An optical lattice clock with a bosonic isotope of mercury. – (Ashley BEGUIN / LCAR / Seminar). – 5/05/2026, 11H

Séminaire LCAR

Ashley BEGUIN, LTE, Observatoire de Paris, Université PSL, Sorbonne Université, Université de Lille, LNE

Summary
Since 1967, time has been defined through atomic transition frequencies, establishing atomic clocks as fundamental tools for science and technology. More recently, optical atomic clocks have surpassed historical atomic microwave clocks, reaching uncertainties close to 10⁻¹⁸ and enabling both applied and fundamental investigations. At this level of performance, atomic clocks serve as sensitive probes for a wide range of applications, such as chronometric geodesy, tests of General Relativity, searches for physics beyond the Standard Model, and the prospective redefinition of the SI second [1]. In addition, frequency ratio measurements would provide powerful tools to constrain possible variations of fundamental constants, such as the fine-structure constant α and the proton-to-electron mass ratio [2].

Among neutral atoms, mercury offers several attractive features for an optical lattice clock, including a low sensitivity to blackbody radiation—16 times lower than ytterbium and 30 times lower than strontium—and a relatively high vapor pressure at room temperature. To date, only the fermionic isotope ¹⁹⁹Hg has been used in mercury clocks. However, the limited lifetime of its excited state constrains the performance achievable with the next generation of ultrastable lasers. In contrast, bosonic isotopes offer a way to overcome this limitation, as they allow for much longer interrogation times, with the normally forbidden ¹S₀–³P₀ transition made accessible through a quenching scheme using a strong external magnetic field [3].

Here, we present the first ¹⁹⁸Hg optical lattice clock and its comparison with a local state-of-the-art ⁸⁷Sr optical lattice clock. This clock is estimated to operate with a relative frequency stability already as low as 6 × 10⁻¹⁶/√(τ/s), and the 198Hg/87Sr optical frequency ratio could be determined for the first time with a preliminary total relative systematic uncertainty of 6.9 × 10⁻¹⁶ [4]. With this magnetically induced transition, both the quadratic Zeeman shift (QZS) and the probe light shift (PLS) play indeed a significant role in the total uncertainty budget. Careful calibration and optimized experimental conditions can reduce the QZS uncertainty below the 10⁻¹⁷ level, whereas the PLS remains a major limitation under our current experimental setup. We are now working on implementing hyper-Ramsey interrogation [5] to suppress the PLS uncertainty to lower levels. Future steps will focus on further improving the clock stability by employing repumping techniques for normalized detection and achieving lower atomic temperatures through a sideband cooling scheme.

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• [3] A. V. Taichenachev et al, Phys. Rev. Lett., 96, 083001 (2006).
• [4] C. Zyskind, et al., arXiv:2512.04920 (2025).
• [5] V. I. Yudin, et al., Phys. Rev. A 82, 011804(R) (2010).