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 $(R_S\ll R_Q\equiv {h/4e}^2)$. 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 $ħ/2\mathrm{}$ when $T\to 0$.