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

携带轨道角动量的光子可能会彻底改变现代自由空间通信技术，因为这种光子能够在其空间波前编码高维希尔伯特空间。然而，无所不在的大气湍流会扭曲波前，从而破坏传输信息的保真度。因此，本项目的第一部分旨在发展大气湍流中光子传播的一般和创新的输运理论。这一理论将对与自由空间量子通信领域最相关的几个基本问题提供新的启示。特别是，它不仅可以让我们更好地理解和定量描述大气退相干对光子轨道角动量态的影响，而且还可以阐明发射光束中发生大强度波动的物理机制。在我们项目的第二部分，我们将致力于展示轨道角动量状态对自由空间量子通信或量子信息技术的有用性。为此，我们将开发有效的纠缠检测协议，确定对大气退相干最稳定的鲁棒纠缠态，并提出实验可行的方法，进一步减少使用自适应光学方法在大气中传播的有害影响。弗赖堡的理论工作将与比勒陀利亚的实验伙伴小组密切合作进行。

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.