Towards an era of extreme precision with the nuclear clock

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The nuclear clock, which uses the thorium-229 isomer and whose precision is greater than the atomic clock, represents a leap forward in temporal precision. Its potential applications, ranging from fundamental physics to geodesy, promise new perspectives in theoretical and applied research. A major advance in timekeeping with thorium-229 at ISOLDE (CERN) paves the way for the design of viable nuclear clocks.

A nuclear clock uses the state transitions of the atomic nucleus to measure time with pinpoint precision, surpassing traditional atomic clocks. Recently, an international team used the unique ISOLDE (Isotope Separator On Line DEvice) facilities at CERN to measure the radiative decay of the metastable state of thorium-229m for the first time. The results have been published in Nature.

This measurement, thanks to a new approach, paves the way for direct laser manipulation of a nuclear state to design an entirely new generation of nuclear clocks. These advances represent a turning point in the quest for precision in metrology and promise immediate applications in advanced navigation and seismic analysis.

Thorium-229: a single isomer

Thorium-229 plays a central role in the development of the nuclear clock due to its metastable excited state, also known as an isomer. What makes it particularly remarkable is its very low excitation energy, measured around 8.19 ± 0.12 eV.

This unique characteristic of thorium-229 is crucial for the nuclear clock. The low excitation energy indeed results in an extremely narrow spectral line. In the context of clocks, a narrow spectral line means that the frequency of the radiation emitted or absorbed by the atom (or nucleus, in this case) is very precise and not subject to much variation. This allows for extremely accurate measurement of time, much more accurate than with atomic clocks.

Advantages over atomic clocks

Atomic clocks, such as those based on cesium-133, work by measuring the atom’s hyperfine transitions. These clocks are extremely accurate, with a margin of error of one second every 300 million years. Optical clocks, which use aluminum ions, go even further in terms of precision, losing or gaining a second every 33 billion years.

In an atomic clock, the electrons are relatively exposed and can be affected by external factors, which can slightly change their transition frequency and thus the accuracy of the clock. In a nuclear clock, on the other hand, transitions occur at the heart of the atomic nucleus, a much more isolated and protected environment. This insulation significantly reduces the impact of external influences, allowing even more accurate time measurement.

Towards the creation of a nuclear clock

The design and implementation of a nuclear clock represents a major technical challenge, mainly due to the unique characteristics of thorium-229 and the specific requirements of its use. One of the most critical aspects is the direct laser excitation of the thorium-229 isomer. This step requires very specific laser technology, which is able to precisely target the low excitation energy of the isomer.

Moreover, the longer lifetime of thorium-229 in the excited state is another crucial factor for the stable operation of the clock. This long lifetime is necessary to maintain the excited state long enough to enable accurate and reliable measurements. This involves delicate management of the isomer to maintain its excited state without excessive disturbance, which could affect the accuracy of the clock.

In a new technique based on vacuum ultraviolet spectroscopy, lead author Sandro Kraemer from KU Leuven (Belgium) and colleagues used ISOLDE to generate an isomer beam with an atomic mass number A = 229, according to the chain of disintegration 229 Fr → 229 Ra → 229 Ac → 229 episodes/ 229m episodes. A fraction of 229 Ac decays to the metastable excited state of 229 Th, the 229m Thi isomer.

ISOLDE vacuum and ultraviolet spectroscopy facility used for the experiment. © S. Kraemer et al., Nature 2023

To achieve this, the team processed the produced 229 Ac into six separate crystals of calcium fluoride and magnesium fluoride with different thicknesses. Using an ultraviolet spectrometer, the researchers measured the radiation emitted when the isomer relaxed to the ground state, finding that the wavelength of the light observed was 148.7 nm. This corresponds to an energy of 8.338 ± 0.024 eV – a precision seven times higher than previous best measurements.

Kraemer said in a statement that their approach also determined the lifetime of the isomer in the magnesium fluoride crystal, which helps predict the accuracy of a thorium-229 core clock based on this system. The result (16.1 ± 2.5 min) indicates that clock accuracy competitive with today’s most accurate atomic clocks is possible, while also being four orders of magnitude more sensitive to a certain number of effects outside the standard model.

Concrete practical applications

The nuclear clock is not just a tool to measure time. Its extreme sensitivity to variations in fundamental constants and to ultra-light dark matter offers perspectives for testing theories of fundamental physics. It could detect minute variations in the fine structure constant, a key element for understanding the universe.

In addition to physics, the nuclear clock could revolutionize areas such as global positioning systems, thanks to its sensitivity to the gravitational shift effect. It could also play a crucial role in detecting fluctuations in Earth’s gravitational potential, which could be useful for monitoring seismic and tectonic activities.

Source: Nature

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