Active Multiplexing of Spectrally Engineered Heralded Single Photons in an Integrated Fibre Architecture This clearly written thesis discusses the development of a highly innovative single photon source that uses active optical switching known as multiplexing to increase the probability of delivering photons into a single mode. Improving single photon sources is critical in advancing the state of the art in photonic quantum technologies for information processing and communications. Introduction Photon Pair Generation via Four Wave Mixing in Photonic Crystal Fibres Numerical Modelling of Multiplexed Photon Pair Sources Design Fabrication and Characterisation of PCFs for Photon Pair Generation Construction of an Integrated Fibre Source of Heralded Single Photons Characterisation of a Multiplexed Photon Pair Source Conclusion.