  The evolution of an entangled photon state propagating through a turbulent
atmosphere is formulated in terms of a set of coupled first order differential
equations, by using an infinitesimal propagation approach. The orbital angular
momentum (OAM) basis is used to described the density matrix of the state.
Although the analysis is done in the paraxial limit for a monochromatic optical
field, the formalism is comprehensive in the sense that it does not require any
assumptions about the strength of the turbulence and it can incorporate any
spectral model for the turbulence. As a comparative example the case of
entangled qubit OAM biphoton states is considered.
