Cassidy's group performs precision optical and microwave spectroscopy of positronium -- a purely leptonic electron-positron atom -- to test bound-state QED to high order and search for new physics, most recently a precision microwave measurement of the 2^3S1 to 2^3P2 fine-structure interval. The group is also developing slow, focused positronium beams toward a laboratory measurement of antimatter's gravitational free-fall, continuing UCL's 50-year history of positron physics.

Chantler's group is built around the idea that X-ray measurements can be made accurate, not just precise: the X-ray Extended Range Technique (XERT) delivers absolute absorption coefficients at the 0.02 per cent level, which in turn allows XAFS to be used for quantitative structure determination and allows high-accuracy tests of atomic theory. The second thread is precision X-ray spectroscopy of highly charged ions and exotic atoms as a test of bound-state QED, where discrepancies between theory and experiment remain unresolved. Positioned against the established body of NV-ensemble quantum sensing work — DEER, nanoscale NMR and T1 relaxometry protocols operating at pT/sqrt(Hz) field sensitivity — this is precision measurement at the other end of the electromagnetic spectrum: the methodological common ground with pT/sqrt(Hz) NV ensemble sensing is the obsessive treatment of systematics and absolute calibration that separates a sensitive measurement from an accurate one. Borderline inclusion, kept because the group's core competency is metrology rather than X-ray applications.