  In a two-level system, constituted by two serially coupled single level
quantum dots, coupled to external leads we find that the current is suppressed
in one direction of biasing caused by a fully occupied two-electron triplet
state in the interacting region. The efficiency of the current suppression is
governed by the ratio between the interdot tunnelling rate and the level
off-set. In the opposite bias direction, the occupation of the two-electron
triplet is lifted which allows a larger current to flow through the system,
where the conductance is provided by transitions between one-electron states
and two-electron singlet states. Is is also shown that a finite ferromagnetic
interdot exchange interaction provides an extended range of the current
suppression, while an anti-ferromagnetic exchange leads to a decreased range of
the blockade regime.
