Explore the vast range of titles available.

The radionuclide thorium-229 features an isomer with an exceptionally low excitation energy that enables direct laser manipulation of nuclear states. It constitutes one of the leading candidates for use in next-generation optical clocks 1 – 3 . This nuclear clock will be a unique tool for precise tests of fundamental physics 4 – 9 . Whereas indirect experimental evidence for the existence of such an extraordinary nuclear state is substantially older 10 , the proof of existence has been delivered only recently by observing the isomer’s electron conversion decay 11 . The isomer’s excitation energy, nuclear spin and electromagnetic moments, the electron conversion lifetime and a refined energy of the isomer have been measured 12 – 16 . In spite of recent progress, the isomer’s radiative decay, a key ingredient for the development of a nuclear clock, remained unobserved. Here, we report the detection of the radiative decay of this low-energy isomer in thorium-229 ( 229m Th). By performing vacuum-ultraviolet spectroscopy of 229m Th incorporated into large-bandgap CaF 2 and MgF 2 crystals at the ISOLDE facility at CERN, photons of 8.338(24) eV are measured, in agreement with recent measurements 14 – 16 and the uncertainty is decreased by a factor of seven. The half-life of 229m Th embedded in MgF 2 is determined to be 670(102) s. The observation of the radiative decay in a large-bandgap crystal has important consequences for the design of a future nuclear clock and the improved uncertainty of the energy eases the search for direct laser excitation of the atomic nucleus. The authors report on the radiative decay of a low-energy isomer in thorium-229 ( 229m Th), which has consequences for the design of a future nuclear clock and eases the search for direct laser excitation of the atomic nucleus.

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.

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.

The radioisotope thorium-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 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 Th isomer are imperative. Here we report the population of the Th isomeric state through resonant X-ray pumping and detection of the radiative decay in a VUV transparent Th-doped CaF 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.