Laser cooling of thulium atoms to ground vibrational state in an optical lattice

The method of laser cooling of atoms, first implemented in the early 1980s at the Institute of Spectroscopy of the Russian Academy of Sciences (ISAN), turned out to be an exceptionally powerful tool that provided revolutionary breakthroughs in such areas as quantum sensorics, the physics of Bose–Einstein and Fermi-condensates, quantum informatics, and many others. It was precisely due to the laser cooling method that atomic fountains—the most accurate microwave clocks—made their appearance, and impetus was lent to the area of optical frequency standards, which today have surpassed the relative error of $10^{-18}$. In this review, dedicated to the 55th anniversary of the founding of ISAN, we will present some modern methods and experimental results aimed at the development of optical clocks on thulium atoms. In addition to the review part, the paper demonstrates a new experimental protocol for the preparation of thulium atoms using sideband cooling of the spectrally narrow 506.2-nm transition. Ensembles of atoms in the initial states of clock transitions in the ground vibrational sublevel of the optical lattice have been attained.