Single-end adaptive optics compensation for emulated turbulence in a Bi-directional 10-Mbit/s per channel free-space quantum communication link using orbital-angular-momentum encoding

A single-end adaptive-optics (AO)module is experimentally demonstrated tomitigate the emulated atmospheric turbulence effects in a bi-directional quantumcommunication link, which employs orbital angularmomentum(OAM) for data encoding. A classical Gaussian beam is used as a probe to detect the turbulence-induced wavefront distortion in the forward direction of the link. Based on the detected wavefront distortion, an AO system located on one end of the link is used to simultaneously compensate for the forward and backward channels. Specifically,with emulated turbulence andwhen the probe is turned on, themode purity of photons carrying OAM ℓ = 1 is improved by ∼ 21 % with AO mitigation. We also measured the performance when encoding data using OAM {ℓ = -1, +2} and {ℓ = -2, +1} in the forward and backward channels, respectively, at 10 Mbit/s per channel with one photon per pulse on average. For this case, we found that the AO system could reduce the turbulence effects increased quantum-symbolerror- rate (QSER) by ∼ 76 % and ∼ 74 %, for both channels in the uni-directional and bi-directional cases, respectively. Similar QSER improvement is observed for the opposite direction channels in the bi-directional case.