  We study counting statistics of electric current pumped by pulses of an
external field. The fluctuations depend on the pulse shape, and can be
minimized by choosing the pulse shape properly. For an optimal pulse shape, the
fluctuations are reduced to the {\it dc} level, i.e., they do not depend on the
duty cycle of the signal. We develop an approach that allows to calculate all
counting statistics for various driving fields, optimal and non-optimal. The
statistics depend in an interesting way on the analytic structure of the field
time dependence, and display an analogy with coherent states and instantons.
