High-Resolution Spectroscopy of 173Yb+ Ions

By Dr. Noopur JainReviewed by Louis CastelFeb 20 2026

Researchers achieved the first coherent excitation and high-resolution spectroscopy of the previously unobserved 436 nm electric quadrupole transition in a single trapped ¹7³Yb? ion, enabling precise measurement of its isotope shift and hyperfine structure. These results improved the determination of the nuclear magnetic octupole moment by over two orders of magnitude and establish a foundation for next-generation optical clocks and precision tests of fundamental physics. The article was published in the journal Physical Review Letters.

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Background

Previous research on Yb⁺ ions with zero nuclear spin has utilized isotope-shift spectroscopy to search for new bosons and study higher-order atomic and nuclear effects. The ¹⁷¹Yb⁺ ion, with a nuclear spin of 1/2, is the foundation for highly accurate optical clocks based on the 2S_1/2→2D_3/2 and 2S_1/2→2F_7/2 transitions, which offer first-order magnetic field immunity. These clocks have been employed for testing local Lorentz symmetry, searching for temporal variations in the fine structure constant, and detecting ultralight dark matter.

The ¹⁷³Yb⁺ ion is expected to extend these studies, for example, by enabling investigations of nuclear-spin-dependent parity-nonconservation (PNC) interactions using the 2S_1/2→2D_3/2 transition, and potentially providing more accurate optical clocks based on the 2S_1/2→2F_7/2 transition with reduced probe light shift. Furthermore, differential studies, such as the PNC ratio and the hyperfine anomaly between ¹⁷³Yb⁺ and ¹⁷¹Yb⁺, benefit from the electronic structural similarity, which simplifies theoretical predictions. Earlier experiments on ¹⁷³Yb⁺ were limited to measuring the 2S_1/2 ground state hyperfine structure using laser-microwave double resonance spectroscopy.

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The Current Study

The experiment utilized a Paul endcap trap confining a single ¹⁷³Yb⁺ ion under ultrahigh vacuum. The trap was operated with radial frequencies of ωx,y/2π=598(5) kHz and an axial frequency of ωz/2π=1197(9) kHz. A constant magnetic field B between 130 μT and 260 μT was applied perpendicular to the trap axis.

A novel laser cooling scheme was implemented that is compatible with 2D_3/2 state detection, unlike previous spectroscopy works. By ensuring that an ion in the 2D_3/2(F=1) state is not pumped back into the cooling cycle, this state can be used as a "dark state," detected by the absence of fluorescence when cooling laser radiation is applied. For efficient laser cooling, spectral components are generated using electro-optic modulators (EOMs) to address all hyperfine levels that might become populated.

The main focus of the spectroscopy was the previously unobserved 2S_1/2→2D_3/2 electric quadrupole (E2) transition at 436 nm. The E2 probe laser was frequency-stabilized by being offset-locked to a ¹⁷¹Yb⁺ optical clock, inheriting a short-term fractional frequency instability of 10−15. The configuration of the probe laser’s wave vector k and polarization Ec was chosen to lie in the same plane as the magnetic field B, with both k and Ec at about 45^° to B. This configuration facilitates ΔmF=0 and ΔmF=2 excitations while suppressing the ΔmF=1 component.

To achieve high-contrast spectroscopy, a projective state preparation (PSP) technique was used to prepare the ion into a specific Zeeman level, specifically 2S_1/2(F=3; mF=0). This involves repeatedly applying 436 nm pulses and detecting 2D_3/2(F=1) population until the ion is successfully transferred to the target state. Following successful preparation, Fourier-limited spectra were obtained using 1 ms π pulses on the 2S_1/2(F=3; mF=0)→2D _3/2 (F=1; mF=0) transition. This method was also used to observe Rabi oscillations on this transition.

Results and Discussion

Using the PSP method, the spectroscopy contrast was increased by more than an order of magnitude, achieving over 80%. Residual deviations from full contrast were attributed to the residual ion temperature after Doppler cooling and the infidelity of the PSP process. Analysis suggested a mean motional state of n¯=18.5(33), corresponding to an ion temperature of T=0.69(11) mK, which is close to the Doppler limit.

The coherent spectroscopy of the 2S_1/2→2D_3/2 E2 transition at 436 nm allowed for the determination of the isotope shift between ¹⁷¹Yb⁺ and ¹⁷³Yb⁺ on this transition with an uncertainty of 1.4 Hz.

Further extending these methods, microwave spectroscopy was performed with hertz-level uncertainties to precisely measure the hyperfine structure (HFS) of the 2D_3/2 state. The measured HFS of the 2D_3/2 state yielded a precise inference of the nuclear magnetic octupole moment for ¹⁷³Yb.

Conclusion

This Letter successfully demonstrated efficient laser cooling, state preparation, and detection of a single trapped ¹⁷³Yb⁺ ion. Coherent spectroscopy of the previously unobserved 2S_1/2→2D_3/2 electric quadrupole transition at 436 nm was achieved, and the isotope shift between ¹⁷¹Yb⁺ and ¹⁷³Yb⁺ was measured with an uncertainty of 1.4 Hz.

By resolving the hyperfine structure of the 2D_3/2 state using microwave spectroscopy with a relative uncertainty below 10⁻⁸, the nuclear magnetic octupole moment for ¹⁷³Yb was inferred with an uncertainty improvement of over two orders of magnitude. These results provide essential high-resolution spectroscopic data for ¹⁷³Yb⁺ and pave the way for its use in applications such as advanced optical clocks, probing new physics.

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