Fluctuation properties and charge resolution of the superconducting SET electrometer in the regime of Cooper-pair tunneling

Single-charge tunneling through a voltage-biased SSS transistor with a capacitively coupled gate and an electromagnetic environment is considered. The dc Cooper-pair current in the blockade region of quasiparticle tunneling is studied. Basing on a two-level approximation. which can be used in the realistic case of low temperatures and small environment resistances, the mean de Cooper-pair current, the noise power spectrum, and the charge resolution are calculated as functions of the driving voltage and the external charge on the island.