Quantum information processing in optical fibres

The use of quantum particles provides unprecedented improvements for some computational tasks and has applications in provably secure communications. The choice of quantum particles for such processes is wide but single photons are a promising candidate due to their extremely low decoherence and light speed trans- mission. Current technology for the generation and processing of single photons is of exceptional experimental interest and forms the basis of my investigations in this thesis. I show the background theory of quantum information and communication, with a general analysis of qubits and optical quantum gates before looking more specifically at different sources of photonic qubits and the generation of photons. I then show recent improvements in photon generation using photonic crystal fi- bres (PCFs), most especially the use of cross-polar phase matching solutions that allow the generation of pure state photons via four wave mixing effects in a X(3} nonlinear medium. These therefore require no spectral filtering to be used for quantum information tasks, thereby improving collection rates and experimental integration times which are crucial when performing multi-photon experiments. The optimization of the parameters for such a source are achieved through the use of an analytical model, and the purity of the photons are tested through the use of quantum interference effects. I move on to show the construction and character- ization of a common two qubit gate, namely the controlled-NOT gate, in optical fibres using fibre based sources and describe it's usefulness and integrability into quantum communication networks. I then calculate bounds on the average gate fidelity of 0.83 < Fave < 0.91 and create a model to show the main sources of error in the controlled-NOT gate operation. If the PCF used in the above sources is pumped in opposite directions in a Sagnac loop configuration, pairs of photons are generated in a maximally entangled Bell state where the polarization of one photon is perfectly correlated with the other photon of the pair. If two such pairs are generated in separate sources and one photon from each pair mixed on a polarizing beam splitter, a fusion operation is performed that entangles those two photons creating a four photon cluster state where all photons are entangled. This cluster state is a universal resource for measurement based quantum computing. In this thesis I show the generation and characterisation of such a cluster state and describe its use to perform a universal set of gates through single qubit measurements. I finally describe future experiments using PCFs and cluster states.