  We derive a first order differential equation for the decoherence of an
orbital angular momentum entangled biphoton state propagating through a
turbulent atmosphere. The derivation is based on the distortion that orbital
angular momentum states experience due to propagation through a thin sheet of
turbulent atmosphere. This distortion is treated as an infinitesimal
transformation leading to a first order differential equation, which we call an
infinitesimal propagation equation. The equation is applied to a simple qubit
case to show how the entanglement decays.
