Browsing by Author "Mariola, Marco."

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The development of a low-cost, handheld quantum key distribution device.
(2017) Pillay, Sharmini.; Petruccione, Francesco.; Mariola, Marco.
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.
Free-space communication in quantum key distribution.
(2015) Mariola, Marco.; Petruccione, Francesco.
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).