Nuclear Clock: The Ticking Nucleus in the Search for Dark Matter express
The most precise modern clocks count time by the oscillations of electrons in atoms. But their rhythm can be thrown off, like a mechanical watch's spring gets jolted by shaking. Physicists found a more reliable pendulum: the atomic nucleus. They took a thorium-229 nucleus, embedded it in a crystal, and coaxed it to oscillate by shining a laser at a very specific frequency. This technique is called spectroscopy — essentially, finding the right rhythm to get the pendulum swinging. And the nucleus responded: it vibrates 2,000 trillion times a second — a million times faster than the quartz in your smartphone.
The precision is staggering: over a day, the error is one billionth of a billionth of a second. Such a clock would be off by just a second over 30 billion years. But the real kicker is that this pendulum is exquisitely sensitive to any 'breath' from the outside world, such as passing particles of dark matter. Scientists searched for its traces — periodic nudges or a smooth drift in frequency. They found nothing, but that's a result too: now we know just how weakly dark matter can interact with ordinary atomic nuclei.
In the future, the accuracy of these solid-state clocks will multiply many times over. They'll test whether the fundamental constants of the standard model are truly constant, and perhaps even notice that time flows differently at different heights, just as Einstein predicted. For now, their main mission is to catch the 'shadow' of dark matter.
Original abstract
as written by the authors arXiv:2606.04997
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 $$3\cdot 10^{-12} \sqrt{\tau/\text{s}}$$ 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.
The transition in the thorium-229 nucleus is the only one in the universe that can be 'caught' with today's laser technology. That's why it was chosen for the first nuclear clock.