1. Department of Applied Physics, Delft University of Technology and Delft Institute for Micro-Electronics and Submicron Technology (DIMES), PO Box 5046, 2600 GA Delft, The Netherlands

Abstract

A HEISENBERG uncertainty relation exists between any two non-commuting variables of a quantum-mechanical system. In a super-conductor, two such variables are the number, n, of Cooper pairs and the phase, , of the superconducting wavefunction. Suppress-ing fluctuations in either variable should lead to enhanced fluctua-tions in the other^1,2. To demonstrate this effect, we have fabricated a structure in which the quantum-mechanical fluctuations in the phase of a superconducting grain can be suppressed. We measure the supercurrent that flows through two Josephson tunnel junctions of small capacitance that are connected to the grain. The capacit-ance of the grain is itself so small that the number of Cooper pairs is well defined—charge transport through the grain is possible only through quantum-mechanical fluctuations in n. The phase of the grain is coupled to a large superconducting reservoir such that the fluctuations in can be controllably suppressed. The enhanced fluctuations in n that result from this coupling give rise to a large increase in the supercurrent through the grain.