Quantum noise limits and squeezed-light enhancement of optical magnetometry
Optically-pumped magnetometers (OPMs), in which an atomic ensemble is optically pumped and the spin precession is optically detected, are among the most sensitive devices to measure low-frequency magnetic fields. As in other atomic quantum sensors, the achievable sensitivity of OPMs is limited by three contributions of quantum noise: photon shot noise, atomic projection noise, and quantum backaction, the latter due to the effective field produced by ac-Stark shift.
Here we first describe a pulsed scalar magnetic gradiometer  that achieves state-of-the-art differential sensitivity of 14 fT/√Hz over a broad dynamic range, including Earth’s field magnitude. We discuss the theoretical Cramer-Rao lower bound, in the presence of non-white spin noise and atomic diffusion, and we compare it against the experimental standard deviation of the estimated frequency difference.
Secondly, we describe the quantum enhancement of an OPM by polarization squeezing of the probe beam . We report an improvement in high-frequency sensitivity and measurement bandwidth with no loss of sensitivity in any region of the frequency spectrum, a direct demonstration of the evasion of measurement backaction.
 V. G. Lucivero et al. “Femtotesla nearly-quantum-noise-limited pulsed gradiometer at Earth-scale fields”, Phys. Rev. Applied 18, L021001 (2022)
 C. Troullinou et al. “Squeezed-light enhancement and backaction evasion in a high-sensitivity optically pumped magnetometer”, Phys. Rev. Lett. 127, 193601 (2021)