Measuring of atmospheric background noise in free-space quantum key distribution systems operating at the 1550 nm wavelength in daylight conditions

In this work we study the level of the background noise at the 1550 nm wavelength, caused by solar radiation, in free-space quantum key distribution systems. We describe the experimental setup for measuring the mean number of photons arriving at a single-photon detector during a gate. We then provide the methodology for recalculating the mean photon number per gate into the absolute value of spectral radiance at the input of the experimental setup. As a result, we obtain experimental data on the background noise spectral radiance coming from the sky and various vertical surfaces scattering solar radiation. The experimental data is obtained for Moscow under various external conditions, such as weather, season and time of day. Since the obtained data does not depend on the specific parameters of the experimental setup, they can be used to estimate the level of the background noise and the signal-to-noise ratio for any receiving optical communication system, including free-space quantum key distribution.