COHERENT STATES OF ALTERNATING-CURRENT

We study counting statistics of electric current pumped by pulses of a n external field. The fluctuations depend on the pulse shape, and can be minimized by choosing the pulse shape properly. For an optimal puls e shape, the fluctuations are reduced to the de level, i.e., they do n ot depend on the duty cycle of the signal. We develop an approach that allows us to calculate all counting statistics for various driving fi elds, optimal and nonoptimal. The statistics depend in an interesting way on the analytic structure of the field time dependence, and displa y an analogy with coherent states and instantons.