For decades, the standard reference tool for ultraprecise timekeeping has been the atomic clock. Scientists have known that an even more precise and reliable timepiece was possible, but technical limitations kept it only a theoretical prospect.
Now, researchers from the U.S. Department of Energy's (DOE) Argonne National Laboratory, Texas A&M University and several European institutions are turning theory into practice. The team has used X-ray beams to excite a long-lived nuclear state in scandium-45, an element used in aerospace components and sports equipment. The work was published in Nature and represents the culmination of a long scientific quest for lead investigator Yuri Shvyd'ko of Argonne.
An atomic clock and a nuclear clock might sound like basically the same thing, but there are differences in how they work. Atomic clocks oscillate based on the quantum transition which occurs when an electron inside an atom is transferred from one energy level to another at a very precise frequency. Accurate to about one second in 300 million years, this is more than enough to serve as the primary time standard for GPS navigation, computer networks and most other human activities.
"For purposes that demand such precision, including the study of certain aspects of relativity, gravitational theory and other physical phenomena such as dark matter, the nuclear clock is the ultimate timepiece."
Olga Kocharovskaya, Texas A&M University
A nuclear clock is based on the natural oscillation of the much smaller nucleus at the very center of an atom, rather than the large cloud of electrons swirling around it. Nuclear clocks are much more immune to disturbances such as temperature changes or electromagnetic fields that can spoil the remarkable precision of an atomic clock. This offers even higher precision in a much more stable form.
Until now, one of the stumbling blocks to the realization of a true nuclear clock has been that existing X-ray sources weren't quite able to provide the necessary kick to start a nucleus oscillating and then detect it. Another has been the identification of a good candidate nucleus. The most promising has generally been considered to be thorium-229.
Scandium-45 has long been considered another promising candidate, ever since Argonne scientists discovered the comparatively long life of its excited state in 1964. With no way to excite the oscillations, however, the material dropped off the radar for decades. In 1990, Shvyd'ko -; then working at an institute in Moscow -; and his colleagues published a paper showing that newly emerging accelerator-based X-ray light sources could be used to power the oscillations.
"In that paper, we showed that light sources could be used," Shvyd'ko said. "Despite the fact that they are broadband sources, they can be used to excite and drive this resonance, and one could also measure the very narrow width of the resonance."
One such light source is the Advanced Photon Source (APS), a DOE Office of Science user facility at Argonne, which saw its first light in 1995. But even the APS X-ray beams do not have the intensity required to accomplish the task. It took the recent advent of advanced X-ray free electron laser (XFEL) sources, such as the European XFEL facility (EuXFEL) in Hamburg, Germany, to turn theory into reality.
"Finding the nuclear resonance within scandium-45 demanded an extremely high-intensity source of X-ray beams along with a specially designed protocol for a very low-noise background detection. Both of these were realized at EuXFEL," said Ralf Röhlsberger of the Helmholtz Institute Jena in Germany, a co-author on the paper
Finding the right resonance energy required a scrupulous tuning of the X-ray energy until the telltale photons from nuclear decay -; which act as a signature of the resonance -; were found.
"We confirmed the detection of approximately 93 nuclear decay events with a high level of confidence," said Peifan Liu of Argonne, a co-author on the paper. "Simultaneously, the energy of the resonance was determined precisely, with an accuracy 250 times higher than that previously known."
Taken together, these results open new prospects for revolutionizing highly sensitive probes of natural properties like gravity and enabling fundamental physics tests that rely on the measurement of time or frequency with utmost precision, researchers said
The success of this experiment is a significant milestone in realizing the long-held potential of a scandium-45 nuclear clock. But this is only the beginning of a long journey, one that will require more breakthroughs in detailed studies of the resonance and the development of even more advanced X-ray sources.