Elementary events of electron transfer in a voltage-driven quantum point contact

We show that the statistics of electron transfer in a coherent quantum point contact driven by an arbitrary time-dependent voltage is composed of elementary events of two kinds: unidirectional one-electron transfers determining the average current and bidirectional two-electron processes contributing to the noise only. This result pertains at vanishing temperature while the extended Keldysh-Green's function formalism in use also enables the systematic calculation of the higher-order current correlators at finite temperatures.
4 pages, 2 figures; to appear in PRL. Version 1 contains additional results for noise and the third cumulant at finite temperatures [Eqs. (19) and (20)]