Coherent states of alternating current

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 dc level, i.e., they do not depend on the duty cycle of the signal. We develop an approach that allows us to calculate all counting statistics for various driving fields, optimal and nonoptimal. 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.