Abstract
Thorium-229 (229Th) possesses an optical nuclear transition between the ground state (229gTh) and low-lying isomer (229mTh). A nuclear clock based on this nuclear-transition frequency is expected to surpass existing atomic clocks owing to its insusceptibility to surrounding fields1,2,3,4,5. In contrast to other charge states, triply charged 229Th (229Th3+) is the most suitable for highly accurate nuclear clocks because it has closed electronic transitions that enable laser cooling, laser-induced fluorescence detection and state preparation of ions1,6,7,8. Although laser spectroscopic studies of 229Th3+ in the nuclear ground state have been performed8, properties of 229mTh3+, including its nuclear decay lifetime that is essential to specify the intrinsic linewidth of the nuclear-clock transition, remain unknown. Here we report the trapping of 229mTh3+ continuously supplied by a 233U source and the determination of nuclear decay half-life of the isolated 229mTh3+ to be \({{\rm{1,400}}}_{-300}^{+600}\,{\rm{s}}\) through nuclear-state-selective laser spectroscopy. Furthermore, by determining the hyperfine constants of 229mTh3+, we reduced the uncertainty of the sensitivity of the 229Th nuclear clock to variations in the fine-structure constant by a factor of four. These results offer key parameters for the 229Th3+ nuclear clock and its applications in the search for new physics.
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Data availability
The data that support the findings of this study are available from the corresponding author on request.
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Acknowledgements
The 233U sample used in this paper was provided by the 233U cooperation project between the Japan Atomic Energy Agency and the Inter-University Cooperative Research Program of the Institute for Materials Research, Tohoku University (proposal nos. 17K0204, 17F0011 and 18F0014). This work was supported by JST PRESTO (grant number JPMJPR1868), JSPS KAKENHI (grant nos. JP19H00685, JP21H04473, JP22H04946 and JP23H00094) and Yamada Science Foundation.
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A.Y. and Y.S. contributed to building the experimental setup, performed the measurements and analysed the data. A.Y., Y.S., H.Ki., K.S. and H.H. contributed to developing the 233U source. M.W. contributed to developing the ion manipulation system, including an RF carpet and a PCB-based quadrupole ion guide. All works were supervised by H.Ka., H.H. and M.W. All authors discussed the results and contributed to the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 Doppler-broadened spectrum of the 5f 2F5/2 ↔ 6d 2D5/2 transition of 229gTh3+.
The red curve represents the background-subtracted fluorescence count rate at 984 nm as a function of the 690-nm laser frequency. Vertical blue bars denote hyperfine transitions \(({F}_{{\rm{g}}}^{{\rm{i}}}\to {F}_{{\rm{g}}}^{{\rm{f}}})\) whose frequencies were calculated on the basis of refs. 7,8. The relative height of the blue bars represents the relative strength of the calculated absorption cross-sections of each hyperfine transition. The black curve represents the summation of Gaussian functions corresponding to each hyperfine transition, for which the width and scaling factor were determined from the fitting of the isolated 5 → 4 peak.
Extended Data Fig. 2 Dependence of decay rate on laser irradiation time.
Dependence of the decay rate on the duty ratio of the laser irradiation time was evaluated for 229gTh3+ (a) and 229mTh3+ (b). Each decay-rate measurement was conducted by repeating a 24-s sequence, in which all three lasers were on for tON s and off for (24 − tON) s. The duty ratio is calculated as tON/24. The error bars represent the standard deviation of the mean. From the linear fitting shown as a solid line, the decay rate for no laser irradiation was estimated to be 8.94(11) × 10−4 s−1 for the ground state and 1.47(15) × 10−3 s−1 for isomer.
Extended Data Fig. 3 Determination of the resonance frequency \(({{\boldsymbol{\nu }}}_{{\bf{m}},{\bf{HF}}}^{690})\) of the |5f 2F5/2, Fm = 1⟩ → |6d 2D5/2, Fm = 2⟩ transition.
a, The spectra of the |5f 2F5/2, Fm = 1⟩ → |6d 2D3/2, Fm = 2⟩ transition (1,088 nm) were measured with several 690-nm laser frequencies around the resonance frequency of the |5f 2F5/2, Fm = 1⟩ → |6d 2D5/2, Fm = 2⟩ transition. From bottom to top, the 690-nm laser frequency was increased by 100 MHz at each step. The spectra were vertically shifted by 1.5 × 105 s−1 at each step for clarity. b, The spectral area was determined by fitting (black line) and plotted as a function of the 690-nm laser frequency. The resonance frequency \({\nu }_{{\rm{m}},{\rm{HF}}}^{690}\) was determined to be the one that gives the largest spectral area. The error bars represent the standard deviation of the mean.
Extended Data Fig. 4 Observations of all 229mTh3+ hyperfine resonances on the 5f 2F5/2 → 6d 2D3/2 and 5f 2F5/2 → 6d 2D5/2 transitions.
a–g, All of the hyperfine transitions on the 5f 2F5/2 → 6d 2D3/2 (1,088 nm) and 5f 2F5/2 → 6d 2D5/2 (690 nm) transitions, which were not contained in Fig. 3b–d, were observed. Definitions of the blue, orange, brown and green curves are same as in Fig. 3b–d. The purple bars indicate the resonance frequencies of each transition calculated using the hyperfine constants and isomer shifts obtained in this study. The width of the purple bar represents the uncertainty of the calculated values. The corresponding hyperfine transitions \(|5f\,{}^{2}{F}_{5/2},{F}_{{\rm{m}}}^{{\rm{i}},690}\rangle \to |6d\,{}^{2}{D}_{5/2},{F}_{{\rm{m}}}^{{\rm{f}},690}\rangle \) (690 nm) and \(|5f\,{}^{2}{F}_{5/2},{F}_{{\rm{m}}}^{{\rm{i}},\,1,088}\rangle \to |6d\,{}^{2}{D}_{3/2},{F}_{{\rm{m}}}^{{\rm{f}},1,088}\rangle \) (1,088 nm) are shown at the top of each figure as \({F}_{{\rm{m}}}^{{\rm{i}},690}\to {F}_{{\rm{m}}}^{{\rm{f}},690}\) and \({F}_{{\rm{m}}}^{{\rm{i}},1,088}\to {F}_{{\rm{m}}}^{{\rm{f}},1,088}\). We measured the spectrum a only with \({\nu }_{{\rm{B}}}^{984}\) because there were no overlapped 229gTh3+ resonances in the scanned frequency range.
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Yamaguchi, A., Shigekawa, Y., Haba, H. et al. Laser spectroscopy of triply charged 229Th isomer for a nuclear clock. Nature (2024). https://doi.org/10.1038/s41586-024-07296-1
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DOI: https://doi.org/10.1038/s41586-024-07296-1
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