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Chinese Physics B

Experimental setup of NTSC SrII optical lattice clock

Feng 峰 Guo 郭, Jia-An 家安 Li 李, Yan-Yan 艳艳 Liu 刘, Xiao-Tong 晓同 Lu 卢, Hong Chang

Published Jul 8, 2025• 0 citations
Publisher

Abstract

Abstract We report the SrII optical lattice clock at the National Time Service Center (NTSC). In this system, a blackbody radiation shield with movable lattice mitigates blackbody radiation shifts through active temperature control. A shallow optical lattice with minimal tunneling minimizes AC Stark shifts. Phase-locked counter-propagating lattice beams and conductive vacuum viewports further reduce systematic uncertainties and a novel initial-state preparation method simplifies the system. Clock transition spectra achieve a linewidth of 2.5 Hz with a 400 ms clock pulse, and self-comparison stability reaches 5.1 × 10 −16 at 1 s. These advancements give this clock the potential to be a critical platform for realizing outstanding systematic uncertainties in the future.

DOI
10.1088/1674-1056/adecfd

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Abstract We report the SrII optical lattice clock at the National Time Service Center (NTSC). In this system, a blackbody radiation shield with movable lattice mitigates blackbody radiation shifts through active temperature control. A shallow optical lattice with minimal tunneling minimizes AC Stark shifts. Phase-locked counter-propagating lattice beams and conductive vacuum viewports further reduce systematic uncertainties and a novel initial-state preparation method simplifies the system. Clock transition spectra achieve a linewidth of 2.5 Hz with a 400 ms clock pulse, and self-comparison stability reaches 5.1 × 10 −16 at 1 s. These advancements give this clock the potential to be a critical platform for realizing outstanding systematic uncertainties in the future.
Controlling the Stark perturbation from ambient thermal radiation is key to advancing the performance of many atomic frequency standards. This thermal radiation perturbs the atomic energy levels, inducing a Stark shift to the clock frequency conventionally known as the BBR shift. For over four decades, the BBR shift has been a persistent challenge in the pursuit of better atomic clock performance. For room-temperature optical lattice clocks (OLCs) based on Yb and Sr, the BBR shift represents the largest uncanceled systematic frequency shift while typically also constituting the largest source of uncertainty in these state-of-the-art clocks at the ≈ 1 × 10−18 level. In this thesis, I report on the design, assembly and integration of a BBR enclosure, or “shield,” that achieves an unprecedented BBR shift uncertainty of 1.7 × 10−20 by furnishing the interrogated atoms with a near-ideal BBR environment at a cryogenic temperature, while still allowing all the critical quantum control functions required for clock operation. Also, I present a novel lattice loading technique, known as ratchet loading, which enables programmable control over the spatial distribution of ultra-cold atoms confined in an optical lattice. This method allows for the loading of a large number of atoms, effectively reducing quantum projection noise (QPN) while also mitigating frequency shifts caused by atomic interactions. Additionally, ratchet loading facilitates the creation of spatially resolved atomic ensembles along the lattice, making it particularly useful for lattice light shift measurements and potentially advantageous for emerging techniques that probe atomic coherence beyond the local oscillator (LO) coherence time. With the near-elimination of blackbody radiation (BBR) uncertainty and improved clock stability for systematic evaluations, the realization of a mid- to low-10−19 uncertainty Yb optical lattice clock is now more attainable than ever.
A major obstacle for optical clocks is the frequency shift due to black body radiation. We discuss how one can tackle this problem in an optical lattice clock; in our case 87-Sr: firstly, by a measurement of the dc Stark shift of the clock transition and, secondly, by interrogating the atoms in a cryogenic environment. Both approaches rely on transporting ultracold atoms over several cm within a probe cycle. We evaluate this approach of mechanically moving the optical lattice and conclude that it is feasible to transport the atoms over 50 mm within 300 ms. With this transport a dc Stark shift measurement will allow to reduce the contribution of the blackbody radiation to the fractional uncertainty below 2 * 10^-17 at room temperature by improving the shift coefficient known only from atomic structure calculations up to now. We propose a cryogenic environment at 77 K that will reduce this contribution to few parts in 10^-18.
We report a transportable one-dimensional optical lattice clock based on 87Sr at the National Time Service Center. The transportable apparatus consists of a compact vacuum system and compact optical subsystems. The vacuum system with a size of 90 cm× 20 cm× 42 cm and the beam distributors are assembled on a double-layer optical breadboard. The modularized optical subsystems are integrated on independent optical breadboards. By using a 230 ms clock laser pulse, spin-polarized spectroscopy with a linewidth of 4.8 Hz is obtained which is close to the 3.9 Hz Fourier-limit linewidth. The time interleaved self-comparison frequency instability is determined to be 6.3 × 10–17 at an averaging time of 2000 s.
We experimentally investigate an optical clock based on ;{171}Yb (I = 1/2) atoms confined in an optical lattice. We have evaluated all known frequency shifts to the clock transition, including a density-dependent collision shift, with a fractional uncertainty of 3.4 x 10;{-16}, limited principally by uncertainty in the blackbody radiation Stark shift. We measured the absolute clock transition frequency relative to the NIST-F1 Cs fountain clock and find the frequency to be 518 295 836 590 865.2(0.7) Hz.
We describe recent experimental progress with the JILA Sr optical frequency standard, which has a systematic uncertainty at the 10-16fractional frequency level, currently limited by frequency shifts due to atomic interactions and room temperature blackbody radiation (BBR). We will discuss systematic measurements of collisional shifts and provide a complete theory to understand these interaction effects at the 10-17level. We will also discuss plans to interrogate atoms inside a cryogenic shield region to eliminate BBR shifts. We will present our recent successful efforts in improving the laser stability with the aim for the JILA Sr standard to reach the standard quantum measurement limit and achieve record levels of stability. Finally, a second Sr optical lattice clock has been constructed at JILA, and we will discuss the results of intercomparison experiments between the two standards.
We experimentally investigate an optical clock based on 171^{171}171Yb (I=1/2I=1/2I=1/2) atoms confined in an optical lattice. We have evaluated all known frequency shifts to the clock transition, including a density-dependent collision shift, with a fractional uncertainty of 3.4×10−163.4 \times 10^{-16}3.4×10−16, limited principally by uncertainty in the blackbody radiation Stark shift. We measured the absolute clock transition frequency relative to the NIST-F1 Cs fountain clock and find the frequency to be 518 295 836 590 865.2(0.7) Hz.