Study of Long-Term Stability of Free-Space Quantum Key Distribution at a Distance of 20 m in a Polarization Coding Scheme

The operation of our atmospheric quantum cryptography system using the BB84 protocol and polarization coding is experimentally studied. The distribution rate of the sifted quantum key and the quantum bit error rate in the key remained constant for 1 hour and were \(7558\pm 83\) bit s \({}^{-1}\) and \(5.1\pm 0.84\%\) , respectively, at the distance of 20 m between the transmitter and receiver. The minimum possible transmittance of a quantum channel, ensuring the security of a quantum key, is calculated for the given parameters of the system