World’s first absolute frequency measurement for the new clock transition at 431 nm in 171Yb

The new clock transition in neutral ytterbium with inner-shell electron excited opens fundamental physics searches with precision spectroscopy.

Highlight

• We observed and measured the absolute frequency of a new clock transition in neutral Yb for the first time. 

• Further precise measurements of the transition pave new ways to search for the time-variation of fundamental constant, new force between an electron and a neutron, and dark matter. 

• Simultaneous operation of the existing clock transition and the new clock transition can improve the stability of the state-of-the-art optical lattice clocks.

Short summary

We observed and performed absolute frequency measurement for a new clock transition in neutral ytterbium. By shining a resonant laser on ultracold atoms, a new clock transition with an inner-shell f electron excited at 431 nm is directly excited, and its absolute frequency is measured to be 695 171 054 858.1(8.2) kHz, whose relative uncertainty improved four orders of magnitude compared to the previous indirect measurement more than 40 years ago. This measurement opens a way to more precise spectroscopy of the transition, leading to various fundamental physics searches, such as searches for dark matter, time variation of the fine-structure constant, new forces through isotope shift measurements, and violation of Einstein’s equivalence principle, as well as a way to improve the stability of the optical lattice clock further. Details of this observation is published on Physical Review A on 2023/6/15, EDT.

Optical lattice clocks, where thousands of neutral atoms are trapped by a light force and interrogated by an extremely stable laser, is one of the world’s most accurate clocks. Particularly, compared to other kinds of atomic clocks, optical lattice clocks have an advantage of higher stability for a fixed average time, thanks to a large number of atoms trapped in the system. A transition in an atom that has a narrow frequency range for a resonance with a laser serves as the ultimate source of the stability, and conventionally, it has a minimum sensitivity to the environment to avoid any potential fluctuations in the transition frequency due to external fields such as magnetic field, electric field, and black body radiation. Such a transition is called a clock transition and can be found in several atoms that have two valence electrons. 

With the development of the precision spectroscopy with clock transitions without sensitivities to the environment, even such transitions turn out to function as sensors for the environment; the level of precision improved so much that faint changes in the environment can affect the resonant frequency of the insensitive transition. Given this high sensitivity, if transitions more sensitive to the external fields are utilized, more precise measurements of the external fields can be performed. In fact, theorists proposed different transitions for measuring different physics. Their predictions are not limited to external fields, and they claim that some transitions are good sensors for exotic fundamental physics.

One of such clock transitions with sensitivities to fundamental physics is a new clock transition at 431 nm in neutral ytterbium. The transition involves an inner-core f orbital electron excited, and this adds some benefits for the purpose of fundamental physics. One feature of this transition is high sensitivity to time variation of the fine-structure constant, the only dimensionless constant that characterizes the strength of electromagnetic force. As the name constant suggests, this quantity is not supposed to change over time or space, but some astronomical observations imply that it changes in distant places in the universe. It is desired to test this on the ground, and the new clock transition has the highest sensitivity among clock transitions in neutral atoms, allowing us to reach the same precision in shorter amount of time compared to previous measurements. Also, it is expected to contribute to searches for new forces between an electron and a neutron through isotope shift measurements, searches for ultralight dark matter, and test of Einstein’s equivalence principle. In spite of these benefits, no direct observation of the transition was reported previously, and exact resonant frequency of the transition was yet to be known, except for some old indirect measurements. 

We searched for this new transition at 431 nm in ytterbium. We trapped and cooled 171Yb atoms down to 30 µK to reduce the Doppler shift and other disturbances against precision spectroscopy. We developed a new laser at 431 nm to address the transition. The laser is locked to our frequency stabilization system originally developed for our optical lattice clock operation, and it is referenced to a a physical realization of Coordinated Universal Time maintained by National Metrology Institute of Japan (UTC(NMIJ)). UTC(NMIJ) is linked to Cs clocks with a relative accuracy of 10-14. This allows us to scan the laser frequency with recording the absolute frequency. Once we found the transition with the initial search, we performed more detailed spectroscopy by shining the 431 nm laser for a short time while the laser and magnetic field for the trap is off. This removes some systematic effects and allows us to apply magnetic field for clear separation of magnetic sublevels.

Fig. 1 shows a spectrum of the transition. When the laser frequency is resonant to the transition, the atoms are excited, and no longer interact with the laser giving fluorescence. This is observed as the decrease in the atom number. Four dips show the four magnetic sublevels and the average frequency of these four dips is regarded as the frequency of the transition. We performed multiple measurements of the absolute frequency to confirm that any single measurement is disturbed by transient frequency shifts. The result of this after compensating the systematic shifts is shown in Fig. 2. We obtained the absolute frequency of 695 171 054 858.1±8.2 kHz. The relative uncertainty for this is 1.2×10-11, which improved by four orders of magnitude from the previous indirect measurement 40 years ago.

This measurement allows everyone in the world to make use of this transition for spectroscopy purpose, and opens a way to more precise spectroscopy, such as 1×10-15 or smaller relative uncertainty similar to other transitions utilized for atomic clocks. The precision spectroscopy of this transition is beneficial for various fundamental physics shown as follows. 

1. Comparison between the transition frequency of the 431 nm transition and that of another clock transition can search for the time variation of the fine-structure constant. This would be a test for the constancy of the fundamental constants. 

2. Frequency comparison between the 431 nm transition and another transition also serves as a search for ultralight dark matter through its coupling to the fine-structure constant. 

3. Isotope shift measurements of this transition provide another set of data for a search for new forces between a neutron and an electron. This also can give some information on the charge radii and deformations of nuclei. 

4. The 431 nm transition is known for high sensitivity to the violation of Einstein’s equivalence principle. 

5. The 431 nm transition can be utilized as a sensor for external fields when we operate an optical lattice clock with the 578 nm clock transition. 

The next steps of our project are as follows. 

1. To observe the transition in all other stable isotopes and determine the isotope shifts in our own system. 

2. To trap atoms in an optical lattice, to observe the transition and to preciesly determine the magic wavelength, a wavelength for the optical lattice that does not change the transition frequency, towards even more precise spectroscopy. 

In the far future, we will perform a search for time variation of the fine-structure constant, and simultaneous operation of the two clock transitions for in-situ calibration of the effect of the external fields. 

This research is supported by JSPS KAKENHI Grant-in-Aid for Research Activity Start-up(21K20359), Grant-in-Aid for Scientific Research(B)(22H01161), Grant-in-Aid for Scientific Research(C)(22K04942), JST FOREST JPMJFR212S, JST-MIRAI JPMJM118A1, and Research Foundation for Opto-Science and Technology.

Paper Information

Journal：Physical Review A 107, L060801 (2023)

Title：Observation of the 4f146s2 1S0 − 4f135d6s2(J = 2) clock transition at 431 nm in 171Yb

Authors：Akio Kawasaki, Takumi Kobayashi, Akiko Nishiyama, Takehiko Tanabe, and Masami Yasuda

Posted on June 16, 2023 (JST)