Optical atomic clocks have already overcome the eighteenth decimal digit of instability and uncertainty, demonstrating incredible control over external perturbations of the clock transition frequency. At the same time, there is an increasing demand for atomic (ionic) transitions and new interrogation and readout protocols providing minimal sensitivity to external fields and possessing practical operational wavelengths. One of the goals is to simplify the clock operation while maintaining the relative uncertainty at a low 10⁻¹⁸ level achieved at the shortest averaging time. This is especially important for transportable and envisioned space-based optical clocks. Here, we demonstrate implementation of a synthetic frequency approach for a thulium optical clock with simultaneous optical interrogation of two clock transitions. Our experiment shows suppression of the quadratic Zeeman shift by at least three orders of magnitude. The effect of the tensor lattice Stark shift in thulium can also be reduced to below 10⁻¹⁸ in fractional frequency units. This makes the thulium optical clock almost free from hard-to-control systematic shifts. The “simultaneous” protocol demonstrates very low sensitivity to the cross-talks between individual clock transitions during interrogation and readout.

We have developed a compact vacuum system for laser cooling and spectroscopy of neutral thulium atoms. Compactness is achieved by obviating a classical Zeeman slower section and placing an atomic oven close to a magneto-optical trap (MOT), specifically at the distance of 11 cm. In this configuration, we significantly gained in solid angle of an atomic beam, which is affected by MOT laser beams, and reached 1 million atoms loaded directly in the MOT with only 15 mW of MOT cooling beams net power. By exploiting Zeeman-like deceleration of atoms with an additional laser beam and tailoring the MOT magnetic field gradient with a small magnetic coil, we demonstrated trapping of up to 13 million atoms. These results show great perspective of the developed setup for realizing a compact high-performance optical atomic clock based on thulium atoms.

Because of the low sensitivity of a thulium optical clock to black-body radiation and good accuracy and stability estimations, it is considered as a promising transportable optical clock. One of the leading systematic effects for Tm clock transition, i.e., the second-order Zeeman effect, can be canceled by probing two clock transitions between different hyperfine levels of ground and metastable states during clock operation. Herein, we demonstrate efficient optical pumping into the $m_F=0$ state of both ground hyperfine levels using single-frequency radiation at 418.8 nm. We prepared the atoms in both target states simultaneously and excited two clock transitions alternately. The resulting population of $m_F=0$ states of ground hyperfine levels was 36% and 3.8% of the initial number of atoms with less than 4% and 0.4% on nonzero magnetic sublevels, respectively. We performed numerical simulations of the optical pumping process, which is able to explain experimental results well.

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Deep laser cooling of atoms, ions, and molecules facilitates the study of fundamental physics as well as applied research. In this work, we report on the narrow-line laser cooling of thulium atoms at the wavelength of $506.2\mathrm{nm}$ with the natural linewidth of $7.8\mathrm{kHz}$, which widens the limits of atomic cloud parameters control. Temperatures of about $400\mathrm{nK}$, phase-space density of up to $3.5\times {10}^{-4}$ and $2\times {10}^6$ number of trapped atoms were achieved. We have also demonstrated formation of double cloud structure in an optical lattice by adjusting parameters of the $506.2\mathrm{nm}$ magneto-optical trap. These results can be used to improve experiments with BEC, atomic interferometers, and optical clocks.

We consider an experiment on trapping and laser cooling of ²²⁹Th³⁺ ions in a linear Paul trap in the presence of undesirable impurities such as ions of the radioactive isotope ²²⁸Th³⁺. We suggest a method of separating these impurities by means of selective laser cooling utilizing the isotope shift of cooling transitions in ²²⁹Th³⁺ and ²²⁸Th³⁺ ions. According to our estimation, the isotope shift is equal to 3.4 GHZ and makes laser separation of these isotopes possible.