Quantum Key Distribution Protocol Using Axially Symmetric Polarization Beams in an Atmospheric Channel

We propose a quantum key distribution protocol using axially symmetric polarization beams invariant to rotation of the radial coordinate in a plane that is normal to the beam propagation axis. A description and a method for generating vector beams with an axially symmetric polarization structure, which are formed by a vector superposition of the Hermite–Gaussian modes with indices 10 and 01 are given. It is shown that axial polarization symmetry renders such beams insensitive to rotations relative to the optical axis, which makes it possible to use them for transmitting information in cryptographic protocols in space communication systems. The mechanism of mode formation using radial polarizers and cube corner reflectors is shown in detail. Although a classical communication channel can be organized using radial polarizers, the inclusion of non-unitary elements in a quantum communication channel destroys its cryptographic strength. Therefore, we consider the procedure for detecting phase shifts in the configuration of the Mach–Zehnder interferometer whose elements are cube corner reflectors.