Transport and entanglement of orbital angular momentum photonic states in turbulent atmosphere

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Photons carrying orbital angular momentum can potentially revolutionise modern free space communication technologies, owing to the ability of such photons to encode highly-dimensional Hilbert spaces in their spatial wave fronts. However, the omnipresent atmospheric turbulence distorts the wave front and thus undermines the fidelity of the transmitted information. The first part of the present project therefore aims at developing a general and innovative transport theory for the propagation of photons in atmospheric turbulence. This theory will shed new light on several fundamental questions most pertinent to the field of free-space quantum communication. In particular, it will not only allow us to better understand and quantitatively describe the effect of atmospheric decoherence on photonic orbital angular momentum states, but also clarify the physical mechanism for the occurrence of large intensity fluctuations in the transmitted beam. In the second part of our project, we will aim at demonstrating the usefulness of orbital angular momentum states for free space quantum communication or quantum information technologies. For this purpose, we will develop efficient protocols for entanglement detection, identify robust entangled states that are most stable against the atmospheric decoherence, and propose experimentally realisable ways of further reducing the detrimental effects of propagation in atmosphere using methods of adaptive optics. The theoretical work in Freiburg will be performed in close cooperation with our experimental partner group in Pretoria.