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Soutenance de thèse

Mardi 28 octobre à 14h00,
Salle des séminaires, Bât. A

Orateur : Thomas WEISSL
"Quantum phase and charge dynamics in Josephson junction chains"

Abstract :

In this thesis entitled ’ Quantum phase and charge dynamics in Josephson junction chains ’ an experimental study and theoretical description of quantum effects of phases and charges in chains of Josephson junctions is presented.

The dynamics of Josephson junction chains are dominated by two different energy scales : the Josephson energy, which is related to the overlap of the superconducting wave functions of the two superconductors forming the junction and the charging energy that is related to the electrostatic energy of the Cooper-pairs on the islands. The realization of a well-defined charge state on a Josephson junction requires a high charging energy to suppress the quantum fluctuations of the charge. In addition, the charge relaxation times must be increased by inserting the junction in a high impedance environment.
We have realized such a well-defined charge state on a Josephson junction in an inductive environment that is formed by a Josephson junction chain. The localized charge state manifest itself by the appearance of a high resistive regime in the current-voltage characteristic.

A Josephson junction chain is however not a perfect inductor. Electromagnetic resonances related with the finite ground capacitance of the superconducting islands influence the charge localization.

We have characterized the effect of losses and nonlinearities on the electromagnetic resonances of Josephson junction chains in microwave spectroscopy measurements.

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