Development of encoding and entanglement for free space quantum key distribution.

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.