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Fluxon cotunneling in coupled Josephson junctions: Perturbation theory
We investigate the effects of fluxon cotunneling and quantum Coulomb drag in a system of two small
Josephson junctions coupled by means of mutual capacitanceCm. Depending on the value ofCm we identify three
different regimes of strong, intermediate, and weak coupling. Focusing our attention on the last two regimes we
develop a perturbation theory in the interaction and explicitly derive fluxon cotunneling amplitudes at sufficiently
small mutual capacitance values. We demonstrate that the Coulomb drag effect survives at any nonzero Cm
and evaluate the nonlocal voltage response that is in general determined by a trade-off between two different
cotunneling processes. Our predictions can be straightforwardly generalized to bilinear Josephson chains and
directly verified in future experiments.