Quantum-Limited Electrometer Based on Single Cooper Pair Tunneling

We propose a system comprising the small-capacitance double Josephson junction shunted as a whole by a small resistance (RSRQ≡h/4e²). We have calculated the dynamic and noise characteristics of the system as functions of a polarization charge on the central electrode. It has been shown straightforwardly that the energy resolution of such an electrometer implying the ``backaction noise'' approaches the fundamental quantum limit /2 when T→0.