[en] The development of the optical frequency comb technique has enabled a wide use of atomic optical clocks by allowing frequency conversion from the optical to the radio frequency range. Today, the fractional instability of such clocks has reached the record eighteen-digit level, two orders of magnitude better than for cesium fountains representing the primary frequency standard. This is paralleled by the development of techniques for transferring accurate time and optical frequency signals, including fiber links. With this technology, the fractional instability of transferred frequency can be lowered to below 10⁻¹⁸ with an averaging time of 1000 s for a 1000 km optical link. At a distance of 500 km, a time signal uncertainty of 250 ps has been achieved. Optical links allow comparing optical clocks and creating a synchronized time and frequency standard network at a new level of precision. Prospects for solving new problems arise, including the determination of the gravitational potential, the measurement of the continental Sagnac effect, and precise tests of fundamental theories. (instruments and methods of investigation)

[en] The role of precision spectroscopic measurements in the development of fundamental theories is discussed, with particular emphasis on the hydrogen atom, the simplest stable atomic system amenable to the accurate calculation of energy levels from quantum electrodynamics. Research areas that greatly benefited from the participation of the Lebedev Physical Institute are reviewed, including the violation of fundamental symmetries, the stability of the fine-structure constant $\mathrm{\alpha <!--\; ??\; -->}$, and sensitive tests of quantum electrodynamics. (conferences and symposia. 300 years of physics in russia and 80 years of fian)

[en] A laser system with a narrow generation line for secondary laser cooling of ⁸⁷Sr atoms has been developed and investigated. It is planned to use ultracold ⁸⁷Sr atoms loaded in an optical lattice in an optical frequency standard. To this end, a 689-nm semiconductor laser has been stabilised using an external reference ultrastable cavity with vibrational and temperature compensation near the critical point. The lasing spectral width was 80 Hz (averaging time 40 ms), and the frequency drift was at a level of 0.3 Hz s^-1. Comparison of two independent laser systems yielded a minimum Allan deviation: 2 × 10^-14 for 300-s averaging. It is shown that this system satisfies all requirements necessary for secondary cooling of 87Sr atoms using the spectrally narrow ¹S₀ — ³P₁ transition (λ = 689 nm). (cooling of atoms)

[en] We review the current research on precision spectroscopy and quantum optics applications of laser-cooled lanthanides. We discuss the specific electronic structure of hollow atoms, which determine prospects for application in optical frequency standards and in quantum simulators based on spin interactions in optical lattices. Using the example of the thulium atom, we describe the specifics of laser cooling, optical lattice trapping techniques, and clock transition spectroscopy using spectrally narrow lasers. (conferences and symposia)

[en] The operation of a 2.8 km fiber link at 1.14 µm for ultrastable optical frequency dissemination, which will allow for the characterization of transportable Yb⁺  based optical clock instability via a comparison to another frequency standard, was demonstrated. Active fiber noise compensation allows for the obtaining of a fractional frequency instability of at a 1100 s averaging time. The performance of the link under the influence of mechanical perturbations was also tested. (paper)

[en] The secondary laser cooling of a cloud of strontium-88 atoms on the "1S_0–"3P_1 (689 nm) intercombination transition captured into a magneto-optical trap has been demonstrated. We describe in detail the recapture of atoms from the primary trap operating on the strong "1S_0–"1P_1 (461 nm) transition and determine the recapture coefficient κ, the number of atoms, and their temperature in the secondary trap as a function of experimental parameters. A temperature of 2 µK has been reached in the secondary trap at the recapture coefficient κ = 6%, which confirms the secondary cooling efficiency and is sufficient to perform metrological measurements of the "1S_0–"3P_1 (698 nm) clock transition in an optical lattice

[en] We describe a method that allows accumulation in a Paul trap of a large number of ions obtained from multiple laser ablation pulses, and therefore solving the problem of a single pulse that usualy gives rise to final trapping of too few ions. The proposed method of the cumulative trap loading was based on interaction with a buffer gas that provided a friction force. The method was applied to triply charged thorium ions and trapped a total of 10⁶ ions. Numerical analysis of the ion motion allowed the experimental parameters to be optimized for the most effective implementation of the technique. (letter)

[en] In the framework of the project aimed at creating an optical standard on cold Sr atoms we have realised sub-Doppler spectroscopy of the intercombination transition "1S_0 – "3P_1 (689 nm) in a cell with Sr vapour and in a cloud of atoms loaded in a magneto- optical trap (MOT). By measuring Zeeman splitting of the "3P_1 level in the magnetic field of the MOT we have succeeded in fine adjustment of the MOT relative to a minimum of the magnetic field, which is necessary for successful secondary-stage cooling on the intercombination transition. In turn, absorption saturation spectroscopy in the vapour cell provides the long-term frequency stability of the second-stage cooling laser at λ = 689 nm. (laser spectroscopy)

[en] The magnetic dipole transition between fine structure sublevels of the ground state of thulium atom 4f"1"3("2F"o)6s"2 has been directly excited in a cloud of ultra-cold atoms by a frequency-stabilised laser. This transition at the wavelength λ = 1.14 μm is planned to be used as the clock one in an optical frequency reference on laser-cooled thulium atoms. (extreme light fields and their applications)