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.
Maisi, V. F., Saira, O-P., Pashkin, Y. A., Tsai, J. S., Averin, D. V., & Pekola, J. P. (2011). Real-time observation of discrete Andreev tunneling events. Physical Review Letters, 106(21), . https://doi.org/10.1103/PhysRevLett.106.217003