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arXiv:2509.13991

Improved systematic evaluation of a strontium optical clock with uncertainty below $1\times 10^{-18}$

Zhi-Peng Jia, Jie Li, De-Quan Kong, Zhang, Xiang, Hai-Wei Yu, Xiaoyong Liu, Yuchen Zhang, Yuanbo Wang, Xian-Qing Zhu, Jiahao Zhang, Ming-Yi Zhu, Pei-Jun Feng, Xing-Yang Cui, Xu Ping, Jiang, Xiao, Xiangpei Liu, Peng Liu, Han-Ning Dai, Yu-Ao Chen, Jian-Wei Pan

Published Sep 17, 2025• 0 citations
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Abstract

We report a systematic uncertainty of $9.2\times 10^{-19}$ 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 $6.3\times 10^{-19}$. 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 $6.3\times 10^{-19}$ by enabling simultaneous determination of key polarizabilities and magic wavelength. Magnetic shifts were resolved below $10^{-18}$ 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 $1\times10^{-18}$ 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.

DOI
10.48550/arxiv.2509.13991

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Optical lattice clocks (OLCs) have achieved an unprecedented relative systematic uncertainty of about 10^{-19}, establishing them as candidates for the upcoming redefinition of the second in the International System of Units. Currently, the accuracy of OLCs is primarily limited by black body radiation (BBR), cold collisions, and lattice light shift effects. Various strategies are being explored to mitigate these limitations, including using cryogenic environments to manage BBR and using new trapping geometries to control atomic distributions and interactions. In this project, we propose a new OLC setup that aims to characterize BBR and density effects below the 10^{-18} level. To achieve this goal, we have developed a copper science chamber that is placed inside a primary vacuum chamber. This configuration minimizes temperature inhomogeneities around the atoms to within a few tens of millikelvin (mK). The system features an adaptable one-dimensional (1D) optical lattice shaped with Laguerre-Gaussian LG_{0ell} modes to increase the number of trapping sites while reducing atomic occupancy per site. We created shaped lattices with values of ell from 0 to 4 whose trap depths were compatible with clock operation. We trapped thousands of atoms in TEM_{00}, LG_{01}and LG{02} shaped lattices and performed low-resolution clock spectroscopy on bosonic ^{88}Sr, yielding 100 Hz-wide transitions. To investigate cold collision effects, we plan to implement a second cooling stage using the ^1text{S}_0 rightarrow ^3text{P}_1 intercombination transition in strontium. This will increase atomic density, enhancing observable collision effects and enabling the detection of motional sidebands. These measurements will provide insight into the anharmonicity of LG-shaped lattices and help validate the lattice light shift model under varying trap conditions. Finally, the clock setup is still under construction. Current work focuses on enclosing the interrogation chamber within the primary vacuum to improve the thermal homogeneity and reduce the uncertainty related to BBR, and at the same time, the instrumentation required for high-resolution spectroscopy is currently being installed.

•Sep 23, 2025•0 citations
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.
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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.
The1S0-3P0transition at 698 nm in neutral strontium has the potential to be an extremely precise optical frequency standard [1] and is one of several candidates for a future redefinition of the second. Recent measurements of this transition at three national laboratories around the world have fractional frequency uncertainties in the 1015range and agree to within a hertz [2,3,4]. To realise fully the potential accuracy of this frequency standard, in depth studies of systematic effects need to be performed in order to characterise them at the level necessary for a redefinition of the second. At the National Physical Laboratory we are developing a compact, diode laser based optical lattice clock. We intend to study the clock transition in both88Sr and87Sr with focus on the uncertainties in the frequency shifts due to blackbody radiation. The setup incorporates a novel permanent magnet based Zeeman slower [5].
We report an optical lattice clock with a total systematic uncertainty of 8.1×10^{-19} in fractional frequency units, representing the lowest uncertainty of any clock to date. The clock relies on interrogating the ultranarrow ^{1}S_{0}→^{3}P_{0} transition in a dilute ensemble of fermionic strontium atoms trapped in a vertically-oriented, shallow, one-dimensional optical lattice. Using imaging spectroscopy, we previously demonstrated record high atomic coherence time and measurement precision enabled by precise control of collisional shifts and the lattice light shift. In this work, we revise the black body radiation shift correction by evaluating the 5s4d ^{3}D_{1} lifetime, necessitating precise characterization and control of many body effects in the 5s4d ^{3}D_{1} decay. Last, we measure the second order Zeeman coefficient on the least magnetically sensitive clock transition. All other systematic effects have uncertainties below 1×10^{-19}.
Rapid progress in the precision and accuracy of optical atomic clocks over the last decade has advanced the frontiers of timekeeping, metrology, and quantum science. However, the stabilities of most optical clocks remain limited by the local oscillator rather than the atoms themselves, leaving room for further progress. Here we implement a "multiplexed" one-dimensional optical lattice clock, in which spatially-resolved, movable ensembles of ultra-cold strontium atoms are trapped in the same optical lattice, interrogated simultaneously by a shared clock laser, and read-out in parallel. By performing synchronized Ramsey interrogations of ensemble pairs we observe atom-atom coherence times up to 26 seconds, a 270-fold improvement over the atom-laser coherence time, demonstrate a relative stability of 9.7(4)×10−18/τ9.7(4)\times10^{-18}/\sqrtτ9.7(4)×10−18/τ​ (where τττ is the averaging time in seconds), and reach a fractional uncertainty of 8.9(3)×10−208.9(3)\times 10^{-20}8.9(3)×10−20 after 3.3 hours of averaging. These results demonstrate that applications requiring ultra-high-precision comparisons between optical atomic clocks need not be limited by the stability of the local oscillator. With multiple ensemble pairs, we realize a miniaturized clock network consisting of 6 atom ensembles, resulting in 15 unique pairwise clock comparisons with relative stabilities below 3×10−17/τ3\times10^{-17}/\sqrtτ3×10−17/τ​. Finally, we demonstrate the capability to simultaneously load spatially-resolved, heterogeneous ensemble pairs of all four stable isotopes of strontium in a lattice. The unique capabilities offered by this platform pave the way for future studies of precision isotope shift measurements, spatially resolved characterization of limiting clock systematics, development of clock-based gravitational wave and dark matter detectors, and novel tests of relativity including measurements of the gravitational redshift at sub-centimeter scales.
Optical lattice clocks (OLCs) have achieved an unprecedented relative systematic uncertainty of about 10^{-19}, establishing them as candidates for the upcoming redefinition of the second in the International System of Units. Currently, the accuracy of OLCs is primarily limited by black body radiation (BBR), cold collisions, and lattice light shift effects. Various strategies are being explored to mitigate these limitations, including using cryogenic environments to manage BBR and using new trapping geometries to control atomic distributions and interactions. In this project, we propose a new OLC setup that aims to characterize BBR and density effects below the 10^{-18} level. To achieve this goal, we have developed a copper science chamber that is placed inside a primary vacuum chamber. This configuration minimizes temperature inhomogeneities around the atoms to within a few tens of millikelvin (mK). The system features an adaptable one-dimensional (1D) optical lattice shaped with Laguerre-Gaussian LG_{0ell} modes to increase the number of trapping sites while reducing atomic occupancy per site. We created shaped lattices with values of ell from 0 to 4 whose trap depths were compatible with clock operation. We trapped thousands of atoms in TEM_{00}, LG_{01}and LG{02} shaped lattices and performed low-resolution clock spectroscopy on bosonic ^{88}Sr, yielding 100 Hz-wide transitions. To investigate cold collision effects, we plan to implement a second cooling stage using the ^1text{S}_0 rightarrow ^3text{P}_1 intercombination transition in strontium. This will increase atomic density, enhancing observable collision effects and enabling the detection of motional sidebands. These measurements will provide insight into the anharmonicity of LG-shaped lattices and help validate the lattice light shift model under varying trap conditions. Finally, the clock setup is still under construction. Current work focuses on enclosing the interrogation chamber within the primary vacuum to improve the thermal homogeneity and reduce the uncertainty related to BBR, and at the same time, the instrumentation required for high-resolution spectroscopy is currently being installed.