  We investigate work extraction from non-interacting fermions under arbitrary
unitary operations and the more restricted class of Gaussian unitary operations
that can be feasibly implemented. We characterize general quantum states in
fermionic systems according to their ability to yield work (or not) under such
transformations and study the limit for which multiple copies of passive states
in fermionic systems can be activated for work extraction. We find that a
sufficient number of copies of non-thermal passive states can achieve this,
yielding an upper bound on the number of copies needed.
