  Control of the Coulomb interaction between single electrons is vital for
realizing quantum information processing using flying electrons and,
particularly, for the realization of deterministic two-qubit operations. 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. Moreover, it would be useful to study such a Coulomb collision in silicon
system, which has been extensively studied for qubit applications but so far
has 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 an important step toward quantum manipulation
of flying single electrons in silicon.
