Recent progress in optically pumped magnetometers

Optically pumped magnetometres (OPM) are known as the most precise instruments for scalar measurement of low magnetic fields, including those of the Earth's magnetic field. For a long time OPM could not compete. in absolute accuracy, with the proton precession magnetometers, being, at the same time. much more sensitive and fast. Nowadays, due to the considerable progress in the OPM performance, several types of OPM became much more accurate than the best proton magnetometer. During the last decade the main progress was associated with OPM of the following three types: (1) The narrow-fine potassium M-x-OMP with the highest shot- noise-limited resolution (about 10 fT/Hz(1/2)) and with an absolute accuracy better than 0.1 nT; (2) The "tandem" M-x-M-z-type magnetometer with a narrow line potassium M, unit of enhanced absolute accuracy (of about 10 pT); (3) The laser pumped He-3 free-precession magnetometer of highest absolute accuracy and lowest energy consumption. Each of these three types has its own specific features, preferable in certain applications.