We have studied both
theoretically and experimentally how an LC series circuit connected in parallel to a Josephson junction influences the Josephson dynamics. The presence of the shell circuit introduces two energy scales, which in specific cases, can strongly differ from the plasma frequency of the isolated junction. Josephson junctions were manufactured using the Nb/Al-AlOx/Nb fabrication technology with various on-chip LC shunt circuits. Spectroscopic measurements in the quantum limit show excellent agreement with theory taking into account the shunt inductance and capacitance in the resistively and capacitively shunted junction model. The results clearly show that the dynamics of the system are two-dimensional, resulting in two resonant modes of the system. These findings have important implications for the design and operation of Josephson junction based quantum bits.
This work was partly supported by the Deutsche Forschungsgemeinschaft Center for Functional Nanostructures (Project No. B3.4), the Swedish Research Council (VR), EU STREP project MIDAS, and the Knut and Alice Wallenberg Foundation. F.L. was supported by a grant from the Knut and Alice Wallenberg Foundation. Furthermore, we would like to thank M. Birk for help with inductor simulation, D. Bruch for the design of the printed circuit board, and J. Czolk and T. Wienhold for help with LC circuit characterization.
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