Observation of an Inner-Shell Orbital Clock Transition in Neutral Ytterbium Atoms

内殻電子が励起する時計遷移の初観測に成功 --新奇な光格子時計を用いた超高感度な新物理探索へ--. 京都大学プレスリリース. 2023-04-17.We observe a weakly allowed optical transition of atomic ytterbium from the ground state to the metastable state 4f¹³5d6s² (J=2) for all five bosonic and two fermionic isotopes with resolved Zeeman and hyperfine structures. This inner-shell orbital transition has been proposed as a new frequency standard as well as a quantum sensor for new physics. We find magic wavelengths through the measurement of the scalar and tensor polarizabilities and reveal that the measured trap lifetime in a three-dimensional optical lattice is 1.9(1) s, which is crucial for precision measurements. We also determine the g factor by an interleaved measurement, consistent with our relativistic atomic calculation. This work opens the possibility of an optical lattice clock with improved stability and accuracy as well as novel approaches for physics beyond the standard model

Observation of Nonlinearity of Generalized King Plot in the Search for New Boson

新粒子探索のための量子センサー --原子スペクトルの精密分光から基礎物理法則に迫る--. 京都大学プレスリリース. 2022-05-12.We measure isotope shifts for neutral Yb isotopes on an ultranarrow optical clock transition ¹S₀ - ³P₀ with an accuracy of a few hertz. Combined with one of the recently reported isotope-shift measurements of Yb⁺ on two optical transitions, the result allows us to construct the King plots --a set of scaled isotope shifts data on two different optical transitions plotted in two-dimensional plane. When only the leading-order terms of isotope shifts are taken into account, a King plot should exhibit a linear relation as a result of elimination of the leading nuclear-size dependence. Extremely large nonlinearity unexplainable by a quadratic field shift is revealed, which was proposed previously as a source of the observed nonlinearity of the King plot. We further construct the generalized King plot with three optical transitions so that we can eliminate the contribution arising from a higher-order effect within the standard model. Our analysis of the generalized King plot shows a deviation from linearity at the 3σ level, indicating that there exist at least two higher-order contributions in the measured isotope shifts. Under reasonable assumptions, we obtain the upper bound of the product of the couplings for a new boson, mediating a force between electrons and neutrons --|yeyn|/(ℏc)<1×10⁻¹⁰ for the mass less than 1 keV-- with the 95% confidence level, providing an important step toward probing new physics via isotope-shift spectroscopy