The deterministic generation of a single excitation of the electromagnetic field in a predefined spectral, spatial and polarisation mode is a long running scientific and technical challenge. Such single-photon sources exhibit properties that cannot be described using classical physics, and have a number of potential applications in sensing and metrology, communications, quantum computing and fundamental tests of quantum science. Heralded photon sources, based on non-linear optical processes such as spontaneous parametric down conversion, are a common method for generating single photons. Heralded sources have a significant shortcoming: single photons are delivered by the source in only a small fraction of attempts (typically around 1\%). Over the past 20 years, source multiplexing is a technique that has arisen to combat this problem. In source multiplexing, the output from multiple sources is routed through a switch network to increase the photon delivery probability in a predefined mode.

Despite significant progress, there are a number of outstanding challenges faced by source multiplexing. Optical losses, especially the losses of individual switches in optical switching networks, severely limits performance. Many multiplexing schemes also require large, and possibly ultimately infeasible, numbers of components.

In this thesis, we attempt to more fully understand the potential of source multiplexing. There are a number of different architectures for source multiplexing, each with different photon statistics and component scaling. We investigate the performance of each technique and outline the device requirements needed for high performance sources.

Of all the available architectures, multiplexing in frequency (as opposed to independent spatial sources or over multiple time bins) has a number of advantages. Non-linear processes can be engineered so that hundreds of frequency modes are readily available and multiplexing can be achieved without detrimental loss and component scaling. We demonstrate and full characterise a frequency multiplexed single-photon source.