Cooper-Pair Box as a Variable Capacitor

Abstract

We calculate the effective capacitance of a Cooper-pair box and demonstrate that the Cooper-pair box can be used as a variable capacitor. The gate voltage modulates the charge transfer through the small junction in the Cooper-pair box, leading to a gate-dependent capacitance. We describe ongoing experiments to use an Al/AlO/sub x//Al Cooper-pair box as a tunable circuit element at milli-Kelvin temperatures and discuss possible applications to quantum computing as a variable coupling element between two Josephson junction qubits.