In free-space, as opposed to fibre-optic communications, OAM is expected to be well-preserved during propagation. However, these states can still be disrupted by atmospheric turbulence, which distorts the mode-shape such that OAM is not preserved. In this work, X. Yan et al. study the effect of turbulence on two photons which are entangled in their OAM. A numerical model is used, with the photons’ propagation through air split into small segments, each of which contains the normal evolution of the mode and an additional randomised, inhomogeneous phase-plate, which plays the part of the turbulence. The authors go beyond previous theoretical works by using a more general non-Kolmogorov spectrum for these spatial phase fluctuations. They plot the decay of entanglement with propagation, considering the effects of the different length scales involved in the turbulence. In particular, it is seen that when the OAM quantum numbers involved are higher, more entanglement is retained – this is attributed to the modes’ increased area and finer spatial structure. It is hoped that by better understanding the effect of turbulence on entangled OAM states, mitigating strategies can be adopted to allow robust quantum communication.
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