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Researchers have successfully demonstrated the first operational thorium-229 nuclear optical clock by stabilizing a laser to the 148 nm nuclear transition in thorium-229 nuclei embedded in a calcium fluoride crystal at room temperature. The clock achieves a fractional frequency instability of 3×10^-12 per square root of averaging time in seconds, reaching 10^-15 level instability after one day of continuous operation. This nuclear clock was used to search for signatures of ultralight dark matter by monitoring the nuclear transition energy for periodic fluctuations and drifts over timescales from 20 seconds to one day.
Why it matters
Nuclear clocks could potentially surpass current atomic clocks in precision while being more robust against external disturbances, enabling improved tests of fundamental physics and better detection of phenomena like dark matter. The enhanced sensitivity of thorium-229 to certain fundamental constants makes it particularly valuable for constraining dark matter models and testing physics beyond the Standard Model.
arXiv:2606.04997v2 Announce Type: replace
Abstract: The laser-accessible nuclear transition in the thorium-229 isotope has been identified as a promising candidate for the realization of an optical nuclear clock. Such a nuclear clock might rival or outperform current optical clocks based on electron-shell transitions in atoms or ions, is expected to be more robust against external perturbations, and provides enhanced sensitivity in clock-based tests of fundamental principles of physics. Here, we implement a thorium-229 nuclear clock by stabilizing a continuous-wave laser to the 148 nm nuclear transition with rapid feedback based on continuous absorption spectroscopy. The thorium-229 nuclei are embedded into a millimeter-sized, room temperature calcium fluoride crystal. A subharmonic of the 148 nm radiation is continuously compared to a Yb+ single-ion clock. The nuclear clock shows a simple shot-noise limited scaling of the fractional frequency instability of $3cdot 10^{-12} sqrt{tau/text{s}}$ where $tau$ is the averaging time, approaching $10^{-15}$ instabilities over 1 day of continuous operation. Improvements of the instability by several orders of magnitude can be projected for future solid-state nuclear clocks. We use the nuclear clock to constrain models of ultralight dark matter by searching for periodic fluctuations and slow drifts in the nuclear transition energy, on time scales between 20 s and 1 day. Drawing benefit from the enhanced sensitivity of the thorium-229 transition, these constraints compete with the best atomic clocks concerning dark matter coupling to photons and go beyond previous measurements regarding coupling to the strong force and quarks.
Source: A thorium-229 optical nuclear clock with feedback loop