Coulomb collisions of hot and cold single electrons in series-coupled silicon single-electron pumps

Precise understanding of the Coulomb interaction between single electrons is vital to achieve accurate single-electron control toward quantum current standards and quantum information processing. Since the strength of the Coulomb interaction increases with decreasing distance, a collision experiment of single electrons would be an ideal way to investigate it. It would be useful to study such a Coulomb collision in silicon single-electron pumps, which can accurately transfer single electrons one by one, while silicon systems have not been used for making Coulomb collisions at the single-electron level. Here, we made two series-coupled tunable-barrier single-electron pumps in silicon and used one to inject a hot single electron into the other pump in each pumping cycle. The hot single electron collides with a cold single electron confined in the other single-electron pump. We observed a current flow due to ejection not only of the hot single electron but also of the confined cold single electron. The latter leads to an excess current at a current plateau at a certain voltage range. We also found that increasing the number of cold electrons from one to two increased the cold-electron current by at least twofold. These results can be explained by a charging effect due to the Coulomb interaction. This observation is valuable to understand single-electron dynamics in the silicon single-electron devices toward accurate current generation and quantum manipulation of flying single electrons.