Thorium Nuclear Clock Ticks with a Whisper of Light ⚡ экспресс
Exciting the nuclear transition in thorium-229 with a continuous-wave laser of less than a nanowatt power and detecting the signal via absorption instead of fluorescence greatly speeds up measurements and simplifies the creation of solid-state nuclear clocks. The laser light at 148 nm was obtained by cascaded frequency doubling of a diode laser. Absorption spectroscopy in a CaF₂ crystal revealed two thorium sites; for one of them, the electric field gradient is record low (0.1 V/Ų), indicating high symmetry and promising nuclear resonance lines that are almost insensitive to lattice vibrations. This paves the way to clocks that could be orders of magnitude more accurate than existing atomic ones.
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.