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, φ, of the superconducting wavefunction. Suppress-ing fluctuations in either variable should lead to enhanced fluctua-tions in the other1,2. 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 φ 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.
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Elion, W., Matters, M., Geigenmüller, U. et al. Direct demonstration of Heisenberg's uncertainty principle in a superconductor. Nature 371, 594–595 (1994). https://doi.org/10.1038/371594a0
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DOI: https://doi.org/10.1038/371594a0
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