade of two-electron tunneling

We study the Coulomb blockade in a superconducting grain, connected to two normal electrodes by tunnel junctions. At small bias, the conductance of this system is due to electrons passing in pairs through the grain. The linear conductance is periodic in the gate voltage. The period and the conductance activation energy are determined by the charge 2e, rather than e. At resonance the current first grows linearly with the applied bias and then drops as the quasiparticle transport channel opens up.