Thorium Nuclear Clock Ticks with a Whisper of Light экспресс
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.
Оригинальная аннотация
как её написали авторы, на английском arXiv:2604.16640
A low-energy nuclear transition in the isotope thorium-229 has been excited in thorium-doped crystals with laser light. This opens the perspective towards a highly stable and robust solid-state optical nuclear clock. The required laser radiation at 148 nm wavelength has so far been produced using pulsed laser systems where only a small fraction of the incident photons has been resonant with the narrow nuclear transition. Here we show that the nuclear resonance can be excited with a continuous-wave narrow-bandwidth laser source with a power of less than 1 nW, and that the resonance signal can be detected in absorption rather than in fluorescence. This eliminates the slow nuclear fluorescence decay from the detection process and offers a considerable advantage for clock operation through fast signal acquisition. The VUV laser source is based on three sequential frequency doublings, starting from a diode laser at 1187 nm that is well suited for linewidth narrowing and for frequency comparisons with optical atomic clocks. We use absorption spectroscopy for the quantitative characterization of two different Th-centers in calcium fluoride crystal and measure the isomeric shift between them. One of the centers shows a very small static electric crystal field gradient 0.1 V/$$\r{A}^2$$, to be compared to gradients in the range of 100 V/$$\r{A}^2$$ observed earlier. This indicates a center with high symmetry of the ions surrounding the Th nucleus, promising nuclear resonance lines that are nearly independent of the lattice spacing.
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.