  Electronic transport through a two-level system driven by external electric
field and coupled to (magnetic or non-magnetic) electron reservoirs is
considered theoretically. The basic transport characteristics such as current
and tunnel magnetoresistance (TMR) are calculated in the weak coupling
approximation by the use of rate equation connected with Green function
formalism and slave-boson approach. The time dependent phenomenon is considered
in the gradient expansion approximation. The results show that coherent beats
pattern can be observed both in current and TMR. The proposed system consisting
of two quantum dots attached to external leads, in which the dots' levels can
be tuned independently, can be realized experimentally to test this well known
physical phenomenon. Finally, we also indicate possible practical applications
of such device.
