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International Conference Laser Optics (ICLO)

Measurement of absolute frequencies of transitions in thulium atoms

Artem A. Golovizin, Dmitry O. Tregubov, M. O. Yaushev, D. Mishin, Daniil I. Provorchenko, N. Kolachevsky

Published Jul 1, 2024• 0 citations
Publisher

Abstract

Clock transition at $1.14 \mu \mathrm{m}$ 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.

DOI
10.1109/iclo59702.2024.10624545

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Results in Physics•Apr 8, 2023•6 citations
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.
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 Ð the most accurate microwave clocks Ð made their appearance, and impetus was lent to the area of optical frequency standards, which today have surpassed the relative error of 10 À18 .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.
We propose to use the magnetic-dipole transition at coupling two ground state fine-structure components of thulium (Tm), as a 'clock transition' in optical clocks. We have demonstrated first stage laser cooling of Tm atoms down to a temperature of 25 K using a strong transition at and we have also shown preliminary results for second stage cooling using a weaker transition at (natural linewidth γ G = 350 kHz). Laser cooled atoms have been trapped in an optical dipole trap and in a 1D optical lattice operating near 532 nm.
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.
The experimental comparison of two thulium optical lattice clocks in a time interval of up to one hour has been carried out. The synchronous comparison of a clock transition in two independent atomic ensembles using a single ultrastable laser has allowed us to eliminate fluctuations of the laser frequency from the measured frequency difference and to reach a relative measurement error of 10–16 after 500-s averaging, which corresponds to a relative instability of 2×10−15/τ2 \times {{10}^{{ - 15}}}{\text{/}}\sqrt \tau 2×10−15/τ​ . The successful demonstration of the long-term operation of two systems using the synchronous comparison of clock transitions opens the possibility of studying systematic shifts in thulium optical clocks with an uncertainty of 10–17.
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.
We experimentally determine the absolute frequencies and hyperfine splittings of the 6s6p 3P0,2 − 6s7s 3S1 transitions of ultracold 171Yb atoms in an optical lattice. A frequency comb referenced to an ultrastable optical cavity is used to measure the electric-dipole transition frequencies. With the normalized shelving detection technique, about 12 MHz and 10 MHz linewidth of atomic spectra are obtained, respectively, which is some greater than the natural linewidth of 1.5 MHz and 5.9 MHz and is limited by the broadening from the probe and lattice light. By reference to our 171Yb 6 s2 1S0 − 6s6p 3P0 clock, the absolute frequencies are found to be 461861338.03 (0.21) MHz and 461871596.07 (0.31) MHz for the 6s6p 3P0 − 6s7s 3S1 (F = 1/2 and F = 3/2) transitions, which are more accurate than the previous measurement by one order of magnitude. For the 6s6p 3P2 (F = 3/2 and F = 5/2) − 6s7s 3S1 (F = 1/2 and F = 3/2) transitions, the measured frequencies are 389257019.71 (0.65) MHz, 389267277.10 (0.37) MHz and 389260569.95 (0.89) MHz, respectively, which are the first measurements to our knowledge. Furthermore, the hyperfine constants for the 6s6p 3P2 and 6s7s 3S1 states are determined to be 2682.86(0.38) MHz and 6838.69(0.25) MHz, both of which agree well with other recent experimental results.