  We propose a mixedness quantifier based on entropy fluctuations. It provides
information about the degree of mixedness either for finite dimensional and
infinite dimensional Hilbert spaces. It may be used to determine the reduction
of the Hilbert space as it becomes maximum when, either the state is maximally
mixed, or, when the Hilbert space effectively reduces its dimensions, such as
in the atom field interaction where the two-level atom dictates the final
dimension of the field.
