Continuous-wave laser absorption spectroscopy of the Thorium-229 nucleus
Vienna Center for Quantum Science and Technology, Atominstitut, TU Wien（维也纳量子科学与技术中心，原子研究所，维也纳工业大学）
I. Morawetz, T. Riebner, L. Toscani De Col, F. Schneider, N. Sempelmann, F. Schaden, M. Bartokos, G. A. Kazakov, S. Lahs, K. Beeks, B. Gerstenecker, A. Grüneis,… 展开作者
I. Morawetz, T. Riebner, L. Toscani De Col, F. Schneider, N. Sempelmann, F. Schaden, M. Bartokos, G. A. Kazakov, S. Lahs, K. Beeks, B. Gerstenecker, A. Grüneis, M. Pimon, T. Schumm, V. Lal, G. Zitzer, V. Petrov, J. Tiedau, M. V. Okhapkin, E. Peik
A low-energy nuclear transition in the isotope thorium-229 has been excited in thorium-doped crystals with laser light. This opens the perspective towards a highly stable and robust solid-state optical nuclear clock. The required laser radiation at 148 nm wavelength has so far been produced using pulsed laser systems where only a small fraction of the incident photons has been resonant with the narrow nuclear transition. Here we show that the nuclear resonance can be excited with a continuous-wave narrow-bandwidth laser source with a power of less than 1 nW, and that the resonance signal can be detected in absorption rather than in fluorescence. This eliminates the slow nuclear fluorescence decay from the detection process and offers a considerable advantage for clock operation through fast signal acquisition. The VUV laser source is based on three sequential frequency doublings, starting from a diode laser at 1187 nm that is well suited for linewidth narrowing and for frequency comparisons with optical atomic clocks. We use absorption spectroscopy for the quantitative characterization of two different Th-centers in calcium fluoride crystal and measure the isomeric shift between them. One of the centers shows a very small static electric crystal field gradient 0.1 V/$Å^2$, to be compared to gradients in the range of 100 V/$Å^2$ observed earlier. This indicates a center with high symmetry of the ions surrounding the Th nucleus, promising nuclear resonance lines that are nearly independent of the lattice spacing.
A possibility of the two-photon excitation of an isomeric state in a nucleus of thorium-229 has been discussed. The fluorescence intensity of the excitation is demonstrated to be identical for the irradiation of nuclei with either monochromatic light or polychromatic radiation consisting of a sequence of short light pulses of the same intensity. The two-photon excitation of Th^{3+} ion in an electromagnetic trap with a focused laser beam with a wavelength of about 320 nm and power of 100 mW can lead to the absorption saturation, at which the fluorescence emission with the frequency of the tran…
Compact and robust ion traps for thorium are enabling technology for the next generation of atomic clocks based on a low-energy isomeric transition in the thorium-229 nucleus. We aim at a laser ablation loading of single triply ionized thorium in a radio-frequency electromagnetic linear Paul trap. Detection of ions is based on a modified mass spectrometer and a channeltron with single-ion sensitivity. In this study, we successfully created and detected 232Th+ and 232Th2+ ions from plasma plumes, studied their yield evolution, and compared the loading to a quadrupole ion trap with Yb. We explor…
The 7.8 eV nuclear isomer transition in 229 Thorium has been suggested as an etalon transition in a new type of optical frequency standard. Here we discuss the construction of a "solid-state nuclear clock" from Thorium nuclei implanted into single crystals transparent in the vacuum ultraviolet range. We investigate crystal-induced line shifts and broadening effects for the specific system of Calcium fluoride. At liquid Nitrogen temperatures, the clock performance will be limited by decoherence due to magnetic coupling of the Thorium nucleus to neighboring nuclear moments, ruling out the common…
The thorium nucleus with mass number $A=229$ has attracted much interest because its extremely low lying first excited isomeric state at about $8$eV opens the possibility for the development of a nuclear clock. However, neither the exact energy of this nuclear isomer nor properties, such as nuclear magnetic dipole and electric quadrupole moment are known to a high precision so far. The latter can be determined by investigating the hyperfine structure of thorium atoms or ions. Due to its electronic structure and the long lifetime of the nuclear isomeric state, Th$^{2+}$ is especially suitable f…
Investigation into thorium sulfate as a spinless crystal for a high-performance solid-state $^{229}Th$ nuclear clock
University of Manchester（曼彻斯特大学）；University of California, Los Angeles（加州大学洛杉矶分校）；Johns Hopkins University（约翰斯·霍普金斯大学）；University of Nevada, Reno（内华达大学里诺分校）；Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室）
Defect assignment of the clock site in $^{229}\text{Th:CaF}_2$
Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室）；University of Manchester（曼彻斯特大学）；University of California, Los Angeles（加州大学洛杉矶分校）；University of Nevada, Reno（内华达大学里诺分校）；Challenge Institute for Quantum Computation（量子计算挑战研究所）；Center for Quantum Science and Engineering（量子科学与工程中心）
A thorium-229 optical nuclear clock with feedback loop
Vienna Center for Quantum Science and Technology, Atominstitut, TU Wien（维也纳量子科学与技术中心，原子研究所，维也纳工业大学）；Bundesamt für Eich- und Vermessungswesen (BEV)（奥地利联邦计量与测量局）；Wolfgang Pauli Institut（沃尔夫冈·泡利研究所）；Institut für Erdmessung, Leibniz Universität Hannover（汉诺威莱布尼茨大学大地测量研究所）；Institute of Scientific Instruments of the CAS, v.v.i.（中国科学院科学仪器研究所）；Physikalisch-Technische Bundesanstalt (PTB)（德国物理技术联邦研究院）；Max-Born-Institute for Nonlinear Optics and Ultrafast Spectroscopy（马克斯·玻恩非线性光学和超快光谱学研究所）
$^{229}$Th is the only nucleus currently under investigation for the development of a nuclear optical clock (NOC) of ultra-high accuracy. The insufficient knowledge of the first nuclear excitation energy of $^{229}$Th has so far hindered direct nuclear laser spectroscopy of thorium ions and thus the development of a NOC. Here, a nuclear laser excitation scheme is detailed, which makes use of thorium atoms instead of ions. This concept, besides potentially leading to the first nuclear laser spectroscopy, would determine the isomeric energy to 40 $μ$eV resolution, corresponding to 10 GHz, which …
A possibility of amplification of the 7.6 eV $γ$-radiation by the stimulated $γ$-emission of the ensemble of the $^{229m}$Th isomeric nuclei in a host dielectric crystal is proved theoretically. This amplification is a result of: 1) the excitation of a large number of the $^{229m}$Th isomers by laser radiation; 2) the creation of the inverse population of nuclear levels in a cooled sample owing to the interaction of thorium nuclei with the crystal electric field or with an external magnetic field; 3) the emissions/absorption of the optical photons by thorium nuclei in the crystal without recoi…
We investigate the process of nuclear excitation via a two-photon electron transition (NETP) for the case of the doubly charged thorium. The theory of the NETP process has been devised originally for heavy helium-like ions. In this work, we study this process in the nuclear clock isotope $^{229}$Th in the $2+$ charge state. For this purpose, we employ a combination of configuration interaction and many-body perturbation theory to calculate the probability of NETP in resonance approximation. The experimental scenario we propose for the excitation of the low lying isomeric state in $^{229}$Th is…
The 7.6 eV electromagnetic transition between the nearly degenerate ground state and first excited state in the Th-229 nucleus may be very sensitive to potential changes in the fine-structure constant, $α= e^2/\hbar c$. However, the sensitivity is not known, and nuclear calculations are currently unable to determine it. We propose measurements of the differences of atomic transition frequencies between thorium atoms (or ions) with the nucleus in the ground state and in the first excited (isomeric) state. This will enable extraction of the change in nuclear charge radius and electric quadrupole…
The process of internal conversion from excited electronic states is investigated theoretically for the case of the vacuum-ultraviolet nuclear transition of $^{229}{\mathrm Th}$. Due to the very low transition energy, the $^{229}{\mathrm Th}$ nucleus offers the unique possibility to open the otherwise forbidden internal conversion nuclear decay channel for thorium ions via optical laser excitation of the electronic shell. We show that this feature can be exploited to investigate the isomeric state properties via observation of internal conversion from excited electronic configurations of ${\ma…
Excitation of the isomeric nuclear state 229m Th in the process of thorium atom irradiation by two-color femtosecond pulse of Ti: Sa laser at the fundamental wavelength and second harmonic is analyzed. It is shown that the rate of isomeric state excitation can be enhanced significantly with respect to other nucleus excitation processes in laser plasma or by an external coherent source at the resonance wavelength. This enhancement is due to the process of nonlinear laser nuclear excitation based on the following. First, the atomic current associated with the motion of valence electrons in stron…
Intense laser technologies generate light with unprecedented and growing intensities. The possibility emerges that a nucleus responds nonlinearly to an intense light field, pointing to a yet little explored research area of nuclear nonlinear optics. We consider two-photon and three-photon absorption (with subsequent disintegration) processes of the deuteron, the simplest and the most fundamental nontrivial nucleus, as prototypes of nuclear nonlinear optical effects. Quantitative calculations are performed on these processes and novel observable effects are predicted.
A solid-state nuclear clock based on VUV absorption spectroscopy of $^{229}$Th
Hefei National Laboratory, University of Science and Technology of China（中国科学技术大学合肥国家实验室）；School of Physical Sciences, University of Science and Technology of China（中国科学技术大学物理学院）
The nucleus of 229Thorium presents a unique isomer state of very low energy and long lifetime, current estimates are around 7.8 eV and seconds to hours respectively. This nuclear transitions therefore is a promising candidate for a novel type of frequency standard and several groups worldwide have set out to investigate this system. Our aim is to construct a "solid state nuclear clock", i.e. a frequency standard where Thorium ions are implanted into Calciumfluoride crystals transparent in vacuum ultraviolet range. As a first step towards an accurate determination of the exact energy and lifeti…
We present a review of recent works devoted to the variation of the fine structure constant alpha, strong interaction and fundamental masses. There are some hints for the variation in quasar absorption spectra, Big Bang nucleosynthesis, and Oklo natural nuclear reactor data. A very promising method to search for the variation of the fundamental constants consists in comparison of different atomic clocks. Huge enhancement of the variation effects happens in transition between accidentally degenerate atomic and molecular energy levels. A new idea is to build a ``nuclear'' clock based on the ultr…
The generation of muon-antimuon pairs is calculated in the collision of an ultrarelativistic bare ion with an intense x-ray laser beam. The reaction proceeds nonlinearly via absorption of two laser photons. By systematic study throughout the nuclear chart we show that the interplay between the nuclear charge and size, along with the possibility of nuclear excitation leads to saturation of the total production rates for high-Z ions, in contrast to the usual Z^2 scaling for pointlike projectiles. The process is experimentally accessible by combining present-day ion accelerators with near-future …
