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.

Gabrielse directs Northwestern's Center for Fundamental Physics at Low Energy, where his group performs some of the most precise measurements of any single particle. Using a one-electron quantum cyclotron in a cylindrical Penning trap, his team measures the electron magnetic moment (g-factor) to sub-part-per-trillion precision, providing the most stringent test of quantum electrodynamics and the Standard Model. A parallel effort (ACME) searches for the electron's electric dipole moment using a cold beam of ThO molecules, and a new cavity-based dark-matter search and antihydrogen/antiproton precision-measurement program are underway. This is precision quantum sensing of fundamental constants rather than sensing of an external field, but it shares with NV-ensemble magnetometry the goal of pushing measurement sensitivity toward the quantum limit through improved back-action evasion.

NON-PREFERRED (borderline precision-measurement pivot, kept for review). Garcia Ruiz develops precision laser spectroscopy of atoms and molecules built from short-lived radioactive nuclei (at CERN-ISOLDE and the new FRIB facility) to measure nuclear charge radii, moments, and to search for symmetry-violating effects (parity/time-reversal violation) analogous to eEDM searches; it is fundamental precision measurement rather than a deployable quantum sensor, but shares techniques and motivation with the eEDM/precision-AMO quantum-sensing cluster.

Hori performs precision two-photon laser spectroscopy of antiprotonic helium and other exotic atoms (much of it carried out at CERN's Antiproton Decelerator), testing CPT symmetry and QED and constraining the antiproton-to-electron mass ratio.