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First self-stabilising thorium-229 nuclear clock runs in Vienna

By CAShorts DeskPublished Sources: PTB release (EurekAlert); TU Wien release (phys.org); Nature

First self-stabilising thorium-229 nuclear clock runs in Vienna
Illustration (AI-generated)

Physicists at TU Wien in Vienna and Germany's Physikalisch-Technische Bundesanstalt (PTB) reported in Nature on October 7, 2026 the first nuclear clock that stabilises itself. A 148-nanometre laser is locked to thorium-229 nuclei held in a calcium fluoride crystal. It ran over 24 hours unattended, at a precision near 10⁻¹⁵, below the best optical atomic clocks.

Syllabus: Prelims · Current Affairs & IR; Prelims · Science & Technology; GS3 · Science & Technology

Why it matters

Anyone who opens a map on a phone relies on atomic clocks. Satellites of the Global Positioning System (GPS) carry them, and a receiver works out its position from the time the signals take to arrive, so a timing error of one-billionth of a second shifts the position by about 30 cm. A nucleus is more than 10,000 times smaller than an atom and reacts far more weakly to outside disturbances, so it can be a steadier timekeeper. Because the thorium sits in a solid crystal, with no atoms held in a vacuum, such a clock could also be made sturdier. Physicists expect to use it to test basic laws of physics; the Vienna clock has already run a first search for dark matter.

Prelims pointers

  • Atomic clock: keeps time by the frequency of radiation that moves electrons between two energy levels of an atom.
  • SI second: defined by the transition between two hyperfine levels of the ground state of the caesium-133 atom, a microwave frequency of 9,192,631,770 hertz.
  • Optical atomic clock: uses a transition at the much higher frequency of visible or ultraviolet light, read with a laser; atoms or ions are held in a vacuum.
  • Nuclear clock: uses a transition between two energy states of an atomic nucleus; the first one is based on the isotope thorium-229.
  • Thorium-229: its nucleus has two energy states so close together that an ultraviolet laser of 148 nanometres can switch it; other nuclei need far more energy.
  • PTB (Physikalisch-Technische Bundesanstalt): Germany's national metrology institute, at Braunschweig. TU Wien: the Vienna University of Technology, Austria.
  • Isotopes: atoms of one element with the same number of protons and different numbers of neutrons; thorium has 90 protons.

Practise a UPSC-style question

With reference to atomic clocks, consider the following statements:

1. The SI unit of time, the second, is defined by a transition in the strontium-87 atom.

2. Optical atomic clocks work at higher frequencies than caesium clocks, which operate in the microwave region.

Which of the statements given above is/are correct?

  1. (a) 1 only
  2. (b) 2 only
  3. (c) Both 1 and 2
  4. (d) Neither 1 nor 2
Show the answer

Answer: (b) 2 only

1 is incorrect: the second is defined by the transition between two hyperfine levels of the ground state of the caesium-133 atom, at 9,192,631,770 hertz; strontium is used in optical clocks. 2 is correct: the caesium transition is a microwave frequency, while optical clocks use the far higher frequencies of visible or ultraviolet light.

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