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Schematic of an optical atomic clock. The laser oscillator frequency is stabilized to the atomic or nuclear resonance frequency and an optical clockwork is employed to produce a time signal.
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The low-energy, long-lived isomer in 229Th, first studied in the 1970s as an exotic feature in nuclear physics, continues to inspire a multidisciplinary community of physicists. It has stimulated innovative ideas and studies that expand the understanding of atomic and nuclear structure of heavy elements and of the interaction of nuclei with bound e...
Citations
... Driven by these compelling prospects [27][28][29], several methods have been employed to measure the energy of the Th-229 isomer transition [6][7][8][9][10][11][12][13][14][15][16], including indirect γ spectroscopy [9,12,13], internal-conversion electron spectroscopy [10,30], and vacuum ultraviolet (VUV) spectroscopy of radiative fluorescence light [14]. Very recently, laser spectroscopy has been finally achieved by two groups based on thorium-doped VUV-transparent crystals [15,16], and the transition wavelength is determined to be 148.4 ...
... This significant linewidth disparity of many orders of magnitude underscores the need for a narrow-line laser for efficient transition driving and precision spectroscopy. A highly coherent frequency comb in the VUV region has been demonstrated via high-order harmonic generation (HHG) from an in- E × 10 4 , cm −1 frared comb [32][33][34] and is being utilized in Th-229 isomer spectroscopy [28,[35][36][37]. Although the linewidth of the comb might be as narrow as kHz, a single comb line with a power of only 1 nW contributes to driving the isomer transition [28,37]. ...
... A highly coherent frequency comb in the VUV region has been demonstrated via high-order harmonic generation (HHG) from an in- E × 10 4 , cm −1 frared comb [32][33][34] and is being utilized in Th-229 isomer spectroscopy [28,[35][36][37]. Although the linewidth of the comb might be as narrow as kHz, a single comb line with a power of only 1 nW contributes to driving the isomer transition [28,37]. ...
We propose to generate continuous-wave vacuum ultraviolet (VUV) laser light at 148.4 nm using four-wave mixing in cadmium vapor for precision spectroscopy of the Th-229 isomer transition. Due to the large transition matrix elements of cadmium, the readily accessible wavelengths for the incident laser beams, and the high coherence of the four-wave mixing process, over 30 $\mu$W of VUV power can be generated with a narrow linewidth. This development paves the way for coherently driving the Th-229 isomer transition and developing the nuclear optical clock.
... These insights encompass the search for elusive dark matter [4] and the exploration of dark energy [5]. Amidst the potential optical clock candidates, 229 Th emerges as an exceptional luminary [6][7][8][9][10][11][12][13][14] due to its unique characteristics, including a nuclear transition energy of a few electron volts (eV), narrow linewidth, and insensitivity to external fields [15][16][17]. The development of a nuclear clock utilizing the 229 Th nucleus holds the potential to achieve accuracy levels on the order of 10 −19 [18,19]. ...
... The nuclear transition frequency of 229 Th has undergone continuous refinement through nuclear physical measurements. Initial measurements indicated a transition energy of 7.6(5) eV [20], evolving into the present measurements of 8.28 (17) eV [8], 8.30(92) eV [9], 8.338 (24) eV [11], and a somewhat inconsistent measurement 8.10 (17) eV [10]. Recently, two new results of 8.355 770(29) eV [12] and 8.355 733(2) stat (10) sys eV [13] were reported, and these measurements represent the most accurate to date. ...
... The nuclear transition frequency of 229 Th has undergone continuous refinement through nuclear physical measurements. Initial measurements indicated a transition energy of 7.6(5) eV [20], evolving into the present measurements of 8.28 (17) eV [8], 8.30(92) eV [9], 8.338 (24) eV [11], and a somewhat inconsistent measurement 8.10 (17) eV [10]. Recently, two new results of 8.355 770(29) eV [12] and 8.355 733(2) stat (10) sys eV [13] were reported, and these measurements represent the most accurate to date. ...
In this paper, we propose the 5f5/2→5f7/2 transition in Th3+229 as an ionic clock with accuracy of 10−18 level, which can be also used together with the nuclear clock transition for measuring variations of fine-structure constant with common-mode rejection of certain systematic effects. The many-body perturbation theory and Dirac-Fock plus core polarizability method are used to perform theoretical calculations. Calculation results show that several systematic frequency shifts can be suppressed to 10−18 level or even below. Assuming the accuracy of ionic clock frequency can be achieved in 10−18 level, Th3+229 potentially offers a precision of drift of fine-structure constant of α̇/α in 10−21−yr−1 level by measuring the frequency ratio of nuclear clock frequency and ionic clock frequency.
... Improving the accuracy of atomic standards is vital for research areas investigating the variability of fundamental constants [1,2], building frequency and time standards [3][4][5], testing new theories that advocate beyond the standard model of physics [1,6,7], investigating many-body dynamics [8], and more [9,10]. Trapped ions are one of the ideal choices for high-precision measurements, and clock standards [2,[11][12][13][14][15]. Ambient electromagnetic fields limit the accuracy of trapped-ion clocks [16,17] and their systematic estimation is necessary for a reliable frequency standard [18]. ...
The oscillating magnetic field produced by unbalanced currents in radio-frequency ion traps induces transition frequency shifts and sideband transitions that can be harmful to precision spectroscopy experiments. Here, we describe a methodology, based on two-photon spectroscopy, for determining both the strength and direction of rf-induced magnetic fields without modifying any DC magnetic bias field or changing any trap RF power. The technique is readily applicable to any trapped-ion experiment featuring narrow linewidth transitions.
... Its excitation energy of 8.4 eV places the nuclear transition in the vacuum-ultraviolet (VUV) spectral range and makes it accessible for experiments with tabletop laser systems and the tools of precision optical frequency metrology. A number of proposals have been put forward based on these exceptional properties (see [8,9] for recent reviews), including the concept of a nuclear optical clock of very high accuracy [10,11] and high sensitivity in tests of fundamental physics [12,13]. Reflecting the inherent robustness of nuclear transitions to external fields and chemical environment, even a solid-state version of an optical clock has been proposed, based on Th-229 doped into a VUV-transparent crystal, with a band gap that is larger than the isomer energy [14,15]. ...
... In conclusion, we have demonstrated the first laser excitation of the Th-229 low-energy nuclear transition, have reduced the uncertainty in the transition frequency by nearly 3 orders of magnitude, and have performed a precision measurement of the isomer lifetime. This opens the way toward nuclear laser spectroscopy of Th-229 in different host crystals and with trapped ions in different charge states, including the study of phenomena like electronic bridge processes [6,7,45], collective effects in nuclear scattering [46], and optical nuclear clocks with applications in tests of fundamental physics [13]. The development of dedicated VUV lasers with narrow linewidth will make it possible to access a new regime of resolution and accuracy in laser Mössbauer spectroscopy and to perform coherent control of a nuclear excitation [13]. ...
... This opens the way toward nuclear laser spectroscopy of Th-229 in different host crystals and with trapped ions in different charge states, including the study of phenomena like electronic bridge processes [6,7,45], collective effects in nuclear scattering [46], and optical nuclear clocks with applications in tests of fundamental physics [13]. The development of dedicated VUV lasers with narrow linewidth will make it possible to access a new regime of resolution and accuracy in laser Mössbauer spectroscopy and to perform coherent control of a nuclear excitation [13]. ...
The 8.4 eV nuclear isomer state in Th-229 is resonantly excited in Th-doped CaF2 crystals using a tabletop tunable laser system. A resonance fluorescence signal is observed in two crystals with different Th-229 dopant concentrations, while it is absent in a control experiment using Th-232. The nuclear resonance for the Th4+ ions in Th:CaF2 is measured at the wavelength 148.3821(5) nm, frequency 2020.409(7) THz, and the fluorescence lifetime in the crystal is 630(15) s, corresponding to an isomer half-life of 1740(50) s for a nucleus isolated in vacuum. These results pave the way toward Th-229 nuclear laser spectroscopy and realizing optical nuclear clocks.
... Extending the operational wavelength range of femtosecond frequency combs (operating at around 100 MHz repetition rates) into the vacuum ultraviolet (VUV) enables high-precision spectroscopy of new important atomic and molecular transitions for testing quantum electrodynamics, photoemission spectroscopy [1,2] or looking for new atomic or nuclear clock transitions [3]. There is a special interest to develop VUV frequency combs in the 150 nm range for high precision spectroscopy of the 229m Th isomer nuclear transition [4][5][6][7][8][9][10] and the atomic transitions of Th ions in a similar wavelength range [11]. This spectral range can be reached by the 5th harmonic of Ti:sapphire or the 7th harmonic of Yb-doped lasers. ...
We report the realization of an intra-oscillator high harmonic source based on a Kerr lens mode locked Ti:sapphire laser running at 80 MHz repetition rate. A nonlinear medium consisting of an AlN nanofilm on a thin sapphire substrate is placed inside the oscillator cavity. The harmonics are generated, in reflection geometry, on the AlN nanofilm, directing the harmonic beam out of the cavity. Exploiting the benefits of this approach, a compact size, tunable, high repetition rate and coherent vacuum ultraviolet light source with a spectrum up to the 7th harmonic has been achieved. In particular, the powerful 5th harmonic covering the 145-163 nm range aims to be an attractive tunable light source for spectroscopical applications.
... Ultrafast-laser-driven high harmonic generation (HHG) [1] has become an unusually polyvalent tool for modern physics. The compact laser setups have largely facilitated lab-scale experiments relying on coherent XUV and soft X-ray radiation [2][3][4][5][6] and provided access to the whole domain of attosecond physics [7][8][9][10]. This tremendous progress has been largely dependent on state-of-the-art lasers. ...
Resonant enhancement inside an optical cavity has been a wide-spread approach to increase efficiency of nonlinear optical conversion processes while reducing the demands on the driving laser power. This concept has been particularly important for high harmonic generation XUV sources, where passive femtosecond enhancement cavities allowed significant increase in repetition rates required for applications in photoelectron spectroscopy, XUV frequency comb spectroscopy, including the recent endeavor of thorium nuclear clock development. In addition to passive cavities, it has been shown that comparable driving conditions can be achieved inside mode-locked thin-disk laser oscillators, offering a simplified single-stage alternative. This approach is less sensitive to losses thanks to the presence of gain inside the cavity and should thus allow higher conversion efficiencies through tolerating higher intensity in the gas target. Here, we show that the intra-oscillator approach can indeed surpass the much more mature technology of passive enhancement cavities in terms of XUV flux, even reaching comparable values to single-pass sources based on chirped-pulse fiber amplifier lasers. Our system operates at 17 MHz repetition rate generating photon energies between 60 eV and 100 eV. Importantly, this covers the highly attractive wavelength for the silicon industry of 13.5 nm at which our source delivers 60 nW of outcoupled average power per harmonic order.
... The structure of the nuclear levels is governed by both Coulomb and nuclear forces [6]. This allows probing of these forces via nuclear spectroscopy, paving the way for new fundamental research-for example, the search for dark matter, or potential drifts in the fine-structure constant [7,8]. 229 Th is required to be in a 3+ or higher charge state to suppress the nonradiative decay [9]. ...
The 8−eV first nuclear excited state in Th229 is a candidate for implementing a nuclear clock. Doping Th229 into ionic crystals such as CaF2 is expected to suppress nonradiative decay, enabling nuclear spectroscopy and the realization of a solid-state optical clock. Yet, the inherent radioactivity of Th229 prohibits the growth of high-quality single crystals with high Th229 concentration; radiolysis causes fluoride loss, increasing absorption at 8eV. These radioactively doped crystals are thus a unique material for which a deeper analysis of the physical effects of radioactivity on growth, crystal structure, and electronic properties is presented. Following the analysis, we overcome the increase in absorption at 8eV by annealing Th229-doped CaF2 at 1250∘C in CF4. This technique allows to adjust the fluoride content without crystal melting, preserving its single-crystal structure. Superionic state annealing ensures rapid fluoride distribution, creating fully transparent and radiation-hard crystals. This approach enables control over the charge state of dopants, which can be used in deep-UV optics, laser crystals, scintillators, and nuclear clocks.
... Ultrafast-laser-driven high harmonic generation (HHG) [1] has become an unusually polyvalent tool for modern physics. The compact laser setups have largely facilitated lab-scale experiments relying on coherent XUV and soft X-ray radiation [2][3][4][5][6] and provided access to the whole domain of attosecond physics [7][8][9][10]. This tremendous progress has been largely dependent on state-of-the-art lasers. ...
Resonant enhancement inside an optical cavity has been a wide-spread approach to increase efficiency of nonlinear optical conversion processes while reducing the demands on the driving laser power. This concept has been particularly important for high harmonic generation XUV sources, where passive femtosecond enhancement cavities allowed significant increase in repetition rates required for applications in photoelectron spectroscopy, XUV frequency comb spectroscopy, including the recent endeavor of thorium nuclear clock development. In addition to passive cavities, it has been shown that comparable driving conditions can be achieved inside mode-locked thin-disk laser oscillators, offering a simplified single-stage alternative. This approach is less sensitive to losses thanks to the presence of gain inside the cavity and should thus allow higher conversion efficiencies through tolerating higher intensity in the gas target. Here, we show that the intra-oscillator approach can indeed surpass the much more mature technology of passive enhancement cavities in terms of XUV flux, even reaching comparable values to single-pass sources based on chirped-pulse fiber amplifier lasers. Our system operates at 17 MHz repetition rate generating photon energies between 60 eV and 100 eV. Importantly, this covers the highly attractive wavelength for the silicon industry of 13.5 nm at which our source delivers 60 nW of outcoupled average power per harmonic order.
... Ultrafast-laser-driven high harmonic generation (HHG) [1] has become an unusually polyvalent tool for modern physics. The compact laser setups have largely facilitated lab-scale experiments relying on coherent XUV and soft X-ray radiation [2][3][4][5][6] and provided access to the whole domain of attosecond physics [7][8][9][10]. This tremendous progress has been largely dependent on state-of-the-art lasers. ...
Resonant enhancement inside an optical cavity has been a wide-spread approach to increase efficiency of nonlinear optical conversion processes while reducing the demands on the driving laser power. This concept has been particularly important for high harmonic generation XUV sources, where passive femtosecond enhancement cavities allowed significant increase in repetition rates required for applications in photoelectron spectroscopy, XUV frequency comb spectroscopy, including the recent endeavor of thorium nuclear clock development. In addition to passive cavities, it has been shown that comparable driving conditions can be achieved inside mode-locked thin-disk laser oscillators, offering a simplified single-stage alternative. This approach is less sensitive to losses thanks to the presence of gain inside the cavity and should thus allow higher conversion efficiencies through tolerating higher intensity in the gas target. Here, we show that the intra-oscillator approach can indeed surpass the much more mature technology of passive enhancement cavities in terms of XUV flux, even reaching comparable values to single-pass sources based on chirped-pulse fiber amplifier lasers. Our system operates at 17 MHz repetition rate generating photon energies between 60 eV and 100 eV. Importantly, this covers the highly attractive wavelength for the silicon industry of 13.5 nm at which our source delivers 60 nW of outcoupled average power per harmonic order.
... Early discussions about the potential use of 229m Th for frequency metrology date back to 1996, when the "development of a high-stability nuclear source of light for metrology" was discussed by Tkalya et al. as an Fig. 2 Illustration of our present knowledge of the nuclear level properties (excitation energy, lifetimes, relative linewidth, nuclear moments (magetic dipole, electric quadrupole) of the almost-degenerate ground-state doublet in 229 Th (characterized by its spin-parity and Nilsson quantum number assignments). Updated from [23] application for 229m Th [26]. The first detailed concept and analysis of a "nuclear clock" was worked out in 2003 by Peik and Tamm [27], addressing both an ion-based as well as a solid-state-based approach of its realization. ...
... As outlined in [23], with different sensitivities to specific effects of new physics beyond the Standard Model, meaningful comparisons can be made between a clock system of high sensitivity and a stable reference of low sensitivity. The 229 Th nuclear clock would be an attractive addition to the already presently used ensemble of atomic clocks used in fundamental physics tests, because it will provide a strongly enhanced sensitivity for variations of fundamental constants. ...
Today’s most precise timekeeping is based on optical atomic clocks. However, those could potentially be outperformed by a nuclear clock, based on a nuclear transition instead of an atomic shell transition. Such a nuclear clock promises intriguing applications in applied as well as fundamental physics, ranging from geodesy and seismology to the investigation of possible time variations of fundamental constants and the search for dark matter. Only one nuclear state is known so far that could drive a nuclear clock: the “Thorium Isomer” $$^{229m}$$ 229 m Th, i.e., the isomeric first excited state of $$^{229}$$ 229 Th, representing the lowest nuclear excitation so far reported in the landscape of nuclear isotopes. Indirectly conjectured to exist already in 1976, decades of experimental efforts were dedicated to unambiguously identify this elusive nuclear state and to characterize its properties. However, for 40 years, these efforts remained inconclusive. The turning point was marked by the first direct detection of $$^{229m}$$ 229 m Th via its internal conversion decay branch in 2016. Since then, remarkable progress could be achieved in characterizing the properties and decay parameters. The half-life of the neutral isomer was determined, the hyperfine structure was measured via collinear laser spectroscopy, providing information on nuclear moments and the nuclear charge radius and also the excitation energy of the isomer could be directly determined with different techniques. In a recent experiment at CERN’s ISOLDE facility, the long-sought radiative decay of the Thorium isomer could be observed for the first time via implantation of ( $$\beta$$ β -decaying) $$^{229}$$ 229 Ac into a vacuum-ultraviolet (VUV) transparent crystal and subsequent fluorescence detection in a VUV spectrometer. Thus, the excitation energy of $$^{229m}$$ 229 m Th could be determined with unprecedented precision to 8.338(24) eV, corresponding to a wavelength of 148.71(42) nm. These achievements, together with ongoing laser developments for the required VUV wavelength, open the door toward a laser-driven control of the isomeric transition and thus to the development of an ultra-precise nuclear frequency standard.
