A thorium-229 optical nuclear clock with feedback loop.

A thorium-229 nuclear clock achieves a fractional frequency instability of 3 × 10^-12/τ, approaching 10^-15 over a day, surpassing current atomic clock performance.

• Why it matters: This work addresses the need for more robust and sensitive timekeeping systems that can improve tests of fundamental physics and detect ultralight dark matter interactions.
• What they did: The team stabilized a continuous-wave laser to the 148-nm nuclear transition in thorium-229 embedded in calcium fluoride, using rapid feedback and absorption spectroscopy, and compared it with a Yb+ single-ion clock.
• The result: The nuclear clock's high stability enables constraints on ultralight dark matter models, competing with the best atomic clocks in photon coupling and surpassing previous limits in strong-force interactions.