Scientists unveil first ultra-accurate nuclear clocks

Scientists have unveiled the first operational nuclear clocks, a milestone in international efforts to upgrade and secure the precision timekeeping vital to the digital systems that underpin modern life.

Devices based on the radioactive element thorium matched existing atomic clock technology in an exacting physics experiment, said researchers, who are now focused on improvements to make them useful in practice.

The development of the clocks by groups in China and Europe is part of a wider quest to safeguard the measurement of time, as risks of both accidental clock outages and sabotage threaten electronic systems vital for communications, financial transactions and operating power plants.

“The nuclear clock offers the potential of the outstanding performance of state of the art optical atomic clocks, but with a massively reduced energy footprint and a simpler and more robust apparatus,” said Thorsten Schumm, a physicist at Vienna’s university of technology, TU Wien. “It’s certain that we will make very fast progress in nuclear clock performance in the next two or three years.”

The nuclear clocks are so-called because they exploit tiny energy shifts in the states of the protons and neutrons that make up nuclei at the centre of atoms. Since these changes are much smaller than those for electrons measured by long-established atomic clock technology, nuclear clocks can in theory make more precise measurements. The thorium clock should also be less prone to disruption by external electromagnetic sources because the nuclei are more resilient to these than electrons are.

Resilient ultra-accurate timekeeping and navigation methods have become a growing national security priority for countries around the world as wartime tampering and unexpected outages have revealed vulnerabilities in the existing Global Positioning System (GPS). Many precision clocks on which fundamental digital services depend are housed on satellites that are vulnerable to interference or damage from sources such as growing volumes of spacecraft debris.

The thorium clocks proved able in a test to detect invisible “dark matter” at a level that compares with the best atomic clocks, the researchers reported in papers published in the journal Nature on Wednesday. The clocks offered further possible practical advantages because they were “solid-state” devices based on crystals of calcium fluoride rather than the gases used in atomic clocks, the scientists said.

The nuclear clock promised to be more compact and easier to operate than state of the art existing optical atomic clocks that are “extraordinarily precise” but remain “complex laboratory systems”, said Shiqian Ding, a researcher at Tsinghua University, Beijing, who led the Chinese group’s work.

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“If that becomes possible, it could bring optical clock-level precision out of specialised laboratories and allow such precision to be deployed much more broadly, including in navigation . . . space-based measurements and precision synchronisation,” Ding said.

Obstacles to the rise of thorium clocks include that they are still much less stable than existing optical atomic clocks, although the researchers say they are confident they can make significant technical improvements. Thorium-229 — the form of the element used in the clocks — is extremely scarce and is also crucial to a promising potential cancer therapy, so supply of it is tightly controlled.

Other new clock technologies are being developed and improved, including devices based on the rare earth element ytterbium. A further class of potential next-generation timepieces is the so-called molecular clock, which exploits the rotational and vibrational motions of individual molecules.