Advances in unshielded optically pumped alkali magnetometers

The development of optically pumped magnetometers (OPMs) capable of operating in fields up to and beyond Earth's field has been accelerated in recent years by advances in the micro-fabrication of components, such as alkali-vapour cells and chip-scale lasers, and the application of spin-exchange relaxation suppression techniques, such as light-narrowing. Using these techniques, OPMs can be built for highly-accurate scalar geo-magnetometry. Sub-picotesla sensitivity can be achieved in compact packages, and commercialisation of these technologies is progressing globally. We review the underlying principles for OPM operation, including optimisation of microfabricated alkali vapour cells, and summarise the current state-of-the-art in their application.