Free-space communication in quantum key distribution.
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Date
2015
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Abstract
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).
Description
Ph. D. University of KwaZulu-Natal, Durban 2015.
Keywords
Quantum theory., Cryptography., Data encryption (Computer science), Computer security., Optical communication., Quantum computers., Theses--Physics., Quantum cryptography., Quantum key distribution.