Direct Observation of Josephson Capacitance

M. A. Sillanpaa, T. Lehtinen, A. Paila, Yu. Makhlin, L. Roschier, P. J. Hakonen

Published 2005-04-20, updated 2006-01-13Version 2

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.