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Figure 4: QSER for (a) the OAM -encoded quantum link as a function of the average photon number per pulse ( ) and (b) the two-OAM-encoded link as a function of OAM mode spacing when . Different cases: back-to-back link (1) with no probe beam and (2) with probe beam; under the influence of turbulence effects (3) without any compensation without the probe beam, (4) without any compensation with the probe beam, and (5) with AO compensation with the probe beam. QSER: the ratio of accumulated error symbols (photons) to the total registered symbols (photons). The dash-dot lines are experimental results from [9], in which the encoding OAM modes are generated and received by SLMs.