Atomic clocks are currently the gold standard of timekeeping. These devices measure time on the basis of transitions between two states of an atom. In two papers in Nature, Masuda et al.1 and Seiferle et al.2 report progress towards a clock that instead uses transitions between two states of an atomic nucleus. Such a nuclear clock could outperform existing atomic timekeepers, and have applications in both fundamental and applied physics.
Humans have been trying to measure the passage of time for thousands of years. From the sundial, to the hourglass, to the pocket watch, we have continually tried to improve our ability to quantify and standardize time. In the early 1900s, scientists struggled to define time consistently, and put forth various standards to help synchronize humanity. What was missing was a natural reference point that could be used, regardless of its location on Earth. We needed to define what a second truly meant: something fundamental that remains accurate and precise across all space, for all millennia.
Scientists realized that the properties of atomic transitions are independent of location in space or time. This recognition led to the idea of using a known transition between two atomic states as a means to define time. If a standardized second could be defined as a specific and agreed-on number of atomic transitions, time could be quantified. Researchers set out to do this in the 1930s, and by the end of the 1940s the world had its first atomic clock3.
Over the past 70 years, atomic clocks have been continually improved and currently have a precision4 of about 1 part in 1018. But what if we could do better than these devices? What if we could make a clock that was 100,000 times smaller, was less susceptible to its environment and possibly had a precision of 1 part in 1019? An atomic nucleus, which is about 100,000 times smaller than an atom, could provide such a device5.
Since 2003, researchers around the world have been trying to make a nuclear clock using the nucleus of a thorium-229 atom6. This nucleus, unlike all others that are known, has an excited state (called an isomeric state) that is only a few electronvolts (eV) in energy above its ground state7. As a result, the transition between these two states is accessible using specialized lasers. The problem is that the exact energy of the isomeric state is currently unknown. Masuda et al. and Seiferle et al. have made progress towards understanding the exact character of the thorium-229 isomeric transition, by carrying out experiments that extend previous work7.
In Masuda and colleagues’ experiment, a high-intensity X-ray beam was passed through a pair of silicon crystals that narrowed the energy range of the X-rays to 0.1 eV. These X-rays were then used to irradiate a thorium-229 nucleus that was in the ground state (Fig. 1). The nucleus transitioned to a second excited state that has an energy much higher than that of the isomeric state. The narrow X-ray energy range allowed the authors to determine the exact energy of this second excited state: 29.19 keV. Finally, the nucleus decayed directly to the isomeric state. The approach of Masuda et al. could enable this state to be produced more efficiently than was previously possible.
In Seiferle and colleagues’ experiment, a beam of thorium-229 ions was generated from the natural decay of uranium-233 ions. About 2% of the thorium ions were in the isomeric state. These ions were then neutralized to allow them to decay to the ground state through a process called internal conversion. In this process, a nuclear decay that would typically produce a γ-ray instead causes the neutral atom to emit an electron (Fig. 1). However, internal conversion is complicated, because the electron can originate from many different energy levels in the neutral atom.
To observe the ejected electrons from internal conversion, Seiferle and co-workers used a magnetic field to bend the trajectory of these particles towards an electron detector. They applied an electric field to the electrons until the voltage associated with this field was large enough to stop the electrons. The final voltage was equal to the initial energy of the electrons. Seiferle et al. then used a theoretical model to interpret the electron energy spectrum, which is the first energy spectrum observed from the decay products of the isomeric state. Their analysis indicated that the energy of the isomeric state is 8.28 ± 0.17 eV.
Although the ultimate and groundbreaking goal of directly observing the thorium-229 isomeric transition remains elusive, substantial progress continues to be made. The results of Masuda et al. and Seiferle et al. are key steps forward. Hopefully, the observation is not too far off, as teams of scientists race to make the world’s first nuclear clock, which would offer unprecedented precision. This finding would enable a whole host of experiments and discoveries in the decades to follow. For instance, a nuclear clock could have applications in dark-matter research8 and in the observation of possible variations in the fundamental constants of physics9.
Nature 573, 202-203 (2019)