ИСТИНА

Quantum key distribution (QKD) is a quantum communication protocol based on principles of quantum mechanics, information carriers in which are quantum objects such as photons. Implementations of QKD utilizing fiber communication lines are widespread. The phase encoding is commonly used in such lines. Meanwhile, in free-space channels, polarization-encoded protocols are usually used due to their relative implementation simplicity and robustness of polarization in free-space propagation. It would seem to be impractical to use a phase encoding across free-space channels due to distortions of temporal and spatial modes of photons during propagation through an atmospheric turbulence. Here, we investigate free-space delay interferometer which performs passive optical correction of multi-mode signal. So far we have observed the visibility ≈ 95 % for single-mode signal and ≈ 93 % for multi-mode signal, which allows us to believe that practical QKD implementation with this interferometer is possible. Also we show that the interference visibility for a multi-mode signal is relatively independent of the length of the multi-mode fiber at the input of the interferometer.