Free-space communication in quantum key distribution.
Quantum cryptography permits the sharing of a secret key hence information between authorized parties such that an unauthorized party is unable to obtain any useful information. A quantum and a classical authenticated channel are used to connect the authorized parties. The security of cryptography is based on the fundamental principles of quantum physics specifically the Heisenberg’s uncertainty principle, entanglement and the no-cloning theorem. Two major quantum channels that are used in quantum cryptography for the transmission of messages are optic fibers and free space. The key is transmitted as a series of single photons and the bits of the key are encoded by the measurement of the quantum state. In recent years, real Quantum Key Distribution (QKD) systems have been built and communication has also been established by using optic fiber networks for the transmission of encoded messages. However, open challenges still remain, for example the distribution of the key over large distances and communication involving moving parties. In the quest to increase the communication distance and have an alignment free reference system, free-space quantum communication has been favoured. Regardless of free-space communication being seen as a good candidate for quantum communication, it suffers from the problem of alignment when communication involves moving objects, for example between Earth and an orbiting satellite. Although vertical communication between an Earth station and a satellite is possible, horizontal path communication still poses a great challenge. This is mainly due to turbulence in the atmosphere, vapor pressure and pollution. In this thesis, it is demonstrated how the problem of alignment might be solved. In particular, we will focus on the ways to obtain an autonomous system, which can be used to align the transmitter and the receiver for free-space quantum communication. We assess the possibility of obtaining a tracking system by using open-source electronics. In order to build our tracking system, it was used an algorithm implemented by a microcontroller mounted on a printed component board. The embedded system can be considered to be a coarse tracking for optical communication and a fine alignment for radio-communications. An angular sensor for the base alignment is plugged into the microcontroller system. To align the polarization bases, the receiver sends a polarized laser beacon to the transmitter and by an angular sensor the transmitter is able to align his bases for the single photon transmission. Then by using a correction algorithm, this system provides an accurate alignment. Moreover, in order to show that our system works, we tested the polarization alignment system in the laboratory. To verify the tracking system, we used both cartography software and a short range experiment. We finally present the results of the above systems as implemented and tested at the University of KwaZulu-Natal (UKZN).
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