Effect of Measurement on the Periodicity of the Coulomb Staircase of a Superconducting Box

Abstract

We report on the effect of the backaction of a single Cooper pair transistor electrometer ($\mathbf{E}$) on the charge state of a superconducting box ($\mathbf{B}$). The charge is $e$ periodic in the gate bias of $\mathbf{B}$ when $\mathbf{E}$ is operated near voltages $2\Delta /e$ or $4\Delta /e$. We show that this is due to quasiparticle poisoning of $\mathbf{B}$ at a rate proportional to the number of quasiparticle tunneling events in $\mathbf{E}$ per second. We are able to eliminate this backaction and recover $2e$-charge periodicity using a new measurement method based on switching-current modulation of $\mathbf{E}$.