  Entanglement in fermion many-body systems is studied using a generalized
definition of separability based on partitions of the set of observables,
rather than on particle tensor products. In this way, the characterizing
properties of non-separable fermion states can be explicitly analyzed, allowing
a precise description of the geometric structure of the corresponding state
space. These results have direct applications in fermion quantum metrology:
sub-shot noise accuracy in parameter estimation can be obtained without the
need of a preliminary state entangling operation.
