  Mesoscopic physics deals with three fundamental issues: quantum coherence,
fluctuations and correlations. Here we analyze these issues for atom optics,
using a simplified model of an assembly of atoms (or detectors, which are
particles with some internal degree of freedom) moving in arbitrary
trajectories in a quantum field. Employing the influence functional formalism,
we study the self-consistent effect of the field on the atoms, and their mutual
interactions via coupling to the field. We derive the coupled Langevin
equations for the atom assemblage and analyze the relation of dissipative
dynamics of the atoms with the correlation and fluctuations of the quantum
field. This provides a useful theoretical framework for analysing the coherent
properties of atom-field systems.
