  Due to atomic interactions and dispersion in the total atom number, the order
parameter of a pair-condensed Fermi gas experiences a collapse in a time that
we derive microscopically. As in the bosonic case, this blurring time depends
on the derivative of the gas chemical potential with respect to the atom number
and on the variance of that atom number. The result is obtained first using
linearized time-dependent Bogoliubov-de Gennes equations, then in the Random
Phase Approximation, and then it is generalized to beyond mean field. In this
framework, we construct and compare two phase operators for the paired
fermionic field: The first one, issued from our study of the dynamics, is the
infinitesimal generator of adiabatic translations in the total number of pairs.
The second one is the phase operator of the amplitude of the field of pairs on
the condensate mode. We explain that these two operators differ due to the
dependence of the condensate wave function on the atom number.
