Quantum versions of de Finetti's theorem are powerful tools, yielding
conceptually important insights into the security of key distribution protocols
or tomography schemes and allowing to bound the error made by mean-field
approaches. Such theorems link the symmetry of a quantum state under the
exchange of subsystems to negligible quantum correlations and are well
understood and established in the context of distinguishable particles. In this
work, we derive a de Finetti theorem for finite sized Majorana fermionic
systems. It is shown, much reflecting the spirit of other quantum de Finetti
theorems, that a state which is invariant under certain permutations of modes
loses most of its anti-symmetric character and is locally well described by a
mode separable state. We discuss the structure of the resulting mode separable
states and establish in specific instances a quantitative link to the quality
of Hartree-Fock approximation of quantum systems. We hint at a link to
generalized Pauli principles for one-body reduced density operators. Finally,
building upon the obtained de Finetti theorem, we generalize and extend the
applicability of Hudson's fermionic central limit theorem.Comment: 15 pages, 1 figure, tiny change