Universality of the Kondo effect in a quantum dot out of equilibrium

We study the Kondo effect in a quantum dot subject to an external ac field. The Kondo effect can be probed by measuring the de current induced by an auxiliary de bias V-dc applied across the dot. In the absence of ac perturbation, the corresponding differential conductance G(V-dc) is known to exhibit a sharp peak at V-dc = 0, which is the manifestation of the Kondo effect. There exists only one energy scale, the Kondo temperature TK, which controls all the low-energy physics of the system; G is some universal function of eV(dc)/T-K. We demonstrate that the dot driven out of equilibrium by an ac field is also characterized by a universal behavior: the conductance G depends on the ac field only through two dimensionless parameters, which are the frequency omega and the amplitude of the ac perturbation, both divided by T-K. We analytically find the large- and small-frequency asymptotes of the universal dependence of G on these parameters. The obtained results allow us to predict the behavior of the conductance in the crossover regime h omega similar to T-K.