Direct Observation of Josephson Capacitance

TL;DR

This paper demonstrates a method to directly measure the Josephson capacitance in a Cooper pair box, enabling non-demolition quantum state readout through reactive low-frequency measurements.

Contribution

It introduces a novel low-frequency reactive measurement scheme to directly observe Josephson capacitance and perform non-demolition quantum state readout in a Cooper pair box.

Findings

Capacitance measured over phase-gate bias plane matches theoretical predictions.

The method allows efficient non-demolition readout of the CPB quantum state.

Quantitative analysis confirms the band-structure-based explanation.

Abstract

The effective capacitance has been measured in the split Cooper pair box (CPB) over its phase-gate bias plane. Our low-frequency reactive measurement scheme allows to probe purely the capacitive susceptibility due to the CPB band structure. The data are quantitatively explained using parameters determined independently by spectroscopic means. In addition, we show in practice that the method offers an efficient way to do non-demolition readout of the CPB quantum state.