  Using an experimental setup that simulates a turbulent atmosphere, we study
the secret key rate for quantum key distribution protocols in orbital angular
momentum based free space quantum communication. The quantum key distribution
protocols under consideration include the Ekert 91 protocol for different
choices of mutually unbiased bases and the six-state protocol. We find that the
secret key rate of these protocols decay to zero roughly at the same scale
where the entanglement of formation decays to zero.
