Thorium nuclear clock ticks steadily for 24 hours

A thorium nuclear clock has kept its own rhythm for more than a day without help from a conventional atomic clock. ScienceDaily reports that the Vienna prototype ran steadily for over 24 hours without intervention, the first demonstration of a nuclear clock that can hold its own stability.

That is a step in precision measurement, or metrology. Nuclear clocks use atomic nuclei as their reference rather than the electron transitions inside atoms, and ScienceDaily reports they could eventually measure time and other physical quantities at accuracy today's technology cannot reach.

Here's the scale: nuclei are more than 10,000 times smaller than atoms, so the outside disturbances that nudge a measurement have far less to push against.

The result is a lab prototype, measured over a single day. ScienceDaily's account does not name a journal where the work appeared.

Why thorium nuclei work

Nuclei sit in different energy states, and jumping between those states usually takes enormous energy. Thorium is the exception. Two of its nuclear states are separated by an unusually small energy gap, small enough that laser light can trigger the jump. That transition gives researchers a reference for marking time.

In April 2024, a team led by Prof. Thorsten Schumm at the Institute of Atomic and Subatomic Physics at TU Wien, working with Prof. Ekkehard Peik's group at PTB Braunschweig, identified that transition experimentally and showed laser beams could excite thorium nuclei. Later that year, the groups connected their thorium apparatus to a conventional optical atomic clock, demonstrating the nuclei could serve as a timekeeping reference. What was still missing was independence.

How the clock steadies itself

The new setup centers on a special crystal containing thorium atoms, manufactured at TU Wien. A laser shines onto the crystal and interacts with the nuclei inside. Thorium nuclei absorb that light only when it arrives at precisely the right frequency. If the laser begins to drift, absorption drops, the system detects the change and steers the laser back. No conventional atomic clock is needed to keep it stable.

"What you really want is a self-stabilizing nuclear clock," Schumm said.

He described the idea as simple: a laser changes the energy state of the thorium nuclei, and the nuclei in turn stabilize the laser's frequency. Temperature swings and other small shifts in the surroundings can push a laser off its rhythm, which is why that feedback loop matters for high-precision work.

One second in 30 million years

Over a day of measurement, the prototype reached a relative precision of about one part in a quadrillion, which ScienceDaily puts at 10 to the power of minus 15 and describes as roughly one second over 30 million years. That figure describes the clock's measured precision, not an actual run lasting millions of years.

"This is not yet at the level of the world's best optical atomic clocks, but for a first prototype it is a fantastic result," Schumm said.

The leading optical atomic clocks remain the benchmark, and the nuclear clock has not caught them yet. Schumm and his colleagues see room to improve. More powerful lasers and higher-quality thorium crystals are the two routes ScienceDaily reports, and better versions could eventually overtake existing atomic clocks and set a new standard for measuring time.

Better timekeeping is not only about clocks. ScienceDaily reports that sharper measurement of time can sharpen the measurement of other physical quantities, giving researchers more sensitive tools for studying nature's fundamental properties. That is the payoff worth watching: not a prettier clock, but better questions.

• metrology
• nuclear clock
• thorium
• Thorsten Schumm
• TU Wien