No abstract

Recently, there is a tremendous attention on the high-dimensional quantum information processing using the orbital angular momentum(OAM) of photons. It provides an efficient method to increase the capacity of quantum channel. However, OAM is very vulnerable to atmospheric turbulence, resulting in phase distortion and therefore, leading to information leakage during quantum communications. On the other hand, quantum secure direct communications (QSDC) defines an efficient way of information exchange through quantum channels directly. Here in this work, by combining the applications of OAM states of photons as the information carrier with the QSDC, we analyze the performance of the high-dimensional quantum secure direct communication protocol based on encoding the information on the single photon OAM state under the condition of free space channel. Numerically, we discussed the performance of OAM states QSDC under atmospheric turbulence, and mainly calculates the influence of cross-talk caused by phase fluctuation on the system. We draw the conclusion that under the weak turbulence, when the beam waist $w^{\prime}$ and the Fried parameter $r$ become comparable, the information capacity will decrease sharply.