Nuclear clocks using thorium-229 transitions realized in two breakthroughs
Two independent research teams have created the first working nuclear clocks by using narrow-linewidth lasers to drive a transition in thorium-229 nuclei embedded in crystals, achieving record stability for new physics searches.
Two independent teams have created the first nuclear clocks using thorium-229 transitions in calcium fluoride crystals, driven by narrow-linewidth UV lasers. The breakthrough, reported in Nature, marks the realization of a concept proposed in 2003 and offers unprecedented stability for testing fundamental physics. One team is led by Ekkehard Peik at Germany's Physikalisch-Technische Bundesanstalt (PTB) and Thorsten Schumm from TU Wien in Austria, with colleagues from Germany, Austria, and Czechia. The other is led by Shiqian Ding from Tsinghua University and includes collaborators at 13 other institutions across China.
Both groups embedded the thorium-229 isotope into millimetre-sized calcium fluoride (CaF2) crystals. They then used continuous-wave vacuum-ultraviolet lasers at a wavelength near 148 nanometers to drive the nuclear transition, which lies 8.4 electron volts above the ground state. Each team employed a different nonlinear process to generate the crucial laser light. They monitored the nuclear resonance through direct absorption spectroscopy in the crystal and used a feedback loop to lock the laser frequency, creating a stabilized clock that could be compared to existing atomic frequency standards.
Technical advances and performance comparison
The teams employed different technical strategies, leading to distinct performance levels. The European team used a crystal with a higher concentration of thorium-229 but operated with a lower-power laser. The Chinese team, facing limited access to the isotope, developed a method to grow crystals using only 1.4 micrograms of thorium-229 and generated laser light at higher powers. This resulted in different fractional frequency instabilities, a key metric for clock stability.
| Team | Fractional Frequency Instability | Equivalent Stability (one second gained/lost over) |
|---|---|---|
| Tsinghua-led (China) | 5 × 10⁻¹³/√τ/s | 19 million years |
| PTB-TU Wien (Europe) | 3 × 10⁻¹²/√τ/s | 3 million years |
While both represent major advances, the Chinese device demonstrated superior stability. For context, the best optical atomic clocks are significantly more stable, gaining or losing a second only over tens of billions of years.
Applications in fundamental physics and future outlook
The European team immediately applied their new clock as a sensor for new physics. They used it to search for couplings of ultralight scalar dark matter by linking it to a remote ytterbium-ion atomic clock via a stabilized optical fiber. The search found no variation in clock frequency ratios over timescales up to a day. The resulting limits on dark matter's coupling to photons and the strong nuclear force are competitive with those set by today's best atomic clocks.
The primary focus of the Chinese team's work was on technical metrology. They demonstrated high-power laser interrogation, clock performance, and inter-crystal reproducibility. The clock transition frequencies they measured in two independently fabricated crystals agreed with each other and with previous measurements made in the United States.
Both groups are now focused on improvements. Ekkehard Peik stated they are working on boosting laser power and improving the detector signal-to-noise ratio. "We have now seen 30 kHz linewidth, but the natural linewidth should be way below 1 Hz, which would enable more stable clock operation," he said. Shiqian Ding identified developing better host crystals than CaF2 as an important next step to narrow the nuclear transition linewidth. Ding is also interested in creating a trapped-ion thorium nuclear clock for greater environmental isolation.
Context and expert validation
Experts not involved in the projects recognized the significance of the demonstrations. Theorist Victor Flambaum called the devices the 'first prototypes of nuclear clocks.' He highlighted their potential to amplify subtle new physics effects, stating, "We have shown that in nuclear clocks these effects are strongly enhanced." Davide Calonico, scientific director of Italy’s National Institute of Metrological Research, called the results a major metrological milestone.
Fiona Kirk of the Weizmann Institute of Science noted that the unusually low transition energy in thorium-229 might result from an accidental cancellation within the nucleus. This same cancellation could act as a powerful amplifier for physics beyond the standard model. Both teams are now refining their systems, aiming for narrower linewidths and the enhanced stability that would unlock the full potential of nuclear timekeeping.
