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The well defined orbital angular momentum states of photons offer a practical realization of quantum digits and a means of secure single-photon optical communication. The orbital angular momentum is associated with the spatial distribution of the wave function and the number of orbital angular momentum eigenstates is unlimited, giving the possibility of arbitrary base-N digits. The free-space optical communications using angular momentum states of single photons is investigated and the effect of slant path atmospheric turbulence on the angular momentum of the photons is modeled with Rytov approximation and modified von Karman spectrum model of index-of-refraction fluctuation. The analytical modeling for the angular momentum scattering due to the atmospheric turbulence and the orbital angular momentum quantum number for photons propagation in slant path atmospheric turbulence are presented. The refractive index fluctuations in the atmosphere perturb the complex amplitude of a propagating beam so that the photons that were launched in an eigenstate of orbital angular momentum are no longer guaranteed to be in the original eigenstate after propagation.