  We investigate the time-dependent transport properties of single and double
quantum-impurity systems based on the hierarchical equations of motion (HEOM)
approach. In the Kondo regime, the dynamical current in both cases is found
oscillating due to the temporal coherence of electrons tunneling through the
device, which shares the same mechanism as the single-level resonance without
e-e interactions but shows some different characteristics. For single
quantum-impurity systems, the temperature T plays an inhibitory action to the
oscillations of dynamic current through its suppression to the Kondo effects.
The amplitude of the current oscillations is attenuated by the e-e interaction
$U$ in the Kondo regime. The frequency of the current oscillation is found
almost independent of T and U. For parallel-coupling double quantum-impurity
systems, the oscillation of the current shows similar behaviors to the single
one, but with two-to-three times larger amplitudes. At the limit of small
inter-impurity coupling the oscillation of the current exhibits enhanced
characters while it is weakened at the other limit.
