Faraday-enhanced Ramsey Interferometry in Ultracold Atoms

This thesis develops new sensing protocols combining weak and strong quantum measurements to resolve noisy and unstable magnetic fields. It describes experiments implementing these protocols on a spinor Bose Einstein condensate, a form of ultracold matter at temperature close to absolute zero. This realises a quantum magnetometer robust to real-world magnetic environments. The magnetometer achieves a record volume-normalised precision, while measuring field magnitude without the ambiguity of conventional interferometric methods. The novel sensing protocols in this thesis should be broadly applicable to other quantum sensors, and may enable applications in fields as diverse as physiology, circuit design, and materials science.