Coulomb Blockade in a One-Dimensional Array of Strong Josephson Junctions

Abstract

We measure current across a voltage-biased, long array of Josephson junctions. Current-voltage characteristics do not reveal any critical current and instead display the presence of threshold voltage and finite activation energy, consistent with the Coulomb blockade. However, the respective energy scale is well below the value prescribed by a single-island geometric capacitances. We explain it by strong quantum fluctuations of charge associated with a large value of Josephson energy, $E_J\gg E_C$, where $E_C$ is the charging energy associated with the inter-island capacitance. Comparison between the data and theory points to an exponential (in parameter $E_J/E_C$) suppression of the threshold. We achieve a quantitative agreement with theory by accounting for charge-$e$ solitons associated with the quasiparticles entering the array from its leads. This scenario however underestimates the current below threshold in comparison with the experiment.