The turbulence induced decay of orbital angular momentum (OAM) entanglement between two photons is investigated numerically and experimentally. To compare our results with previous work, we simulate the turbulent atmosphere with a single phase screen based on the Kolmogorov theory of turbulence. We consider two different scenarios: in the first only one of the two photons propagates through turbulence, and in the second both photons propagate through uncorrelated turbulence. Comparing the entanglement evolution for different OAM values, we found the entanglement to be more robust in turbulence for higher OAM values. We derive an empirical formula for the distance scale at which entanglement decays in term of the scale parameters and the OAM value.

The evolution of high dimensional entanglement in atmospheric turbulence is investigated. We study the effects of turbulence on photonic states generated by spontaneous parametric downconversion, both theoretically and experimentally. One of the photons propagates through turbulence, while the other is left undisturbed. The atmospheric turbulence is simulated by a single phase screen based on the Kolmogorov theory of turbulence. The output after turbulence is projected into a three-dimensional (qutrit) basis composed of specific Laguerre-Gaussian modes. A full state tomography is performed to determine the density matrix for each output quantum state. These density matrices are used to determine the amount of entanglement, quantified in terms of the negativity, as a function of the scintillation strength. Theoretically, the entanglement is calculated using a single phase screen approximation. We obtain good agreement between theory and experiment.

Using an experimental setup that simulates a turbulent atmosphere, we study the secret key rate for quantum key distribution (QKD) protocols in orbital angular momentum based free space quantum communication. The QKD 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.

When a pair of photons that are entangled in terms of their transverse modes, such as an orbital angular momentum (OAM) basis, propagates through atmospheric turbulence, the scintillation causes a decay of the entanglement. Here, we use numerical simulations to study how this decoherence process depends on the various dimension parameters of the system. The relevant dimension parameters are the propagation distance, the wavelength, the beam radius, and the refractive index structure constant, indicating the strength of the turbulence. We show that beyond the weak scintillation regime, the entanglement evolution cannot be accurately modeled by a single phase screen that is specified by a single dimensionless parameter. Two dimensionless parameters are necessary to describe the OAM entanglement evolution. Furthermore, it is found that higher OAM modes are not more robust in turbulence beyond the weak scintillation regime.