Solid-state nuclear clock accuracy

A clock can use a nucleus’s resonant transition as its reference. Thorium-229 nuclei embedded in a crystal are now driven by a laser, with feedback keeping the laser on resonance. The engineering challenge is to know the reference frequency accurately.[1] [2]

A solid-state nuclear clock uses the frequency at which thorium-229 nuclei in a calcium-fluoride crystal respond to light as its reference. The question is how accurately that frequency can be known after accounting for shifts caused by the crystal and its surroundings. The specific engineering target is a validated total systematic fractional uncertainty near $10^{-18}$: an uncertainty in the corrected frequency of roughly one part in a billion billion.

For this $^{229}\mathrm{Th}:\mathrm{CaF}_2$ platform, the published zero-slope working temperature is $196(5)\,\mathrm{K}$. Temperature co-sensing is projected to suppress that contribution below the target, without yet establishing the full uncertainty budget.[1] [2]

The crystal environment can shift the resonance. Averaging a noisy measurement longer does not remove a steady shift caused by temperature, strain or magnetic fields. Those shifts must be measured, controlled and included in the clock’s uncertainty.[1] [2] [3]

Characterize strain and spatial inhomogeneity, Zeeman effects, line selection and interrogation-dependent shifts through controlled perturbations and independent clock comparisons. The measured $1.1\times10^{-13}$ reproducibility and a feedback clock’s near-$10^{-15}$ day-scale instability quantify different limits; neither is a total $10^{-18}$ accuracy result.[1] [2] [3]

A crystal holds many nuclei in a small volume, offering an attractive clock platform. Demonstrating that different conditions give the same corrected frequency would turn a sharp resonance into a dependable measurement standard.[1] [2] [3]

A substantiated budget would make residual drift interpretable as a metrological or physical signal rather than an unknown host-crystal effect. The nuclear reference’s sensitivity to fundamental interactions becomes useful only after ordinary environmental shifts can be separated from such signals.[1] [2] [3]