Abstract
The exceptionally low-energy isomeric transition in 229Th at around 148.4 nm (refs. 1,2,3,4,5,6) offers a unique opportunity for coherent nuclear control and the realization of a nuclear clock7,8. Recent advances, most notably the incorporation of large ensembles of 229Th nuclei in transparent crystals6,9,10,11 and the development of pulsed vacuum ultraviolet (VUV) lasers12,13,14, have enabled initial laser spectroscopy of this transition15,16,17. However, the lack of an intense, narrow-linewidth VUV laser has precluded coherent nuclear manipulation8,18. Here we introduce and report a continuous-wave (CW) laser at 148.4 nm, generated by means of four-wave mixing (FWM)19 in cadmium vapour. The source delivers more than 100 nW of power with a projected linewidth well below 100 Hz and supports broad wavelength tunability. This represents a five-orders-of-magnitude improvement in linewidth over all previous single-frequency lasers below 190 nm (refs. 12,13,14,20). We develop a spatially resolved homodyne technique that places a stringent upper bound on FWM-induced phase noise, thereby supporting the feasibility of sub-hertz VUV linewidths. Our work addresses the central challenge towards a 229Th-based nuclear clock and establishes a widely tunable, ultranarrow-linewidth laser platform for potential applications across quantum information science21,22,23,24, condensed-matter physics25 and high-resolution VUV spectroscopy26.
This is a preview of subscription content, access via your institution
Access options
Access Nature and 54 other Nature Portfolio journals
Get Nature+, our best-value online-access subscription
$32.99 / 30 days
cancel any time
Subscribe to this journal
Receive 51 print issues and online access
$199.00 per year
only $3.90 per issue
Buy this article
- Purchase on SpringerLink
- Instant access to the full article PDF.
USD 39.95
Prices may be subject to local taxes which are calculated during checkout
Similar content being viewed by others
Data availability
The datasets generated and analysed during this study are available in the Figshare repository at https://doi.org/10.6084/m9.figshare.30795137.
Code availability
The plotting scripts used to generate the figures from the datasets are available in the Figshare repository at https://doi.org/10.6084/m9.figshare.30795137.
References
Kroger, L. A. & Reich, C. W. Features of the low-energy level scheme of 229Th as observed in the α-decay of 233U. Nucl. Phys. A 259, 29–60 (1976).
Beck, B. R. et al. Energy splitting of the ground-state doublet in the nucleus 229Th. Phys. Rev. Lett. 98, 142501 (2007).
Seiferle, B. et al. Energy of the 229Th nuclear clock transition. Nature 573, 243–246 (2019).
Masuda, T. et al. X-ray pumping of the 229Th nuclear clock isomer. Nature 573, 238–242 (2019).
Sikorsky, T. et al. Measurement of the 229Th isomer energy with a magnetic microcalorimeter. Phys. Rev. Lett. 125, 142503 (2020).
Kraemer, S. et al. Observation of the radiative decay of the 229Th nuclear clock isomer. Nature 617, 706–710 (2023).
Peik, E. & Tamm, C. Nuclear laser spectroscopy of the 3.5 eV transition in Th-229. Europhys. Lett. 61, 181 (2003).
Campbell, C. J. et al. Single-ion nuclear clock for metrology at the 19th decimal place. Phys. Rev. Lett. 108, 120802 (2012).
Rellergert, W. G. et al. Constraining the evolution of the fundamental constants with a solid-state optical frequency reference based on the 229Th nucleus. Phys. Rev. Lett. 104, 200802 (2010).
Kazakov, G. A. et al. Performance of a 229Thorium solid-state nuclear clock. New J. Phys. 14, 083019 (2012).
Beeks, K. et al. Growth and characterization of thorium-doped calcium fluoride single crystals. Sci. Rep. 13, 3897 (2023).
Jeet, J. Search for the Low Lying Transition in the 229Th Nucleus. Dissertation, Univ. California, Los Angeles (2018).
Thielking, J. et al. Vacuum-ultraviolet laser source for spectroscopy of trapped thorium ions. New J. Phys. 25, 083026 (2023).
Zhang, C. et al. Tunable VUV frequency comb for 229mTh nuclear spectroscopy. Opt. Lett. 47, 5591–5594 (2022).
Tiedau, J. et al. Laser excitation of the Th-229 nucleus. Phys. Rev. Lett. 132, 182501 (2024).
Elwell, R. et al. Laser excitation of the 229Th nuclear isomeric transition in a solid-state host. Phys. Rev. Lett. 133, 013201 (2024).
Zhang, C. et al. Frequency ratio of the 229mTh nuclear isomeric transition and the 87Sr atomic clock. Nature 633, 63–70 (2024).
Beeks, K. et al. The thorium-229 low-energy isomer and the nuclear clock. Nat. Rev. Phys. 3, 238–248 (2021).
Hodgson, R. T., Sorokin, P. P. & Wynne, J. J. Tunable coherent vacuum-ultraviolet generation in atomic vapors. Phys. Rev. Lett. 32, 343–346 (1974).
Scholz, M. et al. 1.3-mW tunable and narrow-band continuous-wave light source at 191 nm. Opt. Express 20, 18659–18664 (2012).
Schmidt, P. O. et al. Spectroscopy using quantum logic. Science 309, 749–752 (2005).
Marshall, M. C. et al. High-stability single-ion clock with 5.5 × 10−19 systematic uncertainty. Phys. Rev. Lett. 135, 033201 (2025).
Schmidt-Kaler, F. et al. Rydberg excitation of trapped cold ions: a detailed case study. New J. Phys. 13, 075014 (2011).
Zhang, C. et al. Submicrosecond entangling gate between trapped ions via Rydberg interaction. Nature 580, 345–349 (2020).
Zhou, X. et al. New developments in laser-based photoemission spectroscopy and its scientific applications: a key issues review. Rep. Prog. Phys. 81, 062101 (2018).
Kostko, O., Bandyopadhyay, B. & Ahmed, M. Vacuum ultraviolet photoionization of complex chemical systems. Annu. Rev. Phys. Chem. 67, 19–40 (2016).
Flambaum, V. V. Enhanced effect of temporal variation of the fine structure constant and the strong interaction in 229Th. Phys. Rev. Lett. 97, 092502 (2006).
Fuchs, E. et al. Searching for dark matter with the 229Th nuclear lineshape from laser spectroscopy. Phys. Rev. X 15, 021055 (2025).
Zhang, C. et al. 229ThF4 thin films for solid-state nuclear clocks. Nature 636, 603–608 (2024).
Higgins, J. S. et al. Temperature sensitivity of a thorium-229 solid-state nuclear clock. Phys. Rev. Lett. 134, 113801 (2025).
Terhune, J. E. S. et al. Photo-induced quenching of the 229Th isomer in a solid-state host. Phys. Rev. Res. 7, L022062 (2025).
Schaden, F. et al. Laser-induced quenching of the Th-229 nuclear clock isomer in calcium fluoride. Phys. Rev. Res. 7, L022036 (2025).
Campbell, C. J., Radnaev, A. G. & Kuzmich, A. Wigner crystals of 229Th for optical excitation of the nuclear isomer. Phys. Rev. Lett. 106, 223001 (2011).
Thielking, J. et al. Laser spectroscopic characterization of the nuclear-clock isomer 229mTh. Nature 556, 321–325 (2018).
Scharl, K. et al. Setup for the ionic lifetime measurement of the 229mTh3+ nuclear clock isomer. Atoms 11, 108 (2023).
Zitzer, G. et al. Sympathetic cooling of trapped Th3+ alpha-recoil ions for laser spectroscopy. Phys. Rev. A 109, 033116 (2024).
Yamaguchi, A. et al. Laser spectroscopy of triply charged 229Th isomer for a nuclear clock. Nature 629, 62–66 (2024).
Leibfried, D., Blatt, R., Monroe, C. & Wineland, D. Quantum dynamics of single trapped ions. Rev. Mod. Phys. 75, 281 (2003).
Thirolf, P. Shedding light on the thorium-229 nuclear clock isomer. Physics 17, 71 (2024).
Mutailipu, M. & Pan, S. Emergent deep-ultraviolet nonlinear optical candidates. Angew. Chem. Int. Ed. 59, 20302–20317 (2020).
Víllora, E. G., Shimamura, K., Sumiya, K. & Ishibashi, H. Birefringent- and quasi phase-matching with BaMgF4 for vacuum-UV/UV and mid-IR all solid-state lasers. Opt. Express 17, 12362–12378 (2009).
Yakar, O., Nitiss, E., Hu, J. & Brès, C.-S. Integrated backward second-harmonic generation through optically induced quasi-phase-matching. Phys. Rev. Lett. 131, 143802 (2023).
Eikema, K. S. E., Walz, J. & Hänsch, T. W. Continuous wave coherent Lyman-α radiation. Phys. Rev. Lett. 83, 3828 (1999).
Kolbe, D., Scheid, M. & Walz, J. Triple resonant four-wave mixing boosts the yield of continuous coherent vacuum ultraviolet generation. Phys. Rev. Lett. 109, 063901 (2012).
Pahl, A. et al. Generation of continuous coherent radiation at Lyman-α and 1S-2P spectroscopy of atomic hydrogen. Laser Phys. 15, 46–54 (2005).
Xiao, Q. et al. Proposal for the generation of continuous-wave vacuum ultraviolet laser light for Th-229 isomer precision spectroscopy. Preprint at https://arxiv.org/abs/2406.16841 (2024).
Benko, C. et al. Extreme ultraviolet radiation with coherence time greater than 1 s. Nat. Photon. 8, 530–536 (2014).
Penyazkov, G., Yu, Y., Skripnikov, L. V. & Ding, S. Theoretical study of transition matrix elements in cadmium for vacuum-ultraviolet generation in 229Th nuclear clock applications. Phys. Rev. A 112, 022807 (2025).
Wang, J. et al. A new instrument of VUV laser desorption/ionization mass spectrometry imaging with micrometer spatial resolution and low level of molecular fragmentation. Rev. Sci. Instrum. 88, 114102 (2017).
Vidal, C. R. in Tunable Lasers (eds Mollenauer, L. F., White, J. C. & Pollock, C. R.) Ch. 3 (Springer, 2005).
Tian, H. et al. Frequency-shifted f-2f interferometer for unveiling the noise performance of carrier-envelope offset in passively stabilized frequency combs. Appl. Phys. Lett. 125, 241107 (2024).
Bodine, M. I. et al. Optical atomic clock comparison through turbulent air. Phys. Rev. Res. 2, 033395 (2020).
von der Wense, L. et al. The theory of direct laser excitation of nuclear transitions. Eur. Phys. J. A 56, 176 (2020).
Hiraki, T. et al. Controlling 229Th isomeric state population in a VUV transparent crystal. Nat. Commun. 15, 5536 (2024).
Matei, D. G. et al. 1.5 μm lasers with sub-10 mHz linewidth. Phys. Rev. Lett. 118, 263202 (2017).
Lal, V. et al. Continuous-wave laser source at the 148 nm nuclear transition of Th-229. Optica 12, 1971–1974 (2025).
Wu, L. et al. 0.26-Hz-linewidth ultrastable lasers at 1557 nm. Sci. Rep. 6, 24969 (2016).
Fienup, J. R. Phase retrieval algorithms: a comparison. Appl. Opt. 21, 2758–2769 (1982).
Riley, D. S. & Karam, S. L. The Allan variance and its applications to frequency stability measurements. Proc. IEEE 82, 1250–1259 (1994).
Riley, W. J. Handbook of frequency stability analysis. National Institute of Standards and Technology https://www.nist.gov/publications/handbook-frequency-stability-analysis (2008).
Makdissi, A., Vernotte, F. & De Clercq, E. Stability variances: a filter approach. IEEE Trans. Ultrason. Ferroelectr. Freq. Control 57, 1011–1028 (2010).
Mandel, L. & Wolf, E. Optical Coherence and Quantum Optics (Cambridge Univ. Press, 1995).
Elliott, D. S., Roy, R. & Smith, S. J. Extracavity laser band-shape and bandwidth modification. Phys. Rev. A 26, 12–18 (1982).
Larkin, K. G. Efficient nonlinear algorithm for envelope detection in white-light interferograms. J. Opt. Soc. Am. A 13, 832–843 (1996).
Rutman, J. Characterization of phase and frequency instabilities in precision frequency sources: fifteen years of progress. Proc. IEEE 66, 1048–1075 (1978).
Domenico, G. D., Schilt, S. & Thomann, P. Simple approach to the relation between laser frequency noise and laser line shape. Appl. Opt. 49, 4801–4807 (2010).
Fortier, T. & Baumann, E. 20 years of developments in optical frequency comb technology and applications. Commun. Phys. 2, 153 (2019).
Photomultiplier Tubes: Basics and Applications 4th edn (Hamamatsu Photonics K. K., 2017).
Acknowledgements
We thank X. Zhang and S. Zhou for discussions, Z. Xu and X. Wen for advice on frequency doubling, M. K. Tey for support on electronics and C. Zhang, H. Wu and J. Ye for comments on the manuscript. This work is supported by the National Natural Science Foundation of China (NSFC no. 12341401), Beijing Science and Technology Planning Project (grant no. Z25110100040000) and the Tsinghua University Dushi Program, as well as by NSFC nos. 12274253, 92265205, 12504306 and 92565202.
Author information
Authors and Affiliations
Contributions
Q.X., G.P., X.L., B.H., W.B., J.S., H.S., G.Y., Y. Li, J.L., L.Y., Y.M. and S.D. designed the experiment, constructed the set-up and carried out the measurements. T.L., H.T., B.L. and Y. Lin built the ULE-cavity-stabilized 1,550-nm laser. Q.X. and S.D. wrote the manuscript, with input from all authors.
Corresponding author
Ethics declarations
Competing interests
The authors declare no competing interests.
Peer review
Peer review information
Nature thanks Tomas Sikorsky and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.
Additional information
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
Extended Data Fig. 1 Measurement of VUV power.
With the 710-nm beam shuttered, the PMT records a background count rate of 1.42(1) × 105 s−1. At t ≈ 3.5 s, the 710-nm beam is unshuttered and the count rate rises to 4.76(34) × 106 s−1. Control measurements verify that the background count is dominated by residual 375-nm stray light, that the PMT is insensitive at 710-nm and that it operates in the linear regime. Using the overall detection efficiency ηabs = 0.21(4) × 10−4 (Extended Data Table 1), we infer a VUV power of 290(60) nW at 148.4 nm.
Extended Data Fig. 2 Linewidth measurements of the 750-nm and 710-nm Ti:sapphire lasers.
a, 750-nm Ti:sapphire laser. b, 710-nm Ti:sapphire laser. Both lasers are Pound–Drever–Hall-locked to a 10-cm-long ULE cavity. Each is heterodyned with an optical frequency comb referenced to a ULE-stabilized fibre laser at 1,550 nm (linewidth 0.25(1) Hz). The optical beats are down-mixed to an intermediate frequency within the span of a dynamic signal analyser and the resulting spectra (blue points) are fitted with Lorentzians (blue lines), yielding FWHM linewidths of Δf750 = 1.05(7) Hz and Δf710 = 1.01(9) Hz after subtracting the comb contribution67.
Supplementary information
Rights and permissions
Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.
About this article
Cite this article
Xiao, Q., Penyazkov, G., Li, X. et al. Continuous-wave narrow-linewidth vacuum ultraviolet laser source. Nature (2026). https://doi.org/10.1038/s41586-026-10107-4
Received:
Accepted:
Published:
Version of record:
DOI: https://doi.org/10.1038/s41586-026-10107-4
