A long-sought photon that is emitted by the nucleus of a thorium isotope has now been observed. The feat is a key step in efforts to build a nuclear clock, a device that is precise enough to probe the Universe’s best-kept secrets.

The most precise timekeepers today are atomic clocks, which measure time using the frequency associated with transitions that electrons make between the different energy levels of an atom. But atomic nuclei make similar transitions, and these jumps could potentially offer an even better way of keeping time. In particular, the nucleus of the isotope thorium-229 undergoes a transition with an energy and a frequency that make it uniquely suitable for very precise timekeeping. But observing this transition and identifying its energy precisely are difficult tasks. Writing in Nature, Kraemer et al.1 have detected the photon that is emitted in this transition, an advance that is crucial for the development of nuclear clocks.

Originally discovered in a mineral found off the Norwegian coast in 1828, thorium is named after Thor, the Norse god of thunder. It would take another century and a half for scientists to determine that one specific thorium isotope displays an anomaly that sets it apart from the rest2 — and perhaps makes the element worthy of its other-worldly name. The thorium nucleus in question has 229 nucleons (protons and neutrons), and can transition to an excited state that is only around 8 electronvolts more energetic than its lowest energy (ground) state. This difference is so tiny by nuclear-physics standards that the two states could barely be distinguished when they were first reported2. And it is the transition between these states that could make extraordinary nuclear timekeeping possible.

The working principle behind the nuclear clock closely resembles that of its atomic siblings3. The idea is that a light wave can induce a nucleus to jump between energy levels; the light’s frequency simply must precisely match that corresponding to the energy difference between the levels. This can be achieved with a laser, and — for optimal timekeeping — the ratio between the tuning range (the band of frequencies that can drive the jump) and the transition frequency itself should be very small. For thorium-229, this ratio is minuscule, and the transition is also better protected against stray photons that could affect the signal than are atomic transitions. Unfortunately, the laser required to drive the thorium-229 transition is yet to be built, in part because the exact value of the nuclear-transition energy was, for a long time, not known4–6...