The CASPEr Experiment and Searches for Axions using Atoms, Molecules, and Solids*

Laboratory-scale precision experiments have developed an extensive set of tools for exploring fundamental physics. This motivation provided the early stimulus for studies of the quantum limits of measurements, seeding the field of quantum sensing and metrology. I will focus on how precision magnetic resonance can be used to search for for ultra-light axion-like dark matter. The Cosmic Axion Spin Precession Experiment (CASPEr-e) uses nuclear magnetic resonance to search for the defining gluon coupling of the Quantum Chromodynamics (QCD) axion, which solves the strong-CP problem of QCD. The prototype CASPEr-e experiment has achieved design sensitivity in the nano-electronvolt mass range. To reach the sensitivity to the QCD axion, the CASPEr team is developing a precision magnetic resonance detector, limited by the quantum spin projection noise of a macroscopic ensemble of nuclear spins.