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TT: Fachverband Tiefe Temperaturen

TT 22: Transport: Poster Session

TT 22.15: Poster

Mittwoch, 27. Februar 2008, 14:00–18:00, Poster B

Effective capacitance of charge qubits undergoing Landau-Zener transitions — •Georg Reuther, Peter Hänggi, and Sigmund Kohler — Institut für Physik, Universität Augsburg, 86135 Augsburg

A Cooper pair box in the charging regime has been used for the implementation of solid-state qubits in various experiments. The state of such a qubit can be probed by a weak rf signal. At the charge degeneracy point, the phase of the reflected signal depends sensitively on the effective capacitance of the qubit which, in turn, depends on the energy of the qubit state. For an ac-driven qubit undergoing multiple Landau-Zener transitions, the tunnelling dynamics has been monitored in this way [1]. Relating the measured data to the qubit state, however, was based on the assumption that the qubit relaxation is either very slow or very fast. We work beyond this limit and study the effective capacitance in the crossover regime of intermediate relaxation times. Our results allow for relating the measured phase shift to the actual state of the qubit and to estimate the measurement fidelity.

[1] M. Sillanpää et al, Phys. Rev. Lett. 96, 187002 (2006)