Quantum sensing is one of the four pillars of the European roadmap on quantum technologies. In particular, atomic sensors, including magnetometers, clocks, gyroscopes and gravimeters, are of interest to many areas of physics and they are useful in many applications.
Optically-pumped magnetometers (OPMs), in which an atomic ensemble is optically pumped and its spin-dynamics optically detected, are the most sensitive devices to measure low-frequency magnetic fields. This is a paradigmatic quantum sensing technology, which applies to medical diagnosis, geophysics, navigation and searches beyond the standard model.
In this talk, after giving a general overview on optical magnetometry , I will describe a number of experiments [2, 3] that I performed as postdoctoral research associate at Princeton University. This is a class of scalar optically-pumped magnetic gradiometers that reach femtotesla sensitivity over a broad dynamic range, including Earth’s field magnitude, and led to the first detection of human biomagnetism in unshielded environment .
In the second part, I will describe the quantum noise contributions to the sensitivity of OPMs. Once atomic sensors reach fundamental sensitivity, the only way to improve their performance is to use quantum resources as optical and atomic spin squeezing or entanglement, which is one of the major goals in atomic quantum sensing. In particular, I will describe the first quantum enhancement of high-density atomic sensors by using squeezed-light, first applied to an unpolarized ensemble, i.e. to spin noise spectroscopy (SNS) , secondly to a high sensitivity OPM, thanks to the evasion of quantum backaction .
Finally, I will give an overlook on two current projects I am leading at ICFO towards miniaturization of atomic sensors by using MEMS technology, within the EU quantum flagship project macQsimal , as well as a new class of vapour cells written by a femtosecond laser.
 D. Budker and M. Romalis “Optical Magnetometry”, Nature Physics 3, 227–234 (2007)
 M. E. Limes et al. “Portable magnetometry for detection of biomagnetism in ambient environments”, Phys. Rev. Applied 14, 011002 (2020)
 V. G. Lucivero et al. “Femtotesla direct magnetic gradiometer using a single multipass cell”, Phys. Rev. Applied 15, 014004 (2021)
 V. G. Lucivero et al. “Squeezed- light spin noise spectroscopy”, Phys. Rev. A 93, 053802 (2016)
 C. Troullinou et al. “Squeezed-light enhancement and backaction evasion in a high-sensitivity optically pumped magnetometer”, Phys. Rev. Lett. 127, 193601 (2021)