Solid-State Nuclear Clocks Move Closer with New Nanophotonic Designs

A new nanophotonic platform creates compact, solid-state nuclear clocks utilising the $^{229}$Th nuclear isomer, a pursuit that has remained challenging despite recent observations of laser-induced excitation. Sandro Kraemer and colleagues from Max Planck Institute for the Science of Light, Keio University, Ghent University, Institute for Nuclear and Radiation Physics, Johannes Gutenberg University, and 9 other institutions, demonstrate a platform integrating thorium nuclei with high-$Q$ fluoride photonic resonators to sharply enhance nuclear excitation rates and enable practical optical interrogation. The research outlines a technological roadmap encompassing resonator fabrication, thorium implantation, laser integration, and on-chip detection, representing a key step towards scalable and compact nuclear frequency standards by using advances in materials integration and nanophotonics.

Thorium-229 nuclear clock realised within a fluoride whispering-gallery-mode resonator

Accuracy potentially approaching $10^{-19}$ is now achieved with an all-solid-state nuclear clock, exceeding the precision of previous gas-based methods. This breakthrough relies on embedding thorium-229 nuclei within high-$Q$ fluoride photonic resonators. These structures trap light to amplify the signal from the nucleus, a technique previously impractical due to signal weakness. The $^{229}$Th isotope possesses a unique low-energy nuclear transition at a frequency of approximately 149kHz, making it an ideal candidate for a nuclear clock. However, directly interrogating this transition requires extremely weak signals, necessitating innovative approaches to enhance light-matter interaction. Confining light and enhancing nuclear excitation rates enables compact, scalable frequency standards, vital for applications ranging from fundamental physics, such as tests of fundamental symmetries, to advanced navigation systems requiring highly precise timing. The development of such standards also has implications for geophysics, allowing for improved measurements of gravitational potential and Earth’s rotation.

As a proof of concept, a fluoride whispering-gallery-mode resonator was successfully implanted with thorium-229, validating material compatibility and opening new avenues for solid-state timekeeping. Simulations utilised a crystalline fluoride whispering-gallery-mode resonator, implanted with thorium at a fluence of $10^{13}$ cm$^{-2}$ and doping concentrations up to 2 × $10^{17}$ cm$^{-3}$ to enhance the signal. This doping level represents a careful balance between maximising the number of thorium nuclei available for excitation and minimising the detrimental effects of crystal damage caused by the implantation process. Resonant field build-up within the cavity substantially enhances nuclear excitation rates and enables optical interrogation at practical laser intensities, suggesting this approach overcomes limitations of previous methods. The enhancement arises from the increased interaction time between photons and the thorium nuclei, effectively amplifying the weak nuclear transition signal. This is crucial for achieving the necessary signal-to-noise ratio for accurate frequency measurements.

Fluoride materials were chosen for their broad transparency into the vacuum-ultraviolet, low scattering losses, and high optical damage thresholds, allowing quality factors exceeding $10^{8}$ in the visible spectrum. The broad transparency is particularly important as the excitation and detection schemes may eventually require wavelengths in the vacuum-ultraviolet range. Sustained oscillation or long-term stability of the nuclear transition is not yet achieved, representing a key hurdle before a fully functional device can be realised. Achieving sustained oscillation requires careful control of the laser frequency and power, as well as mitigation of environmental noise sources. This demonstration of a solid-state nuclear clock platform establishes a pathway beyond gas-based atomic clocks, offering potential for miniaturisation and wider deployment. Successfully implanting thorium into these resonators validates material compatibility and opens possibilities for on-chip detection of vacuum-ultraviolet photons, light wavelengths beyond the visible spectrum. This capability could lead to the development of integrated photonic circuits for nuclear clock operation, further reducing size and power consumption.

Enhanced nuclear transition rates via thorium-229 confinement in high-Q fluoride resonators

A tiny, highly reflective cavity made of fluoride crystals, the high-$Q$ fluoride photonic resonator, was central to this development, trapping light similarly to how a perfectly shaped echo chamber amplifies sound. These resonators are typically micro- or nano-scale structures designed to confine light through total internal reflection. The high ‘Q’ factor indicates that the resonator can store light for a relatively long time, enhancing the interaction between the light and the embedded thorium nuclei. This technique overcomes limitations of previous approaches by confining light around the thorium, sharply increasing the rate at which the nuclei can absorb energy and transition between states. The circular structure, where light travels around the edge, bouncing repeatedly, was utilised to maximise this light-matter interaction. The geometry of the whispering-gallery-mode resonator is crucial for achieving high Q-factors and efficient light confinement.

Embedding thorium within nanophotonic structures despite induced crystal damage

The pursuit of miniature atomic clocks promises a major revolution in navigation, secure communication, and fundamental scientific inquiry. Current atomic clocks, while highly accurate, are often bulky and require significant power. Miniaturisation through solid-state technologies offers the potential to create clocks that are portable, robust, and energy-efficient. Translating recent laboratory successes with thorium-229 into a practical, chip-scale device presents formidable challenges, however. The important thorium isotope has now been successfully embedded within a nanophotonic resonator, but a critical tension remains regarding the impact of the implantation process itself.

Thorium introduction into the fluoride crystal inevitably causes damage, potentially degrading the very optical properties, high quality, or ‘Q’ factor, needed to amplify the faint nuclear signal. Ion implantation, while a standard technique in materials science, introduces defects into the crystal lattice, disrupting the periodicity and introducing scattering centres for light. It is important to acknowledge that thorium implantation demonstrably causes some crystal damage for refining this technology. The extent of damage is dependent on the implantation energy, dose, and the material’s inherent resistance to radiation damage. Further work focusing on minimising implantation harm and optimising resonator design will be essential to unlock the full potential of this approach to precision timing. This could involve exploring alternative implantation techniques, such as lower-energy implantation or the use of annealing processes to repair crystal damage. This work highlights the need to balance the benefits of thorium inclusion with the unavoidable consequences of material disruption, a crucial consideration for the future development of solid-state nuclear clocks.

Researchers successfully embedded thorium-229 within a nanophotonic resonator, representing a step towards creating compact nuclear frequency standards. This achievement matters because current atomic clocks are often large and power-hungry, and a solid-state alternative could enable portable and energy-efficient timing devices. The study demonstrated that while introducing thorium causes some damage to the fluoride crystal, it is still possible to create a functional resonator. The authors outline a roadmap for future work including optimising resonator fabrication and minimising implantation-induced damage to improve performance.