High-precision timekeeping: first prototype of an atomic clock

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Atomic clocks only deviate by one second in millions of years. Nevertheless, even more precise timepieces are sometimes needed for basic research. Researchers at the Vienna University of Technology (TU Wien) and JILA/NIST in the USA have made an important advance in precision time measurement: The team combined a high-precision optical atomic clock with a high-energy laser system and successfully coupled these with a crystal containing thorium-229 atomic nuclei. This enabled them to measure the frequency ratio between the energy transition within an atomic nucleus and the atomic clock, as the scientists report in the journal Nature.

In an accompanying Nature article, Prof. Dr. Adriana Pálffy-Buß from the Thorium Nuclear Clock Project and her co-author José R. Crespo López-Urrutia from the Max Planck Institute for Nuclear Physics see this as "a promising step towards a timepiece that could help clarify fundamental questions about the universe".

Excited atomic nuclei

Current atomic clocks measure the transitions of the orbital electrons of cesium-133, for example. The second, for example, is defined as "9192631770 times the period of the radiation corresponding to the transition between the two hyperfine levels of the ground state of atoms of the nuclide cesium-133".

For even more precise timers, higher frequencies are inevitably required, such as those that occur during the change of excited states in the atomic nuclei themselves. The energy levels in atomic nuclei are much less sensitive to external influences due to the strong nuclear forces, which leads to greater accuracy and stability of the frequency measurements. In addition, isomeric states in atomic nuclei, as in the case of thorium-229, often have extremely long half-lives, which further increases the stability of the transition frequency. This leads to a lower frequency drift and thus to a higher stability over longer periods of time.

Unique properties of thorium-229

The weakly radioactive thorium-229 is particularly exciting because it is the only known isomer (an atomic nucleus with a deviating internal energy state) that has an extremely low-energy transition (8.355743 electron volts) that can be excited with laser light in the ultraviolet spectrum. This makes it an ideal candidate for a nuclear optical clock that would enable ultra-high precision measurements and outperform today's best atomic clocks. In fact, a research group from PTB Braunschweig and TU Vienna recently succeeded for the first time in specifically exciting this nuclear transition. This was preceded by a decades-long search for the exact amount of energy required for the so-called thorium transition.

Combed frequencies and the seventh harmonic

For the current publication, the scientists went to great lengths to specifically excite the atomic nucleus. They used a femtosecond laser frequency comb, which they coupled with a strontium-87 atomic clock, one of the most accurate timekeepers in the world. They then coherently converted the basic comb light to the seventh harmonic frequency in the vacuum ultraviolet range. This enabled the researchers to directly excite the nuclear transition in a thorium-doped calcium fluoride crystal and determine its absolute frequency. "In a way, this crystal is the heart of the experiment," says Prof. Thorsten Schumm from TU Wien, whose team produced the thorium crystal.

No more major obstacles

The researchers hope that their prototype will lay the foundation for new possibilities in fundamental physics and precision metrology.

However, the development of a practical atomic nucleus clock still faces various challenges. So the prototype does not yet provide an increase in precision. Furthermore, the exceptionally long lifetime of the isomeric state is advantageous for stability, but makes it difficult to detect the clock signal. According to the team, they are therefore working on more efficient excitation and readout schemes to solve this problem.

Thorsten Schumm sees the prototype as proof that thorium can be used as a timer for ultra-high-precision measurements. "All that remains to be done is technical development work, with no major obstacles to be expected."

(vza)