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The development of a low-cost, handheld quantum key distribution device.

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Quantum Key Distribution (QKD) is an emerging field of information security. To date, this technology has been implemented for large scale financial and voting purposes, but QKD is a versatile solution which can also be utilised to secure personal transactions. The development of low cost, portable QKD devices can further promote the use of quantum encryption in commercial security systems. Research has been done to design hand-held QKD devices for personal use with ATMs. These devices use a short-range free space channel to produce a secret key using the polarisation of single photons as qubits. Free space applications of QKD usually utilise polarisation encoding of single photons since the polarisation states do not deteriorate in the turbulent atmosphere. Recent research also shows the feasibility of using quantum coherent states with continuous variable QKD in free space. The proposed device uses the Coherent One Way (COW) protocol to exchange a secret key between the two authenticated parties. The COW protocol is a simple, practical protocol which uses the time of arrival of consecutive weak coherent pulses as the bit encoding. The security of this protocol lies in preserving the coherence between consecutive laser pulses. Should decoherence be observed in the monitoring line, the presence of an eavesdropper is inferred. An advantage of using the COW protocol is the small size and low cost of the setup. This is ideal for a hand-held device used for short-range QKD. The COW protocol is not traditionally used for a free space channel due to the fragility of coherence in a turbulent medium. Since this is a short-range device which will not encounter any turbulence, the coherence of the laser beam is not compromised. It is therefore suitable to use the COW protocol under these conditions. We present in this thesis, the design of the system, in particular, the conversion from a fibre channel to a free space channel. A low cost optical synchronisation system is presented for use in a laboratory environment and the system is characterised with respect to the efficiency of the source, synchronisation and detection components. The bit generation rate and quantum bit error rate of the system are measured and discussed. Synchronisation techniques for long range free space implementation of the COW protocol, using radio transmission, are presented with a simulation. The simulation is used to demonstrate the compensation for Doppler effects required for communication between a Low Earth Orbit satellite and a ground station.


Doctoral Degree. University of KwaZulu-Natal, Durban.