  We investigate effects of thermal and quantum fluctuations of the superfluid
order parameter in $^1S_{0}$ superfluids in neutron stars. We construct a
separable potential to reproduce the $^1S_{0}$ phase shift reconstructed by
using the partial wave analysis from nucleon scattering data. We include
superfluid fluctuations within a strong-coupling approximation developed by
Nozi\`eres and Schmitt-Rink and determine self-consistently the superfluid
order parameter as well as the chemical potential. We show that the quantum
depletion, which gives a fraction of noncondensed neutrons at zero temperature
due to quantum pairing fluctuations, plays an important role not only near the
critical temperature from superfluid states to normal states but also at zero
temperature. We derive the dispersion relation of Anderson-Bogoliubov modes
associated with phase fluctuations, and show also that there is a nonzero
fraction of noncondensed components in the neutron number as a result of the
strong-coupling effect. Our results indicate that superfluid fluctuations are
important for thermodynamic properties in neutron stars.
