The development of a low-cost, handheld quantum key distribution device.
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Date
2017
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Abstract
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.
Description
Doctoral Degree. University of KwaZulu-Natal, Durban.