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Nuclear laser spectroscopy at the 10 −12 precision level (Nature 633.8028 (2024): 63–70) determined the fractional change in the nuclear quadrupole moment of 229 Th upon excitation, Δ Q 0 / Q 0  = 1.791(2)%. Such high-accuracy nuclear parameters enable stringent tests and refinement of 229 Th nuclear models. Using a semi-classical prolate-spheroid model, we quantify the transition frequency’s sensitivity to fine-structure constant variations as K  = 5900(2300), with uncertainty dominated by the measured charge-radius change Δ〈 r 2 〉. This supports the predicted higher α -sensitivity of nuclear clocks over atomic clocks, important for new-physics searches. We find Δ Q 0 strongly dependent on nuclear volume, challenging the constant-volume approximation. The deviation between measured and predicted Δ Q 0 / Q 0 underlines the need for improved modeling and measurement of additional nuclear parameters. We explicitly assess the octupole contribution to α -sensitivity. The authors report on new developments on the sensitivity of the nuclear clock transition in Th229 for new physics searches involving variations of the fine-structure constant. This highlights the need for developing of advanced nuclear models and parameter searches relating to experimental measurements.

Optical atomic clocks 1 , 2 use electronic energy levels to precisely keep track of time. A clock based on nuclear energy levels promises a next-generation platform for precision metrology and fundamental physics studies. Thorium-229 nuclei exhibit a uniquely low-energy nuclear transition within reach of state-of-the-art vacuum ultraviolet (VUV) laser light sources and have, therefore, been proposed for construction of a nuclear clock 3 , 4 . However, quantum-state-resolved spectroscopy of the 229m Th isomer to determine the underlying nuclear structure and establish a direct frequency connection with existing atomic clocks has yet to be performed. Here, we use a VUV frequency comb to directly excite the narrow 229 Th nuclear clock transition in a solid-state CaF 2 host material and determine the absolute transition frequency. We stabilize the fundamental frequency comb to the JILA 87 Sr clock 2 and coherently upconvert the fundamental to its seventh harmonic in the VUV range by using a femtosecond enhancement cavity. This VUV comb establishes a frequency link between nuclear and electronic energy levels and allows us to directly measure the frequency ratio of the 229 Th nuclear clock transition and the 87 Sr atomic clock. We also precisely measure the nuclear quadrupole splittings and extract intrinsic properties of the isomer. These results mark the start of nuclear-based solid-state optical clocks and demonstrate the first comparison, to our knowledge, of nuclear and atomic clocks for fundamental physics studies. This work represents a confluence of precision metrology, ultrafast strong-field physics, nuclear physics and fundamental physics. A vacuum ultraviolet frequency comb is used to directly excite the narrow 229 Th nuclear clock transition in a solid-state CaF 2 host material, marking the start of nuclear-based solid-state optical clocks.

After nearly 50 years of searching, the vacuum ultraviolet 229 Th nuclear isomeric transition has recently been directly laser excited 1 , 2 and measured with high spectroscopic precision 3 . Nuclear clocks based on this transition are expected to be more robust 4 , 5 than and may outperform 6 , 7 current optical atomic clocks. These clocks also promise sensitive tests for new physics beyond the standard model 5 , 8 , 9 , 10 , 11 – 12 . In light of these important advances and applications, a substantial increase in the need for 229 Th spectroscopy targets in several platforms is anticipated. However, the growth and handling of high-concentration 229 Th-doped crystals 5 used in previous measurements 1 , 2 – 3 , 13 , 14 are challenging because of the scarcity and radioactivity of the 229 Th material. Here we demonstrate a potentially scalable solution to these problems by performing laser excitation of the nuclear transition in 229 ThF 4 thin films grown using a physical vapour deposition process, consuming only micrograms of 229 Th material. The 229 ThF 4 thin films are intrinsically compatible with photonics platforms and nanofabrication tools for integration with laser sources and detectors, paving the way for an integrated and field-deployable solid-state nuclear clock with radioactivity up to three orders of magnitude smaller than typical 229 Th-doped crystals 1 , 2 – 3 , 13 . The high nuclear emitter density in 229 ThF 4 also potentially enables quantum optics studies in a new regime. Finally, we present the estimation of the performance of a nuclear clock based on a defect-free ThF 4 crystal. Laser excitation of the  229 Th isomer, potentially relevant for nuclear clocks, is reported in thorium fluoride thin films, which are less radioactive and amenable to integration compared with existing thorium-doped crystals.

After nearly 50 years of searching, the vacuum ultraviolet Th nuclear isomeric transition has recently been directly laser excited and measured with high spectroscopic precision . Nuclear clocks based on this transition are expected to be more robust than and may outperform current optical atomic clocks. These clocks also promise sensitive tests for new physics beyond the standard model . In light of these important advances and applications, a substantial increase in the need for Th spectroscopy targets in several platforms is anticipated. However, the growth and handling of high-concentration Th-doped crystals used in previous measurements are challenging because of the scarcity and radioactivity of the Th material. Here we demonstrate a potentially scalable solution to these problems by performing laser excitation of the nuclear transition in ThF thin films grown using a physical vapour deposition process, consuming only micrograms of Th material. The ThF thin films are intrinsically compatible with photonics platforms and nanofabrication tools for integration with laser sources and detectors, paving the way for an integrated and field-deployable solid-state nuclear clock with radioactivity up to three orders of magnitude smaller than typical Th-doped crystals . The high nuclear emitter density in ThF also potentially enables quantum optics studies in a new regime. Finally, we present the estimation of the performance of a nuclear clock based on a defect-free ThF crystal.

Optical atomic clocks use electronic energy levels to precisely keep track of time. A clock based on nuclear energy levels promises a next-generation platform for precision metrology and fundamental physics studies. Thorium-229 nuclei exhibit a uniquely low-energy nuclear transition within reach of state-of-the-art vacuum ultraviolet (VUV) laser light sources and have, therefore, been proposed for construction of a nuclear clock . However, quantum-state-resolved spectroscopy of the Th isomer to determine the underlying nuclear structure and establish a direct frequency connection with existing atomic clocks has yet to be performed. Here, we use a VUV frequency comb to directly excite the narrow Th nuclear clock transition in a solid-state CaF host material and determine the absolute transition frequency. We stabilize the fundamental frequency comb to the JILA Sr clock and coherently upconvert the fundamental to its seventh harmonic in the VUV range by using a femtosecond enhancement cavity. This VUV comb establishes a frequency link between nuclear and electronic energy levels and allows us to directly measure the frequency ratio of the Th nuclear clock transition and the Sr atomic clock. We also precisely measure the nuclear quadrupole splittings and extract intrinsic properties of the isomer. These results mark the start of nuclear-based solid-state optical clocks and demonstrate the first comparison, to our knowledge, of nuclear and atomic clocks for fundamental physics studies. This work represents a confluence of precision metrology, ultrafast strong-field physics, nuclear physics and fundamental physics.

Optical atomic clocks\\(^{1,2}\\) use electronic energy levels to precisely keep track of time. A clock based on nuclear energy levels promises a next-generation platform for precision metrology and fundamental physics studies. Thorium-229 nuclei exhibit a uniquely low energy nuclear transition within reach of state-of-the-art vacuum ultraviolet (VUV) laser light sources and have therefore been proposed for construction of the first nuclear clock\\(^{3,4}\\). However, quantum state-resolved spectroscopy of the \\(^{229m}\\)Th isomer to determine the underlying nuclear structure and establish a direct frequency connection with existing atomic clocks has yet to be performed. Here, we use a VUV frequency comb to directly excite the narrow \\(^{229}\\)Th nuclear clock transition in a solid-state CaF\\(_2\\) host material and determine the absolute transition frequency. We stabilize the fundamental frequency comb to the JILA \\(^{87}\\)Sr clock\\(^2\\) and coherently upconvert the fundamental to its 7th harmonic in the VUV range using a femtosecond enhancement cavity. This VUV comb establishes a frequency link between nuclear and electronic energy levels and allows us to directly measure the frequency ratio of the \\(^{229}\\)Th nuclear clock transition and the \\(^{87}\\)Sr atomic clock. We also precisely measure the nuclear quadrupole splittings and extract intrinsic properties of the isomer. These results mark the start of nuclear-based solid-state optical clock and demonstrate the first comparison of nuclear and atomic clocks for fundamental physics studies. This work represents a confluence of precision metrology, ultrafast strong field physics, nuclear physics, and fundamental physics.

Solid-state \\(^229\\)Th nuclear clocks are set to provide new opportunities for precision metrology and fundamental physics. Taking advantage of a nuclear transition's inherent low sensitivity to its environment, orders of magnitude more emitters can be hosted in a solid-state crystal compared to current optical lattice atomic clocks. Furthermore, solid-state systems needing only simple thermal control are key to the development of field-deployable compact clocks. In this work, we explore and characterize the frequency reproducibility of the \\(^229\\)Th:CaF\\(_2\\) nuclear clock transition, a key performance metric for all clocks. We measure the transition linewidth and center frequency as a function of the doping concentration, temperature, and time. We report the concentration-dependent inhomogeneous linewidth of the nuclear transition, limited by the intrinsic host crystal properties. We determine an optimal working temperature for the \\(^229\\)Th:CaF\\(_2\\) nuclear clock at 195(5) K where the first-order thermal sensitivity vanishes. This would enable in-situ temperature co-sensing using different quadrupole-split lines, reducing the temperature-induced systematic shift below the 10\\(^-18\\) fractional frequency uncertainty level. At 195 K, the reproducibility of the nuclear transition frequency is 280 Hz (fractionally \\(1.410^-13\\)) for two differently doped \\(^229\\)Th:CaF\\(_2\\) crystals over four months. These results form the foundation for understanding, controlling, and harnessing the coherent nuclear excitation of \\(^229\\)Th in solid-state hosts, and for their applications in constraining temporal variations of fundamental constants.

Quantum state-resolved spectroscopy of the low energy thorium-229 nuclear transition was recently achieved. The five allowed transitions within the electric quadrupole structure were measured to the kilohertz level in a calcium fluoride host crystal, opening many new areas of research using nuclear clocks. Central to the performance of solid-state clock operation is an understanding of systematic shifts such as the temperature dependence of the clock transitions. In this work, we measure the four strongest transitions of thorium-229 in the same crystal at three temperature values: 150 K, 229 K, and 293 K. We find shifts of the unsplit frequency and the electric quadrupole splittings, corresponding to decreases in the electron density, electric field gradient, and field gradient asymmetry at the nucleus as temperature increases. The \\(\\textit{m}\\) = \\(\\pm 5/2 \\rightarrow \\pm 3/2\\) line shifts only 62(6) kHz over the temperature range, i.e., approximately 0.4 kHz/K, representing a promising candidate for a future solid-state optical clock. Achieving 10\\(^{-18}\\) precision requires crystal temperature stability of 5\\(\\mu\\)K.

Quantum state-resolved spectroscopy of the low energy thorium-229 nuclear transition was recently achieved. The five allowed transitions within the electric quadrupole splitting structure were measured to the kilohertz level in a calcium fluoride host crystal, opening the field of nuclear-based optical clocks. Central to the performance of solid-state clock operation is an understanding of systematic shifts such as the temperature dependence of the clock transitions. In this work, we measure the four strongest transitions of thorium-229 in the same crystal at three temperature values: 150 K, 229 K, and 293 K. We find shifts of the unsplit frequency and the electric quadrupole splittings, corresponding to decreases in the electron density, electric field gradient, and field gradient asymmetry at the nucleus as temperature increases. The \\(m\\) = \\( 5/2 3/2\\) line shifts only 62(6) kHz over the temperature range, i.e., approximately 0.4 kHz/K, representing a promising candidate for a future solid-state optical clock. Achieving 10\\(^-18\\) precision requires crystal temperature stability of 5\\(\\)K.

Ultralight dark matter is expected to induce oscillations of nuclear parameters. These oscillations are characterized by extremely weak couplings or high suppression scales, with the Planck scale - the characteristic scale of quantum gravity - serving as a natural benchmark. Probing this phenomenon requires systems with exceptional sensitivity to shifts in nuclear energies. The uniquely low-energy nuclear isomeric transition in \\(^229\\)Th provides such sensitivity: it directly probes the nuclear interaction and, owing to a near cancellation between electromagnetic and nuclear contributions, its response to changes in nuclear structure is greatly amplified. We devise and perform a new type of ultrasensitive search for dark matter which uses the precision nuclear spectroscopy at JILA to set the strongest bounds in the mass range \\(10^-21\\, eV m_ DM 10^-19\\, eV\\). Our results probe effective interaction scales exceeding \\(10^6\\) times the Planck scale (the Mega-Planck scale) and establish the \\(^229\\)Th system as the leading probe of dark matter couplings to the nuclear sector.