ar X iv : c on d - m at / 9 51 11 28 v 1 2 7 N ov 1 99 5 Supercurrent noise in quantum point contacts

Spectral density of current fluctuations in a short ballistic superconducting quantum point contact is calculated for arbitrary bias voltages V. Contrary to a common opinion that the supercurrent flow in Josephson junctions is coherent process with no fluctuations, we find extremely large current noise that is caused by the supercurrent coherence. An unusual feature of the noise, besides its magnitude, is its voltage dependence: the noise decreases with increasing V , despite the fact that the dc current grows steadily with V. At finite voltages the noise can be qualitatively understood as the shot noise of the large charge quanta of magnitude 2∆/V equal to the charge transferred during one period of Josephson oscillations.