Development of encoding and entanglement for free space quantum key distribution.
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Abstract
It is established that the manipulation of quantum information is bound only by the laws
of physics and thus information can be characterised, quantified and processed as a physical
entity using the basic properties of quantum mechanics. The most advanced quantum
information related technology at present is Quantum Key Distribution (QKD). QKD
encodes information into a quantum data carrier, in particular, a single photon that is
transported through a quantum channel to produce secure key. To date QKD has been
demonstrated across two types of channels which is generally fibre or free space. Free
space QKD is beneficial since it provides various mediums for the encoding. When considering
the atmosphere as a medium for free space QKD, one of the challenges is the
effects of turbulence. Apart from the turbulence effects, one of the other challenges faced
with implementing a QKD system, is the use of an appropriate source of single photons
for the encoding. A good implementation of a source of single photons would be to make
use of entanglement. This thesis addresses the above two aspects of QKD
Entanglement is essentially at the core of quantum mechanics and deals with the ability
to couple two or more particles in time and space. It is relevant to all sub-atomic
particles which include photons, electrons and ions. One of the techniques to obtaining
an entangled photon pair lies in the successful implementation of a second-order nonlinear
process which is referred to as Spontaneous Parametric Down Conversion (SPDC).
The objective of this study was to build a polarisation encoded entangled single photon
source using high powered lasers. The reason for this is that, for the successful implementation
of free space QKD using entangled single photons, a bright source with high
quality entanglement is required. We reach a power output of 400 mW and a violation of
the Clauser, Horne, Shimony and Holt (CHSH) inequality, 0.8 % less than the theoretical
limit.
Free space QKD is challenged by the effects of atmospheric turbulence due to additional
noise introduced to the system. Hence it is of relevance hence to study the effects that turbulence
poses on photons. Within this study we simulate atmospheric turbulence which
was applied to two well known methods of encoding namely, polarisation and states carrying
Orbital Angular Momentum (OAM). Polarisation encoding, when subjected to simulated
turbulence, suffered phase dependence on the coincidence. This was due to the inefficient
coupling of single mode fibre within the detection scheme. These results suggest
that the design of an entangled source is crucial when utilised within non-ideal conditions.
An alternative method of encoding is to make use of photons carrying OAM. Here
we investigated an alternative approach to encoding of OAM states in the form of higher
Development of Encoding and Entanglement for Free Space Quantum Key Distribution
order Bessel beams. Although OAM suffers much loss due to the break down of the phase
in the presence of atmospheric turbulence, it is promising method of encoding to reach
hyper entanglement states. Furthermore this will provide a means to attain dense coding.
Encoding and an appropriate single photon source is significant for the implemantation
of free space QKD. The aforementioned investgations carried out within this thesis is
promising for the further development of free space QKD.
Description
Ph. D. University of KwaZulu-Natal, Durban 2014.
Keywords
Quantum entanglement., Theses -- Physics.