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arXiv:2008.02050 · Physical Review A

Simultaneous preparation of two initial clock states in a thulium optical clock

E.S. Fedorova, Artem A. Golovizin, Dmitry O. Tregubov, D. Mishin, Daniil I. Provorchenko, В. Н. Сорокин, Ksenia Yu. Khabarova, N. Kolachevsky

Published Dec 17, 2020• 22 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physreva.102.063114

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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 mF=0{m}_{F}=0mF​=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 mF=0{m}_{F}=0mF​=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.
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 and ionic transitions with minimal sensitivity to external fields, with practical operational wavelengths and robust readout protocols. One of the goals is to simplify clock's operation maintaining its relative uncertainty at low 10-18 level. It is especially important for transportable and envisioned space-based optical clocks. We proved earlier that the 1.14um inner-shell magnetic dipole transition in neutral thulium possesses very low blackbody radiation shift compared to other neutrals. Here we demonstrate operation of a bi-colour thulium optical clock with extraordinary low sensitivity to the Zeeman shift due to a simultaneous interrogation of two clock transitions and data processing. Our experiment shows suppression of the quadratic Zeeman shift by at least three orders of magnitude. The effect of tensor lattice Stark shift can be also reduced to below 10-18 in fractional frequency units. All these features make thulium optical clock almost free from hard-to-control systematic shifts. Together with convenient cooling and trapping laser wavelengths, it provides great perspectives for thulium lattice clock as a high-performance transportable system.
Clock transition at 1.14μm1.14 \mu \mathrm{m}1.14μm in neutral thulium is a promising candidate for compact and robust transportable optical clock due to its extremely low sensitivity to external electric and magnetic fields. Cold Tm atoms are now used in optical lattice clocks, quantum simulators and for the study of Bose-Einstein condensation and cold collision physics. In this work, we discuss experimentally measured absolute frequencies of the two transitions at 410.6 nm and 530.7 nm, used for laser cooling of thulium atoms, transition at 418.8 nm used for optical pumping and clock transition at 1140 nm. Using accurate laser spectroscopy in an ensemble of cold thulium atoms, we measured frequencies of these transitions using an optical frequency comb, which was referenced to an active hydrogen maser. Transition frequencies were measured with an uncertainty at one tenth of their natural linewidth, which is sufficient for cooling and pumping of thulium atoms. The uncertainty of the clock transition frequency is 30 Hz, which was limited by the maser frequency calibration with GNSS. Our results can find application in existing experiments with cold thulium atoms and especially in new projects.
Abstract A population transfer from the upper level of the clock transition in thulium atoms to the ground level is demonstrated by using radiation with a wavelength of 402 nm. The method for detecting the clock transition excitation that is not affected by technical fluctuations of the number of atoms in an optical lattice is investigated. The hyperfine splitting of the |4f12(3F2)5d5/2 6s2; J = 7/2〉 level in the thulium atom is measured to be 1480(9) MHz. The suggested method for population transfer can be applied to both the hyperfine components of the clock transition intended for compensating Zeeman effect in a thulium optical clock.
Abstract 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.
The deterministic excitation of thulium atoms from the ground vibrational state of the optical lattice potential to the first and second vibrational levels with efficiencies of 40 and 24%, respectively, has been demonstrated. The purity of the prepared vibrational states is at least 93 and 95% in the former and latter cases, respectively, and is limited by both the purity of the initial state and the non-resonant excitation of the clock transition carrier frequency. The obtained high efficiency and quality of the preparation of atoms in a given vibrational level make it possible to characterize and control the frequency shift of the clock transition due to the interaction of thulium atoms with the field of the optical lattice, which is necessary to achieve an uncertainty of 10–17 for the thulium optical clock.