Research

Currently, my research project is focused on precision spectroscopy of ytterbium and its application to fundamental physics. (Please refer to Past Research for my previous works.)

Ytterbium (Yb) is one of the popular atoms in the field of atomic physics. It has seven stable isotopes, two fermions and five bosons, and a couple of transitions that are strong enough for laser cooling and atom detections. Besides these basic requirements, ytterbium has a feature of multiple narrow transitions. Particularly, the 578 nm transition is implemented in the state-of-the-art optical lattice clocks because of its narrow linewidth and insensitivity to external fields. The 507 nm transition is another narrow-linewidth transition for ytterbium that is already observed. Two metastable states for these two transitions have the same electronic structure, and lie in different fine structure. 

In addition,  Yb has another narrow-linewidth transition at 431 nm. This transition is first theoretically predicted to be highly sensitive to various fundamental physics searches [1,2], due to an electron in the inner-shell f orbital excited by this transition. One is the high sensitivity to the variation of the fine structure constant. Another is the high sensitivity to the new particle search with isotope shift measurements [3], because of the large difference of the electronic structure from the known narrow-linewidth transitions. Further application of this transition can be possible, if the absence of an electron from the f orbital and non-zero orbital angular momentum is utilized. However, the transition had not been directly observed at the point of the theoretical proposals in 2018. 

In my project, I observed this 431 nm transition and performed an absolute frequency measurement for the first time in the world [4]. Subsequently, I performed absolute frequency measurements for multiple isotopes, and analyzed the isotope shift data. Together with a theoretical calculation for the electronic structure of ytterbium, I obtained charge radii difference between isotopes and put a constraint on the existence of bosons mediating a force between an electron and a neutron [5].

Currently, I am working on improving the accuracy of the absolute frequency measurement to <1 Hz by establishing an optical lattice clock with the 431 nm transition. This will improve the sensitivity to the bosons mediating forces between an electron and a neutron by more than three orders of magnitude.  Also, this opens ways to a search for ultralight dark matter and variation of the fine structure constant with higher precision. Also, the transition adds extra sensitivity to the environment such as magnetic and electric fields to the 578 nm transition used for optical lattice clocks. This can be utilized for in-situ characterization of systematic shifts for an optical lattice clock. 

Please contact Akio Kawasaki for more details and open positions. Currently, a graduate student position is available, and I am searching for a postdoc. 

[1] Safronova, et al.: Phys. Rev. Lett. 120, 173001 (2018)

[2] Dzuba, et al.: Phys. Rev. A 98, 022501 (2018)

[3] Hur*, Aude Craik*, Counts*, ..., AK, ...: Phys. Rev. Lett. 128, 163201 (2022) and many other references. 

[4] AK, et al.:Phys. Rev. A 107, L060801 (2023)

[5] AK, et al.:Phys. Rev. A 109, 062806 (2024)

Last update: January 3, 2025