  In this work we show how the criterion of asymptotic Hamiltonian
diagonalization originated in hybrid quantum cosmology serves to pick out a
unique vacuum for the Dirac field in de Sitter, in the context of quantum field
theory in curved spacetimes. This criterion is based on the dynamical
definition of annihilation and creationlike variables for the fermionic field,
which obey the linearized dynamics of a Hamiltonian that has been diagonalized
in a way that is adapted to its local spatial structure. This leads to
fermionic variables that possess a precise asymptotic expansion in the
ultraviolet limit of large wavenumbers. We explicitly show that, when the
cosmological background is fixed as a de Sitter solution, this expansion
uniquely selects the choice of fermionic annihilation and creationlike
variables for all spatial scales, and thus picks out a unique privileged Fock
representation and vacuum state for the Dirac field in de Sitter. The explicit
form of the basis of solutions to the Dirac equation associated with this
vacuum is then computed.
