Readout scheme for the solid-state 229Th nuclear clock

The low-energy excited state of the Thorium-229 (229Th) nucleus has fascinated researchers for decades. With the excitation energy within the ultraviolet radiation range, it is the only known nuclear state accessible to laser manipulation. This system opens up many novel applications, ranging from tests on temporal variations of the fundamental interaction constants to technological implementations as an ultra-precise "optical nuclear clock". While the exact value of the first excitation energy of the 229Th nucleus is still unknown, significant progress has been made in narrowing down the uncertainty interval. All recent results place the energy between 7.8 eV to 8.3 eV. This is within the transmission region of large-band-gap VUV materials such as fluoride crystals. Hence, it becomes possible to embed 229Th inside a solid-state matrix and address a large number of nuclei optically. For the atomic clock operation, it is not only necessary to be able to manipulate the state of the atom, but also to have the ability to read the state. For the 229Th nucleus, only two energy levels are accessible for optical manipulation, the standard readout schemes used in atomic clocks will not work. This project aims to develop a readout scheme for a solid-state nuclear clock based on nuclear quadrupole resonance spectroscopy (NQRS). The interaction of the nuclear quadrupole moment with the electric field gradient of the crystal causes the nuclear states` splitting. Because the excited nuclear state has a different nuclear spin than the ground state, the splittings are unique for each nuclear state. NQRS can be used for non-destructive readout of the nuclear state during clock operation. Moreover, the NQRS will provide valuable information about the microscopic structure of 229Th atoms doped into the host fluoride crystal`s crystal lattice.

The main goal of this project was to develop a new type of "solid-state nuclear clock" using a uniquely low-energy nuclear transition in Thorium-229 (229Th). Such a clock would surpass today's most precise timekeeping devices by relying on a nuclear transition that is much less sensitive to external disturbances than atomic transitions. Achieving this goal required a series of steps, from growing crystals containing 229Th to designing a novel spectrometer for reading out nuclear signals. To embed the isotopes, we grew calcium fluoride (CaF) crystals doped with 229Th, as well as Uranium-235 (235U) and Neptunium-237 (237Np) for comparison and calibration. This process demanded precise temperature controls to ensure that the resulting crystals were transparent and uniformly doped. We achieved doping levels of approximately 510^18 atoms/cm for Thorium and 10^19 atoms/cm for Uranium and Neptunium. Although the uranium-doped crystals showed significant paramagnetism-making the nuclear signal impossible to detect under our conditions-the experiment helped us understand how doping concentration and impurities affect our measurements. Parallel to the crystal growth efforts, we performed theoretical calculations (using density functional theory) to predict the nuclear quadrupole resonance (NQR) frequency of 229Th in CaF. We then conducted Mössbauer spectroscopy on the neptunium-doped samples, revealing inhomogeneous broadening of spectral lines. This pointed to the inhomogeneous broadening probably caused by too high concentration of dopants. Understanding and eliminating this effect is vital for clock applications. We also partnered with researchers at JILA to use vacuum ultraviolet (VUV) laser comb spectroscopy, resolving the 229Th hyperfine splitting in a Calcium Fluoride environment. Based on these measurements, our lab in Vienna performed NQR scans around 234 MHz at low temperature (15 K), where the resonance is expected. While we identified a few possible signals, we are limited by ring-down artifacts from our detection circuit, which look similar to the resonance signal. Ongoing work aims to refine the detection scheme by increasing measurement repetitions, adjusting the frequency range, and decreasing the crystal doping to lengthen the nuclear relaxation times. A key technological milestone was the development of a super-regenerative receiver (SRR) NQR spectrometer. This compact, power-efficient design cycles through oscillation and quench phases, amplifying weak rf nuclear signals. In the future, a solid-state nuclear clock using the SRR-based approach could operate-potentially enabling high precision in fields such as satellite navigation, telecommunications, and fundamental physics experiments.