  Many fundamental and applied experiments in quantum optics require
transferring nonclassical states of light through large distances. In this
context the free-space channels are a very promising alternative to optical
fibers as they are mobile and enable to establish communications with moving
objects, using satellites for global quantum links. For such channels the
atmospheric turbulence is the main disturbing factor. The statistical
properties of the fluctuating transmittance through the turbulent atmosphere
are given by the probability distribution of transmittance (PDT). We derive the
consistent PDTs for the atmospheric quantum channels by step-by-step inclusion
of various atmospheric effects such as beam wandering, beam broadening and
deformation of the beam into elliptic form, beam deformations into arbitrary
forms. We discuss the applicability of PDT models for different propagation
distances and optical turbulence strengths in the case when the receiver module
has an annular aperture. We analyze the optimal detection and correction
strategies which can improve the channel transmission characteristics. The
obtained results are important for the design of optical experiments including
postselection and adaptive strategies and for the security analysis of quantum
communication protocols in free-space.
