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A Clock inside the Nucleus
Modern atomic clocks are already astonishingly stable: the best lose only about one second every 100 million years. Yet they still rely on electrons in an atom’s outer regions, where stray electric and magnetic fields can disturb the measurement. Physicists have long wanted to move the clock’s reference point into the much smaller, denser, and better-shielded atomic nucleus.
Allison Parshall explains that atomic and nuclear clocks share the same basic idea. A precisely tuned laser excites an atom only when its light has exactly the right frequency. Because identical atoms always respond at the same frequency, the laser’s oscillations provide an exceptionally regular tick. A nuclear clock instead tunes the laser to a transition within the nucleus, which is roughly 100,000 times smaller than the atom while holding nearly all its mass. In principle, that compactness should make the clock less vulnerable to outside noise and capable of even finer measurements.
The obstacle is energy. Most nuclei require photons far beyond the reach of tabletop lasers. Thorium-229 is the rare exception: one of its nuclear transitions occurs at an unusually low energy that lasers can reach. Even so, researchers spent years narrowing down the exact frequency needed to trigger it.
Finding the Nuclear Tick
The breakthrough joined two teams. Thorsten Schumm’s group at the Vienna University of Technology grew transparent crystals containing quadrillions of thorium-229 nuclei, holding them fixed in place. The crystals then went to Jun Ye’s laboratory at JILA in Boulder, Colorado, home to one of the world’s most accurate atomic clocks and a laser system capable of searching the remaining frequency range.
Researchers swept the laser through candidate frequencies and watched for the faint ultraviolet glow that would signal a successful transition. After weeks of searching, they saw the signal, confirmed that they had excited the thorium nuclei, and synchronized the resulting nuclear frequency with the laboratory’s atomic clock. That synchronization produced the first working nuclear clock.
The prototype measured 12 digits of the nuclear frequency, compared with 18 digits for the atomic clock beside it. Its importance, however, was not immediate superiority but proof that the system could run at all. Experts expect engineering improvements could eventually extend nuclear clocks to 20 or 21 digits of precision.
More Than Better Timekeeping
A thorium crystal may also offer a practical advantage. Many atomic clocks suspend isolated atoms with electromagnetic fields, making their equipment delicate and difficult to move. A solid crystal could lead to a smaller, tougher clock suitable for use outside the laboratory. On GPS satellites or deep-space missions, such clocks could improve navigation and locate objects to centimeter or even millimeter scales.
The deeper payoff may be new physics. Atomic clocks depend primarily on electromagnetism, whereas nuclear clocks probe the strong force that binds nuclei. Comparing the two over time would test whether these supposedly constant forces truly remain constant. If their ticks ever drifted relative to each other, the first nuclear clock would have done more than sharpen humanity’s measure of time - it would have revealed that the underlying rules of nature are changing.