Nuclear clocks could be the best timepiece in the universe

Atomic clocks work by using a laser to bounce the electrons in an atom at a given frequency, while nuclear clocks would theoretically do the same for atomic nuclei, and we are a step closer to building one.

The discovery of an elusive flash of light from the nucleus of the element thorium, which physicists have been hunting for decades, brings us a step closer to building a nuclear clock. This could be the most precise timepiece in the cosmos and help probe our understanding of the universe.

The nucleus of a thorium atom could make a useful clock

You may have already heard of atomic clocks, which use the energy levels of electrons around a nucleus to tell time – each electron can only have certain, fixed energy levels. Tuning a laser to just the right frequency lets researchers bounce an electron between two of these energy levels. It is this frequency that serves as the tick of an atomic clock, keeping time to an accuracy of only a few lost seconds every billion years.

Atomic nuclei can also jump between different energy levels. In particular, the nucleus of radioactive thorium has an unusually small gap between energy levels that would make for an extraordinarily accurate clock and would also teach us more about the inner workings of the nuclei.

Physicists had been unable to identify the precise laser frequency necessary to build such a clock, but now Sandro Kraemer at Ludwig Maximilian University of Munich in Germany and his colleagues have pinned it down.

Ordinarily, thorium is a billion times more likely to emit an electron than a photon, but, by embedding the nuclei in a crystal lattice of calcium and magnesium fluoride, the team was able to change the odds. With the ability to produce photons over electrons, the researchers could measure this flash of light precisely enough to determine its frequency with seven times less uncertainty than previous measurements.

It is this frequency that would be required to fire a laser at a thorium nucleus to make a clock. “Showing that you can control it to such a degree that you can see the signal of photons coming out is a big milestone for thinking about building a nuclear clock,” says Kraemer.

To produce the radioactive thorium, which doesn’t occur in nature, Kraemer and his team fired protons into a uranium target at a facility in CERN in Switzerland to produce a beam of radioactive actinium ions. They then fired actinium ions into calcium and magnesium fluoride crystals, where they then decayed into photon-producing radioactive thorium nuclei.

The work means we now know the energy levels of the thorium nucleus almost well enough to build a working nuclear clock, says Kraemer, and there are research groups already trying to construct a laser that can excite the nucleus – but the final hurdle might involve years of fine-tuning the frequency to the perfect pitch.

If successful, a nuclear clock could tell us about the inner workings of nuclei and probe for discrepancies in the universe’s fundamental forces, as well as improve on the already incredibly precise atomic clocks in existence.

“The different feature that it relies on – a nuclear resonance – would make it interesting to compare this nuclear clock with established atomic clocks in order to look for effects of new physics in the quantum domain or in relativity,” says Ekkehard Peik at the German National Metrology Institute. The forces that operate on the nuclear scale, such as the strong and weak force, are different from those acting on atomic clocks, so any discrepancy might hint at new physics, he says.

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