Single photons in orbital angular momentum (OAM) modes have attracted attention largely because of their potential use in free-space quantum communication. While traditional quantum communication protocols encode information using two states of a photon, such as two orthogonal polarization states, there are an infinite number of orthogonal OAM states, meaning that information can be encoded in a high-dimension space and each photon can carry several bits of information. This is especially useful over longer distances when many photons are lost in transmission, so as to increase bit rate while restricted to a low flux of photons.

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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