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.