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Chinese Physics Letters

Improved Evaluation of BBR and Collisional Frequency Shifts of NIM-Sr2 with 7.2 × 10−18 Total Uncertainty

Bing-Kun Lu, Zhen Guo Sun, Tao Yang, Yige Lin, Qiang Wang, Ye Li, Fei Meng, Bai-Ke Lin, Tianchu Li, Zhanjun Fang

Published Jul 1, 2022• 26 citations
Publisher

Abstract

NIM-Sr2 optical lattice clock has been developed on the Changping campus of National Institute of Metrology (NIM). Considering the limitations in NIM-Sr1, several improved parts have been designed including a differential pumping stage in the vacuum system, a permanent magnet Zeeman slower, water-cooled anti-Helmholtz coils, an extended viewport for Zeeman slower, etc. A clock laser with a short-time stability better than 3 × 10−16 is realized based on a self-designed 30-cm-long ultra-low expansion cavity. The systematic frequency shift has been evaluated to an uncertainty of 7.2 × 10−18, with the uncertainty of BBR shift and the collisional frequency shift being an order of magnitude lower than the last evaluation of NIM-Sr1.

DOI
10.1088/0256-307x/39/8/080601

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NIM-Sr2 optical lattice clock has been developed on the Changping campus of National Institute of Metrology (NIM). Considering the limitations in NIM-Sr1, several improved parts have been designed including a differential pumping stage in the vacuum system, a permanent magnet Zeeman slower, water-cooled anti-Helmholtz coils, an extended viewport for Zeeman slower, etc. A clock laser with a short-time stability better than 3 × 10−16 is realized based on a self-designed 30-cm-long ultra-low expansion cavity. The systematic frequency shift has been evaluated to an uncertainty of 7.2 × 10−18, with the uncertainty of BBR shift and the collisional frequency shift being an order of magnitude lower than the last evaluation of NIM-Sr1.
Improved systematic evaluation of a strontium optical clock with uncertainty below 1×10−181\times 10^{-18}1×10−18
We report a systematic uncertainty of 9.2×10−199.2\times 10^{-19}9.2×10−19 for the USTC Sr1 optical lattice clock, achieving accuracy at the level required for the roadmap of the redefinition of the SI second. A finite-element model with {\it in situ}-validated, spatially-resolved chamber emissivity reduced blackbody radiation shift uncertainty to 6.3×10−196.3\times 10^{-19}6.3×10−19. Concurrently, an externally mounted lattice cavity combined with a larger beam waist suppressed density shifts. Enhanced lattice depth modulation consolidated lattice light shift uncertainty to 6.3×10−196.3\times 10^{-19}6.3×10−19 by enabling simultaneous determination of key polarizabilities and magic wavelength. Magnetic shifts were resolved below 10−1810^{-18}10−18 via precision characterization of the second-order Zeeman coefficient. Supported by a crystalline-coated ultra-low-expansion cavity-stabilized laser and refined temperature control suppressing BBR fluctuations, the clock also achieves a frequency stability better than 1×10−181\times10^{-18}1×10−18 at 30,000-s averaging time. These developments collectively establish a new benchmark in USTC Sr1 clock performance and pave the way for high-accuracy applications in metrology and fundamental physics.
The current status of an Yb optical lattice clock at the Korea Research Institute of Standards and Science (KRISS) is reported. The systematic uncertainty of the Yb clock in the first accuracy evaluation was 1.5 × 10 −14 [Park et al. , Metrologia 50, 119 (2013)]. The uncertainty was dominated by the large uncertainties in the lattice ac Stark shift and the collisional shift, which were mainly limited by the large linewidth and jitter of the clock laser. Recently, a highly stable clock laser at 578 nm was developed with a short-term linewidth of 3.5 Hz and a frequency jitter of about 25 Hz at 1 s and 10 s measurement times, respectively. The long-term frequency drift showed only a linear dependence on time, confirming that the temperature of the super-cavity was maintained a zero coefficient of thermal expansion. The frequency of the lattice laser at 759 nm was phase locked to the optical frequency comb and could be stabilized at the “absolute” frequency of the “magic wavelength”, to within a 1-MHz uncertainty. This improvement greatly reduced the fractional uncertainty due to the lattice ac Stark shift down to 2 × 10 −16 . The systematic uncertainty of the clock is currently 5.3 × 10 −15 and is dominated by the collisional frequency shift.
A clock laser based on a 30-cm-long ultrahigh finesse optical cavity was developed to improve the frequency stability of the Sr optical lattice clock at the National Institute of Metrology. Using this clock laser to probe the spin-polarized Sr87 atoms, a Rabi transition linewidth of 1.8 Hz was obtained with 500 ms interrogation time. Two independent digital servos are used to alternatively lock the clock laser to the S01 (mF=+9/2)→P03 (mF=+9/2) transition. The Allan deviation shows that the short-term frequency stability is better than 3.2×10−16 and averages down followed by 1.8×10−15/τ.
Optical atomic clocks have proven to be the most stable clocks with the lowest systematic uncertainty which has now reached a record level of (9.4 X10/(^{19}/)). Thus optical clocks now surpass the best fountain Cs atomic clocks by three orders of magnitude which opens the prospect of using optical clocks as frequency standards for timekeeping in the near future. \n \nThis thesis describes the progress that has been made towards a stationary optical lattice clock based on strontium atoms, and reports the results of an ongoing characterization. \n \nAn experimental apparatus was designed and built to efficiently slow, cool, and trap the strontium atoms which represent the heart of the strontium optical lattice clock discussed in this thesis. The novel aspects of the experimental development described in the thesis comprise a frequency doubling cavity for producing 461 nm laser light employed for cooling and trapping the strontium atoms in the first stage cooling. This cavity provided sufficient power for cooling and trapping the strontium atoms for up to 300 mW of 461 nm laser light. Furthermore, a new design of Zeeman slower based on spherical permanent magnets was produced, built and characterized. \n \nThe slower is compact, tunable, easy to assemble and maintain, and cost-effective as the permanent magnets do not require any power supply or water cooling. It is suggested that this kind of Zeeman slower constitutes an ideal solution for portable optical clock systems. A blue magneto-optical trap was realized for the first time with such a Zeeman slower. With about 30 mW of a 461 nm laser, up to 6X10/(^8/) atoms were captured.
Optical clocks serve as the benchmark devices for the next-generation definition of the second and have significant applications in testing the theories beyong the standard models, dark matter research, and related fields.The ytterbium ion optical clock, featuring two clock transition lines, is among the most precise optical clocks with the lowest uncertainties.The National Institute of Metrology, China (NIM), initiated research on ytterbium ion optical clocks in 2021.Based on an end-cap ion trap, a vacuum system has been established.A 370 nm thirdharmonic fiber laser, along with 760 nm and 935 nm DFB lasers, ensures a stable operation of the optical system.To date, both the electric quadrupole and electric octupole clock transitions have been observed.A 2.8 Hz linewidth of the electric octupole transition has been achieved, with a stability of 1.7E-15 τ -1/2 after closed-loop locking.The uncertainties due to AC Starks shift from clock laser and second-order Doppler shifts are being evaluated, with an expected total uncertainty below 5E-18.
We report an uncertainty evaluation of an171Yb optical lattice clock with a total fractional uncertainty of 3.6 × 10-16, which is mainly limited by the lattice-induced light shift and the blackbody radiation shift. Our evaluation of the lattice-induced light shift, the density shift, and the second-order Zeeman shift is based on an interleaved measurement where we measure the frequency shift using the alternating stabilization of a clock laser to the 6s21S0-6s6p3P0clock transition with two different experimental parameters. In the present evaluation, the uncertainties of two sensitivity coefficients for the lattice-induced hyperpolarizability shift d incorporated in a widely used light shift model by RIKEN and the second-order Zeeman shift aZare improved compared with the uncertainties of previous coefficients. The hyperpolarizability coefficient d is determined by investigating the trap potential depth and the light shifts at the lattice frequencies near the two-photon transitions 6s6p3P0-6s8p3P0, 6s8p3P2, and 6s5f3F2. The obtained values are d=-1.1(4) μHz and aZ=-6.6(3) Hz/mT2. These improved coefficients should reduce the total systematic uncertainties of Yb lattice clocks at other institutes.