  Majorana fermions often coexist with other low-energy fermionic degrees of
freedom. In such situation, topological quantum computation requires the use of
fermionic zero modes of a many-body system. We classify all such modes for
interacting fermions and show how to select the mode that maximizes the
decoherence time. We find that in a typical interacting system the maximal
decoherence time is within one order of magnitude from the decoherence time of
a qbit based on the local part of the fermion parity operator.
