Thorium Nuclear Clock Ticks with a Whisper of Light ⚡ экспресс
In calcium fluoride crystals doped with thorium-229, the nuclear transition was excited for the first time by continuous-wave laser light at 148 nm with less than 1 nW of power. Resonance detection was performed via absorption rather than fluorescence, which eliminates the slow nuclear decay from the detection process and benefits clock operation through fast signal readout. The VUV light source is based on three consecutive frequency doublings starting from a diode laser at 1187 nm, which is well suited for linewidth narrowing and frequency comparisons with optical atomic clocks. Absorption spectroscopy characterized two distinct thorium sites in the CaF₂ crystal and measured the isomer shift between them. For one of the sites, an extremely small static electric field gradient of the crystal — 0.1 V/Ų — was found, in contrast to typical values on the order of 100 V/Ų, indicating a high symmetry of the thorium nuclear environment and promising nuclear resonance lines that are almost independent of the lattice period.
A laser beam as faint as a whisper of light has stirred a thorium-229 nucleus into a higher energy state. Most nuclei demand a blast of energy, but thorium-229 has such a tiny energy gap that even this gentle nudge works. Instead of waiting for the nucleus to emit light, scientists used absorption spectroscopy—measuring the fraction of the whisper absorbed, giving a fast, clean signal. They placed the thorium in a calcium fluoride crystal, in a spot so quiet that the crystal's hum was almost inaudible.
A clock based on this could be so steady it would miss less than a second over the age of the universe. It might reveal whether the standard model, the rulebook of fundamental forces, ever drifts. And the laser's power? A firefly's brief glow outshines it by far.
🎯 Thorium-229 is the only nucleus that responds to such a faint whisper of light: its excitation energy is a mere 8 electronvolts, compared to the millions required by other nuclei. That’s the difference between a soft sigh and a thunderclap.