Jo Feb 28, 2024

Satellite-based free space optical quantum key distribution system is being considered as a very promising technique which is capable of overcoming the transmission limits of fiber-based QKD system and constructing a global quantum communication network.

It was demonstrated that laser beam has orbital angular momentum generated from spiral topological structure of itself in addition to spin angular momentum polarization property. In this case, unlike polarized quantum state, a photon may have a number of quantum states. Using optical orbital angular momentum as information carriers, information can be modulated in the infinite dimensional space. Furthermore, there is a possibility of increasing data throughput efficiently and enhancing the stability of system. Consequently, applying the orbital angular momentum to quantum key distribution, it is possible to increase the quantum key generation rate remarkably and this mode is suitable for free space optical quantum communication.

Some researchers considered polarization coding in FSO QKD system which is one of the good ways to realize quantum communication in free space. For convenience of analysis, they assumed that the air is in a steady state when QKD is performed between two parties in free space. However, the air is in a dynamic state of continuous flow rather than in a fixed state. It is known that there is a possibility of upgrading the performance of FSO QKD system when orbital angular momentum is applied to QKD.

O Ju Hyok, a researcher at the Faculty of Communications, has investigated more suitable QKD protocols for FSO quantum communication and analyzed the impact of atmospheric turbulence on quantum bit error rate of QKD system. He has also developed two QKD protocols suitable for FSO QKD system (one formed by combining random phase modulation with BB84 protocol and the other by combining entanglement modulation using photon orbital angular momentum with BB84 protocol) and analyzed the effects of atmospheric turbulence on the performance of FSO QKD system.

The simulation results demonstrate two facts. First, QBER of FSO QKD system increases with increase in air refraction index structural constant, that is, the intensity of atmospheric turbulence. Second, for FSO QKD, entanglement-based OAM QKD is more suitable than random phase modulation method.