We provide a direct proof of two-electron Andreev transitions in a superconductor - normal metal tunnel junction by detecting them in a real-time electron counting experiment. Our results are consistent with ballistic Andreev transport with an order of magnitude higher rate than expected for a uniform barrier, suggesting that only part of the interface is effectively contributing to the transport. These findings are quantitatively supported by our direct current measurements in single-electron transistors with similar tunnel barriers.
@article{arxiv.1012.5750,
title = {Real-time observation of discrete Andreev tunneling events},
author = {V. F. Maisi and O. -P. Saira and Yu. A. Pashkin and J. S. Tsai and D. V. Averin and J. P. Pekola},
journal= {arXiv preprint arXiv:1012.5750},
year = {2012}
}