Two first Nuclear Clocks Kick Off a Precision Race with atomic clocks

Amazing stuff! Which of the two new nuclear clocks is more precise?

From the abstract (1):

"The laser-accessible nuclear transition in the thorium-229 isotope has been identified as a candidate for realizing an optical nuclear clock that might outperform current optical clocks based on electron-shell transitions in atoms or ions. 

It is expected to be more robust against external perturbations and to provide enhanced sensitivity in clock-based tests of the fundamental principles of physics. Here we realize a thorium-229 nuclear clock by stabilizing a continuous-wave laser to the 148-nm nuclear transition with rapid feedback based on absorption spectroscopy.

The thorium-229 nuclei are embedded in a millimetre-sized, room-temperature calcium fluoride crystal.

A subharmonic of the 148-nm radiation is continuously compared with a Yb+ single-ion clock. The nuclear clock shows a shot-noise-limited fractional frequency instability of  where τ is the averaging time, approaching 10−15 instabilities over 1 day of operation.

Improvements to the instability by several orders of magnitude are projected for future devices.

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 timescales between 20 s and 1 day.

Benefitting 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."

From the abstract (2):

"Atomic clocks have made time and frequency the most precisely measured quantities in physics, progressing from microwave standards that realize the SI second to optical clocks with unprecedented precision. 

A nuclear clock transfers the frequency reference from an electronic to a nuclear transition and the uniquely low-lying, laser-accessible, isomeric transition in 229Th currently offers the most practical route to compact, robust timekeeping and sensitive tests of fundamental physics.

Realizing such a clock requires turning spectroscopy of the 229Th nuclear resonance into a stable discriminator for steering a traceable oscillator.

Here we demonstrate a 229Th nuclear clock by stabilizing a continuous-wave, narrow-linewidth 148.4 nm vacuum-ultraviolet (VUV) laser to a resolved, weakly temperature-sensitive nuclear transition in 229Th:CaF2 crystals. A 10-μW VUV source generated by four-wave mixing in cadmium vapour and phototube-based frequency modulation absorption readout provide a fast, high-signal-to-noise nuclear discriminator. The clock reaches a fractional frequency instability of 

 for averaging time τ.

Clock-transition frequencies measured in two independently fabricated crystals agree at the 10−13 level and are consistent with previous VUV-comb measurements on other 229Th:CaF2 crystals.

These results establish laser-addressed nuclei as operational clock references and provide a reproducible solid-state platform for compact nuclear clocks, nuclear quantum sensors and precision tests of fundamental physics."

Physics - First Nuclear Clocks Kick Off a Precision Race "Two independent teams have built operating nuclear clocks—devices that might one day rival the best atomic clocks and offer new ways to probe fundamental physics."

A thorium-229 optical nuclear clock with feedback loop (1, open access)

A nuclear clock synchronized to 229Th (2, open access)

Photograph of the thorium-229-doped calcium fluoride crystal at the heart of the European team’s nuclear clock

Fig. 1: Experimental platform for a solid-state 229Th nuclear clock.