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Owing to its low excitation energy and long radiative lifetime, the first excited isomeric state of thorium-229, 229m Th, can be optically controlled by a laser 1 , 2 and is an ideal candidate for the creation of a nuclear optical clock 3 , which is expected to complement and outperform current electronic-shell-based atomic clocks 4 . A nuclear clock will have various applications—such as in relativistic geodesy 5 , dark matter research 6 and the observation of potential temporal variations of fundamental constants 7 —but its development has so far been impeded by the imprecise knowledge of the energy of 229m Th. Here we report a direct measurement of the transition energy of this isomeric state to the ground state with an uncertainty of 0.17 electronvolts (one standard deviation) using spectroscopy of the internal conversion electrons emitted in flight during the decay of neutral 229m Th atoms. The energy of the transition between the ground state and the first excited state corresponds to a wavelength of 149.7 ± 3.1 nanometres, which is accessible by laser spectroscopy through high-harmonic generation. Our method combines nuclear and atomic physics measurements to advance precision metrology, and our findings are expected to facilitate the application of high-resolution laser spectroscopy on nuclei and to enable the development of a nuclear optical clock of unprecedented accuracy. The transition energy of the first excited state of 229 Th to the ground state is determined through the measurement of internal conversion electrons to correspond to a wavelength of 149.7 ± 3.1 nanometres.

We study thorium-doped CaF 2 crystals as a possible platform for optical spectroscopy of the 229 Th nuclear isomer transition. We anticipate two major sources of background signal that might cover the nuclear spectroscopy signal: VUV-photoluminescence, caused by the probe light and radioluminescence, caused by the radioactive decay of 229 Th and its daughters. We find a rich photoluminescence spectrum at wavelengths above 260 nm and radioluminescence emission above 220 nm. This is very promising, as fluorescence originating from the isomer transition, predicted at a wavelength shorter than 200 nm, could be filtered spectrally from the crystal luminescence. Furthermore, we investigate the temperature-dependent decay time of the luminescence, as well as thermoluminescence properties. Our findings allow for an immediate optimization of spectroscopy protocols for both the initial search for the nuclear transition using synchrotron radiation, as well as future optical clock operation with narrow-linewidth lasers.

We have grown 232 Th:CaF 2 and 229 Th:CaF 2 single crystals for investigations on the VUV laser-accessible first nuclear excited state of 229 Th, with the aim of building a solid-state nuclear clock. To reach high doping concentrations despite the extreme scarcity (and radioactivity) of 229 Th, we have scaled down the crystal volume by a factor 100 compared to established commercial or scientific growth processes. We use the vertical gradient freeze method on 3.2 mm diameter seed single crystals with a 2 mm drilled pocket, filled with a co-precipitated CaF 2 :ThF 4 :PbF 2 powder in order to grow single crystals. Concentrations of 4 · 10 19  cm - 3 have been realized with 232 Th with good (> 10%) VUV transmission. However, the intrinsic radioactivity of 229 Th drives radio-induced dissociation during growth and radiation damage after solidification. Both lead to a degradation of VUV transmission, currently limiting the 229 Th concentration to < 5 × 10 17  cm - 3 .

The 229Th nucleus has an isomeric state at an energy of about 8 eV above the ground state, several orders of magnitude lower than typical nuclear excitation energies. This has inspired the development of a field of low-energy nuclear physics in which nuclear transition rates are influenced by the electron shell. The low energy makes the 229Th isomer accessible to resonant laser excitation. Observed in laser-cooled trapped thorium ions or with thorium dopant ions in a transparent solid, the nuclear resonance may serve as the reference for an optical clock of very high accuracy. Precision frequency comparisons between such a nuclear clock and conventional atomic clocks will provide sensitivity to the effects of hypothetical new physics beyond the standard model. Although laser excitation of 229Th remains an unsolved challenge, recent experiments have provided essential information on the transition energy and relevant nuclear properties, advancing the field.A clock using the excitation of a low-energy excited state in the 229Th nucleus promises high accuracy and sensitivity to new physics. The recently measured properties of this nucleus will lead to nuclear laser spectroscopy with trapped Th ions and Th-doped crystals.

The radioisotope thorium-229 ( 229 Th) is renowned for its extraordinarily low-energy, long-lived nuclear first-excited state. This isomeric state can be excited by vacuum ultraviolet (VUV) lasers and 229 Th has been proposed as a reference transition for ultra-precise nuclear clocks. To assess the feasibility and performance of the nuclear clock concept, time-controlled excitation and depopulation of the 229 Th isomer are imperative. Here we report the population of the 229 Th isomeric state through resonant X-ray pumping and detection of the radiative decay in a VUV transparent 229 Th-doped CaF 2 crystal. The decay half-life is measured to 447(25) s, with a transition wavelength of 148.18(42) nm and a radiative decay fraction consistent with unity. Furthermore, we report a new “X-ray quenching” effect which allows to de-populate the isomer on demand and effectively reduce the half-life. Such controlled quenching can be used to significantly speed up the interrogation cycle in future nuclear clock schemes. Thorium-229 has the extraordinarily low-energy nuclear state and therefore has potential in atomic clocks. Here, the authors measured the radiative branching fraction of the “clock state”, which is consistent with unity in crystals, and found that this state can be de-populated by X-ray beam irradiation.

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.

Precision laser spectroscopy of the 229-thorium nuclear isomer transition in a solid-state environment would represent a significant milestone in the field of metrology, opening the door to the realization of a nuclear clock. Working toward this goal, experimental methods require knowledge of various properties of a large band-gap material, such as calcium fluoride doped with specific isotopes of the heavy elements thorium, actinium, cerium, neptunium, and uranium. By accurately determining the atomic structure of potential charge compensation schemes by using a generalized gradient approximation within the ab-initio framework of density functional theory, calculations of electric field gradients on the dopants become accessible, which cause a quadrupole splitting of the nuclear-level structure that can be probed experimentally. Band gaps and absorption coefficients in the range of the 229-thorium nuclear transition are estimated by using the G0W0 method and by solving the Bethe–Salpeter equation.

The generation of high-order harmonics in solid crystals has received considerable attention recently. Using a driver laser with 0.8 µm wavelength and 28 fs ultrashort pulses, we present experimental results, accompanied with theoretical considerations, suggesting that the actual sources of the harmonics are nanometer-sized localized and transient electronic states on the surface of the materials when the laser intensity is in the non-perturbative regime. Adaptation of the bond model of the harmonic generation into the non-perturbative regime and including the quantum features of the process provide a localized excitation approach that correctly describes the measured polarization dependence of the harmonic signal, reflecting the microscopic surface structure and symmetries of the examined materials.