Dynamique quantique dans un dcSQUID : du qubit de phase à l'oscillateur quantique bidimensionnel

Abstract : This thesis focuses on the quantum dynamics in inductive dcSQUID. This device is a superconducting loop interrupted by two Josephson junctions. Its dynamics can be described as a massive fictitious particle in a two dimensional potential. A dcSQUID behaves as an artificial atom with two degrees of freedom, controlled by current and flux bias. When the loop inductance is smaller than the Josephson inductance, the junctions are strongly coupled. The device is then described as a one dimensional quantum anharmonic oscillator. In the limit of the two lowest energy levels, a dcSQUID is a phase qubit. Until now decoherence was dominated by the current noise. We show by spectroscopic measurement and coherent oscillations measurement that the effect of the current noise vanishes at zero current bias, enabling longer coherence times. When the loop inductance is larger than the Josephson inductance, the dynamics becomes two dimensional. The device exhibits a rich energy spectrum which can be describe as the one of two coupled anharmonic oscillators, corresponding to symmetric and antisymmetric oscillations modes of the phases across each junctions. We present spectroscopic measurement of this spectrum. We demonstrate the coherent manipulation of the quantum states of each mode. We show evidence of non linear coupling between the modes, in the strong coupling regime. This coupling enables the measurement of coherent oscillations between the internal modes of this artificial atom. In addition we present a novel fabrication technique that allows metallic junction fabrication by angle evaporation without the use of suspended bridge of resist. We propose also a simple model based on heating effects that explain for the first time a frequent anomaly in the IV characteristic of dcSQUID.
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Theses
Other [cond-mat.other]. Université de Grenoble, 2011. French. <NNT : 2011GRENY019>


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Florent Lecocq. Dynamique quantique dans un dcSQUID : du qubit de phase à l'oscillateur quantique bidimensionnel. Other [cond-mat.other]. Université de Grenoble, 2011. French. <NNT : 2011GRENY019>. <tel-00601382v3>

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