Reduction of photon-losses caused by turbulence using spatial diversity in free-space optics quantum communications

Spatial diversity is a technique widely used in wireless communications to enhance the signal quality at the receiver. We propose a multiple-input single-output system that utilizes this technique to enhance a free-space optics quantum communication link by reducing the amount of photon losses caused by atmospheric turbulence, thus increasing the capacity of the quantum channel. The system consists of two transmitters with uncorrelated optical paths, and a single receiver. A 515-nm quantum signal is transmitted through the transmission path with the highest gain, dynamically chosen by comparing the signal distortions of a 660-nm classical signal. Preliminary experiments with a single transmitter have been conducted in a laboratory environment with atmospheric turbulence generated via heat guns. We observed that the single-photon channel is highly correlated with the fluctuations of the 660-nm classical signal, so that an improvement in the former is expected when selecting the path with highest gain. The number of photon counts received was compared with the turbulence-free scenario, revealing that the mean number of counts decreased, and its standard deviation increased when turbulence is present, as expected.