We consider nanojunctions in the single-electron tunnelling regime which, due to a high degree of spatial symmetry, have a degenerate many body spectrum. As a consequence, interference phenomena which cause a current blocking can occur at specific values of the bias and gate voltage. We present here a general formalism to give necessary and sufficient conditions for interference blockade also in the presence of spin polarized leads. As an example we analyze a triple quantum dot single electron transistor (SET). For a set-up with parallel polarized leads, we show how to selectively prepare the system in each of the three states of an excited spin triplet without application of any external magnetic field.
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