Coulomb Blockade and Coherent Single-Cooper-Pair Tunneling in Single Josephson Junctions

Abstract

We have measured the current-voltage characteristics of small-capacitance single Josephson junctions at low temperatures ( $T\le 0.04\mathrm{K}$), where the strength of the coupling between the single junction and the electromagnetic environment was controlled with one-dimensional arrays of dc SQUIDs. We have clearly observed Coulomb blockade of Cooper-pair tunneling and even a region of negative differential resistance, when the zero-bias resistance of the SQUID arrays is much higher than the quantum resistance ${h/e}^2\approx 26\mathrm{k}\Omega$. The negative differential resistance is evidence of coherent single-Cooper-pair tunneling in the single Josephson junction.