An optical lattice clock with a bosonic isotope of mercury

 Optical clocks have now reached accuracies close to 1×10⁻¹⁸. They are used for various applications, such as chronometric geodesy, tests of General Relativity, the search for physics beyond the Standard Model, and the redefinition of the SI second. 

Among neutral species, mercury has several attractive properties for an optical lattice clock, including a low sensitivity to blackbody radiation and a high vapor pressure at room temperature. Until now, the ¹⁹⁹Hg fermionic isotope was the only isotope used in mercury clocks. However, its limited lifetime in the excited state restricts the full potential of the upcoming generation of ultrastable lasers. Using bosonic isotopes instead offers a way to overcome this limitation, thanks to their potentially unlimited lifetime.

This thesis reports the first observation of the ¹⁹⁸Hg bosonic transition in an optical lattice clock, which was achieved through several key experimental advancements and a challenging search for a narrow transition across a wide uncertainty range. The bosonic clock transition is forbidden but it can become weakly allowed via a high magnetic field, a technique known as the quenching method. This approach enables longer probing times that can be adjusted to the laser properties. Therefore, the first critical step was developing a setup capable of generating a sufficiently large magnetic field to induce the bosonic transition with the highest possible coupling. Another challenge involved implementing a widely tunable and flexible probe laser while preserving its ultra-low noise characteristics, allowing the probing of any mercury isotope without introducing  additional noise. Since the coupling also increases with probe power, a major milestone was significantly boosting the power of our deep UV ultrastable light source.

Despite these experimental improvements, our calculations indicated that the coupling remained relatively weak, leading to a narrow-line transition that needed to be found over a broad frequency range. We conducted various measurements and checks, to optimize our chances of finding the transition. Thanks to these cumulative efforts, the search for the ¹⁹⁸Hg transition was successful, marking the first observation of a bosonic mercury isotope transition.

Building on this achievement, we established an operational optical lattice clock with the bosonic ¹⁹⁸Hg, already achieving a stability of 10⁻¹⁵ at 1 s. We have undertaken several studies of this new transition, including measuring the quadratic Zeeman shift coefficient with sufficient precision to control this shift to 10⁻¹⁷ or better. We have also begun investigating other systematic effects, such as the light shift, cold collisional shift, and lattice light shift, along with measuring the ¹⁹⁸Hg magic wavelength. We made a first series of comparing to ⁸⁷Sr and obtained a stability of 1.2×10⁻¹⁵ at 1 s for this comparison, paving the way for a first measurement of the ¹⁹⁸Hg/⁸⁷Sr optical frequency ratio. The work on the bosonic isotope will shortly lead to the possibility to implement more sophisticated probing methods (Hyper-Ramsey spectroscopy) that will improve the uncertainty to the limit of our current experimental setup.

This thesis also presents analyses and results obtained with the ¹⁹⁹Hg fermionic isotope during a fiber link clock comparison with several European institutes conducted in March/April 2023. 

optical lattice clock, mercury, fermionic and bosonic isotope

Full document (EN) : TEL-05034816