Cooper-pair tunneling in small junctions with tunable Josephson coupling

We investigate Cooper-pair tunneling in a circuit consisting of two dc-superconducting quantum inter- ference devices in series, with a gate capacitively coupled to the central island. Measurements cover a wide range of values of the ratio between Josephson coupling energy {ital E}{sub {ital J}} and charging energy {ital E}{sub {ital C}}. The {ital E}{sub {ital J}}/{ital E}{sub {ital C}} ratio dependence of the supercurrent is well described by the orthodox theory provided that strong fluctuations of the Josephson phase due to the electromagnetic environment are taken into account. Our data can be interpreted in terms of squeezing of the charge fluctuations with decreasing {ital E}{sub {ital J}}/{ital E}{sub {ital C}} ratio. {copyright} {ital 1996 The American Physical Society.}