Intro; Supervisor's Foreword; Abstract; Acknowledgements; Contents; 1 Introduction; 1.1 A Hitchhiker's Guide to this Thesis; 1.2 A Brief History of Narrowband Single Photon Sources from Spontaneous Parametric Down-Conversion; References; 2 Theoretical and Experimental Foundations; 2.1 Optical Cavities; 2.1.1 Resonator Theory; 2.1.2 Mode-Matching; 2.2 Stabilisation Techniques; 2.2.1 Pound-Drever-Hall Frequency Stabilisation; 2.2.2 Reference Systems; 2.2.3 Proportional-Integral-Derivative Controller; 2.3 Nonlinear Optical Processes; 2.3.1 Second Harmonic Generation

3.2 Photon Pair Filtering3.2.1 Mode-Cleaning Cavity; 3.2.2 Clustering Effect; 3.3 Electronics and Control; 3.3.1 Field-Programmable Gate Array; 3.3.2 Control Software; 3.3.3 Temperature Stabilisation; 3.4 Frequency Stabilisation; 3.4.1 Laser; 3.4.2 Cavities; 3.4.3 Atomic System; 3.5 Detectors; 3.5.1 Universal Photodetectors; 3.5.2 Single Photon Counting; 3.6 Experimental Setup; 3.6.1 Laser and Absolute Frequency Reference; 3.6.2 Conversion Setup; 3.6.3 Mode-Cleaning Setup; 3.6.4 Feedback Loops; References; 4 Single Photon Characterisation; 4.1 Classical Characterisation