Femtotesla Atomic Magnetometer Employing Diffusion Optical Pumping to Search for Exotic Spin-Dependent Interactions.

Searching for beyond-the-standard-model interactions has been of interest in quantum sensing. Here, we demonstrate a method, both theoretically and experimentally, to search for the spin- and velocity-dependent interaction with an atomic magnetometer at the centimeter scale. By probing the diffused optically polarized atoms, undesirable effects coming along with the optical pumping, such as light shifts and power-broadening effects, are suppressed, which enables a 1.4  fT_{rms}/Hz^{1/2} noise floor and the reduced systematic errors of the atomic magnetometer. Our method sets the most stringent laboratory experiment constraints on the coupling strength between electrons and nucleons for the force range λ>0.7  mm at 1σ confidence. The limit is more than 3 orders of magnitude tighter than the previous constraints for the force range between 1  mm∼10  mm, and one order of magnitude tighter for the force range above 10 mm.