Atomic clocks are renowned for their unparalleled precision in timekeeping. By utilizing periodic transitions between two electronic states of an atom, they can accurately measure the passage of time with an astonishing precision of up to one part in a quintillion. This means that over a span of 30 billion years, they would neither lose nor gain a single second, surpassing the age of the universe by twofold.
Recently, a significant milestone in the realm of timekeeping was achieved by an international team at CERN’s nuclear physics facility, ISOLDE. Their findings, published on May 24 in the journal Nature, lay the foundation for constructing a clock based on periodic transitions between two states of an atomic nucleus—specifically, the nucleus of thorium-229, an isotope of the element thorium.
The potential of a nuclear clock surpasses that of existing atomic clocks due to the distinct size and constituents of a nucleus compared to an atom. Moreover, it holds promise as a sensitive instrument to explore phenomena that lie beyond the boundaries of the Standard Model, the present leading theory in the domain of subatomic particles. For instance, a nuclear clock could facilitate the search for temporal variations in fundamental constants of nature and aid in the quest for ultralight dark matter.
The pursuit of observing and characterizing the transition between the ground state and the first, higher-energy state (referred to as an isomer) of the thorium-229 nucleus, proposed by Ekkehard Peik and Christian Tamm in 2003, has been fiercely contested by researchers over the past two decades. During this time, scientists have steadily enhanced the precision of measuring the isomer’s energy, a vital factor for the development of lasers that drive the transition to the isomer. Despite their relentless efforts, they have yet to witness the emission of light during the transition from the isomer to the ground state, known as the radiative decay of the isomer. This particular phenomenon, characterized by a relatively extended lifespan, is crucial for establishing a nuclear clock as it would enable a higher precision in determining the energy of the isomer, among other essential aspects.
The team at ISOLDE has accomplished a remarkable breakthrough by generating thorium-229 nuclei in the isomeric state using an innovative method and investigating them through vacuum-ultraviolet spectroscopy. By employing this technique, they have determined that the wavelength of the observed light corresponds to an energy of 8.338 electronvolts (eV) for the isomer, with an uncertainty of 0.024 eV. This measurement is seven times more precise than previous records.
A pivotal factor in the team’s success was the production of isomeric thorium-229 nuclei through the beta decay of actinium-229 isotopes, which were synthesized at ISOLDE and incorporated into crystals of calcium fluoride or magnesium fluoride.
Sandro Kraemer, the lead author of the paper, states, “ISOLDE currently stands as one of the two facilities worldwide capable of generating actinium-229 isotopes. By integrating these isotopes into calcium fluoride or magnesium fluoride crystals, we were able to generate a greater quantity of isomeric thorium-229 nuclei, significantly enhancing our chances of observing their radiative decay.”
This novel approach to producing thorium-229 nuclei also facilitated the determination of the isomer’s lifetime within the magnesium fluoride crystal. Understanding this lifespan is crucial for predicting the precision of a thorium-229 nuclear clock built upon this solid-state system. The measured long lifetime of the isomer, specifically 16.1 minutes with an uncertainty of 2.5 minutes, aligns with theoretical estimates and indicates the potential for achieving clock precision that rivals today’s most precise atomic clocks.
Piet Van Duppen, the team’s spokesperson, highlights the significance of solid-state systems like magnesium fluoride crystals as one of the potential platforms for constructing a future thorium-229 nuclear clock. He states, “Our study represents a crucial advancement in this direction and will facilitate the development of lasers necessary to drive the periodic transition that would enable such a clock to function.”