Effects of atmospheric turbulence on polarization entanglement in free-space quantum communication links

Quantum entanglement is essential for building the backbone of quantum information systems. Our particular interest lies in long-range distribution of entangled photons in free space. To achieve this, we rely on photon pairs representing qubits with polarization correlation. The main focus of this paper is integrity of the quantum states in free-space links. When transmitted in atmosphere, classical signals suffer from wave front distortions caused by turbulence; however, its mechanism does not have the same bearing on qubit values. We study the effects of turbulence on quantum states by utilizing a laboratory testbed that includes an atmospheric chamber. It uses a system of controlled components capable of creating various turbulence conditions. When polarized signals are passed through the atmospheric chamber, we analyze the corresponding quantum states and evaluate the degree of entanglement using our mathematical models and existing metrics, such as the coincidence-to-accidental ratio.