Transmission of single photons through atmospheric turbulence with spatial diversity

A multiple-input single-output configuration is used to study the effectiveness of spatial diversity in reducing single-photon losses caused by atmospheric turbulence in quantum communications over free-space. The system consists of two transmitters and a single receiver, where two parallel beams are transmitted, each comprising a 660-nm beacon and a 515-nm collinear single-photon signal. The beacon is intended to compare the turbulence conditions on each path to determine the most suitable transmitter for the single-photon signal. An outdoor experimental campaign with a propagation distance of 500 meters was conducted to obtain results under real turbulence conditions. First, the correlation between the 515-nm and 660-nm collinear signals was measured at 0.82, showing that the beacon signal accurately represents the state of turbulence affecting the single-photon signal most of the time. The spatial correlation was measured at separation distances between the transmitters of 12 and 20 centimeters, yielding correlation coefficients of 0.35 and 0.17, respectively. As expected, the spatial correlation decreases with increasing separation distance. Finally, a slight improvement in the detection of the single-photon signal was observed when using spatial diversity, but further improvements to the system and longer propagation distances are needed to obtain more promising results.