Kernuhr: Researchers work on the most accurate clock in the world

In 30 billion years, they go wrong for just a few seconds: The best atomic clocks are among the most accurate instruments that man has ever invented. But researchers are already working on much more precise timepieces: core clocks.

Physicists have now made a decisive breakthrough in their development, as reported in the journal "Nature". For the first time, they have been able to measure the exact amount of energy released by the decay of an excited thorium-229 nucleus.

The idea: In contrast to ordinary atomic clocks use nuclear clocks not oscillations in the electron shell as a clock, but in the atomic nucleus. This is about 100,000 times smaller than the atomic shell and thus less susceptible to interference.

"The wavelength must be exactly right"

However, there is a problem: Both atomic and possible nuclear clocks are based on vibrations caused by transitions between energy levels within the atom. But it is much more difficult to induce such transitions in the atomic nucleus than in the atomic envelope.

The only exception is Thorium-229. To excite the atomic nucleus, in this case, enough ultraviolet radiation, which can be produced with lasers, as they occur in atomic clocks.

"The energy or wavelength of the laser light must be exactly matched to the energy of the nuclear transition," says study author Benedict Seiferle from the Ludwig-Maximilians-University Munich. Also involved in the project were researchers from the Max Planck Institute for Nuclear Physics in Heidelberg, the GSI Helmholtz Center for Heavy Ion Research Darmstadt, the University of Mainz, the Helmholtz Institute Mainz, the University of Bonn and the Vienna University of Technology.

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However, it was not clear which wavelength the laser must have. Exactly this problem the researchers could fix now. They first prepared thorium-229 ions that are positively charged because they lack electrons. Then they shot them through a graph of graphene.

In doing so, a domino effect occurs: the ion recovers the missing electrons from the foil. The atomic nucleus then releases energy in millionths of a second to an electron, which is thereby ejected from the atomic shell. A new ion is created. With the help of an electron spectrometer, the researchers were able to determine exactly how much energy the atomic nucleus must have delivered to the electron.

From this, the researchers concluded that a laser must have a wavelength of around 150 nanometers to excite a thorium-229 nucleus. If this laser is first constructed, it is not far to the core clock.

For the average consumer, it may not matter if a clock runs in billions of years to the second, but many questions of basic research can be answered according to the physicists only with an extremely precise timing.