We have performed an interference experiment with two tunnel junctions between a thin normal metal wire and a superconducting fork. We have found that the subgap current is strongly modulated by the flux through the resulting normal-superconducting loop. Our results agree with the recent prediction that multiple tunnel attempts of electron pairs, which occur when electrons are confined near the junction, can add coherently if the pair consists of nearly time-reversed states.

• Received 26 April 1994

DOI:https://doi.org/10.1103/PhysRevLett.73.2488