A step closer to the optical nuclear clock
Researchers report a significant advance toward the realization of the world’s first nuclear clock. They have succeeded in characterizing fundamental features of the thorium-229 isomer that could provide the core of such an instrument.
Precise time measurements play a vital role in our daily life. They allow reliable navigation, permit extremely accurate experimentation and provide the basis for synchronised world-wide exchange of data. A team of researchers of the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Ludwig-Maximilians-University Munich (LMU), Johannes Gutenberg University Mainz (JGU), the Helmholtz Institute Mainz (HIM), and the GSI Centre for Heavy Ion Research in Darmstadt now reports on a decisive step toward the potential development of a "nuclear" clock, which has the potential to significantly outperform the best current atomic clocks. The only known excited state of an atomic nucleus whose excitation energy is sufficiently low to be accessible by optical techniques comparable to those used in current atomic clocks, exists in thorium-229. Fundamental properties of thorium-229 in this state have now been determined by experiments, in which PD Dr. Peter Thirolf with his team at the Chair of Experimental Physics (Medical Physics) at LMU played a leading role. The researchers report their new findings in the current issue of the journal Nature.
All atomic clocks in operation today use the frequency of a transition between two energy levels in the outer electron shells of atoms as their basic timekeeper. Some 15 years ago, researchers at the PTB developed a concept for the design of a novel optical clock with unique characteristics. They proposed that, instead of exploiting oscillations in the electron shell, one could make use of a transition between energy levels within an atomic nucleus as the basis for a nuclear clock. Because the protons and neutrons in the nucleus are orders of magnitude more densely packed and much more tightly bound than the electrons in the outer electron shells, they are much less susceptible to perturbation by external forces that might affect their transition frequencies. Therefore, a nuclear clock should be far more stable and precise than present-day optical atomic clocks. However, the typical frequencies of nuclear transitions are much higher than those that occur in electron shells and lie in the X-ray region of the electromagnetic spectrum. This means that they cannot serve as the basis for an optical atomic clock, as all such clocks so far are based on excitation by microwaves or laser light. The exception to this rule is found in an unstable isotope of thorium, thorium-229 (229Th), which exhibits a quasi-stable ‘isomeric’ nuclear state with an extraordinarily low excitation energy. The frequency of the transition between the ground state and this isomeric state corresponds to that of ultraviolet light. This transition can therefore be induced by means of a laser-based technique similar to that used in state-of-the-art optical atomic clocks. More than 10 research groups worldwide are now working on the foundations for the realization of a nuclear clock based on the 229Th isomer. Experimentally speaking, this is an exceedingly challenging endeavor. Finally, in 2016, Peter Thirolf and his team, together with groups based in Mainz and Darmstadt, were able to directly detect the clock transition in 229Th for the first time, and they subsequently succeeded in measuring its half-life. However, it has not been possible to observe the nuclear transition by optical means yet, as the exact excitation energy of the isomer has not been determined with sufficient precision. The transition itself is extremely sharp – as required for timing purposes – and can only be induced if the frequency of the laser light corresponds exactly to the difference in energy between the two states. The quest for the magic frequency may be compared to the proverbial search for a needle in a haystack.
A collaborative effort by the researchers has now achieved an important breakthrough in this search. They have now measured some of the basic features of the 229Th isomer, such as the size of its nucleus and the general form of the distribution of protons. “In our work, the nuclei were not excited from the ground state by means of laser light, as they would be in a future clock. Instead, the isomer was produced by the alpha-decay of uranium-233, and decelerated in a device developed at LMU, extracted and stored in an ion trap as Th2+ ions,” Thirolf explains. The 233U source was provided by the groups in Mainz und Darmstadt. With the aid of laser systems specifically developed for spectroscopic analyses of this ionic species at the PTB, researchers have now been able to determine the transition frequencies in the electron shell of Th2+. These parameters are directly influenced by the state of the nucleus, and encode valuable information on its physical properties.
On the basis of theoretical modeling alone, it has not been possible to predict how the structure of the 229Th nucleus in this unusually low-excited isomer might behave. Furthermore, it is now possible to probe the structure of the electron shell to confirm a successful laser excitation of the nucleus into the isomer. The hunt for determining the optical resonance frequency that triggers the transition to the isomeric first excited state of the 229Th nucleus is not yet over. But researchers now have a far better idea of what the needle in the haystack really looks like.