An international team of researchers has taken a decisive step towards a new generation of more accurate atomic clocks. At the European XFEL X-ray laser, researchers have created a more precise pulse generator based on the element scandium that can be accurate to one second in 300 billion years, which is about a thousand times more accurate than the current standard cesium-based atomic clock. The team presented its results on September 27 in the journal Nature.
Artist's rendering of a scandium nuclear clock: Scientists used X-ray pulses from the European XFEL to stimulate a process in the scandium nucleus that generates a clock signal with an unprecedented accuracy of one second in 300 billion years. Source: European XFEL/Helmholtz Institute Jena, Tobias Wüstefeld/Ralf Röhlsberger
Current atomic clock mechanisms
Atomic clocks are currently the most accurate timekeepers in the world. These clocks use electrons in atomic layers of chemical elements, such as cesium, as pulse generators to define time. These electrons can be boosted to higher energy levels using microwaves of known frequencies. In the process, they absorb microwave radiation.
The atomic clock emits microwaves at cesium atoms and adjusts the frequency of the radiation to maximize absorption of the microwaves; experts call this resonance. The quartz oscillators that generate microwaves can be stabilized with the help of resonance, allowing cesium clocks to be accurate to within one second for 300 million years.
Crucial to the accuracy of an atomic clock is the width of the resonance used. Current cesium atomic clocks already use very narrow resonances; strontium atomic clocks are even more accurate, with a precision of just one second every 15 billion years. Further improvements are virtually impossible to achieve using this electronic excitation method. So teams around the world have been working for years on the concept of "nuclear" clocks, which use transitions in atomic nuclei as pulse generators, rather than transitions in atomic shells. Nuclear resonances are much more violent than the resonances of electrons in atomic shells, but they are also more difficult to excite.
Breakthrough brought by scandium
At European XFEL, the team can now inspire promising transformations in the nuclei of the element scandium, which is readily available in the form of high-purity metal foils or the compound scandium dioxide. This resonance requires X-rays with an energy of 12.4 keV (about 10,000 times the energy of visible light) and a width of only 1.4 femtoelectronvolts (feV). This is 1.4 trillionth of an electron volt, about one-tenth the excitation energy (10-19). This makes an accuracy of 1:10,000,000,000,000 possible.
"This is equivalent to one second in 300 billion years," said DESY researcher Ralf Röhlsberger, who works at the Helmholtz Institute in Jena, a joint institution of the GSI Helmholtz Center for Heavy Ion Research, the Helmholtz Zentrum Dresden-Rosendorf (HZDR) and the Helmholtz Zentrum. .
Applications and future potential
Atomic clocks have many applications that benefit from increased accuracy, such as precise positioning using satellite navigation. “The scientific potential of scandium resonance was discovered more than 30 years ago,” Yuri Shvyd’ko, the project leader of the experiment and Argonne National Laboratory in the United States, reported. "However, so far, no X-ray source has been able to emit light brightly enough within the narrow 1.4feV line of scandium," said Anders Madsen, chief scientist of the European XFELMID experimental station where the experiment was conducted. "Only X-ray lasers such as the European XFEL have changed this situation."
In this groundbreaking experiment, the team illuminated a 0.025 mm thick scandium foil with an X-ray laser and were able to detect the characteristic afterglow emitted by excited nuclei, clear evidence of scandium's extremely narrow resonance lines.
Also important for the construction of atomic clocks is an accurate knowledge of the resonance energy, in other words, the energy of the X-ray laser radiation at which the resonance occurs. Advanced extreme noise suppression and high-resolution crystal optics allowed the scandium resonance energy value in the experiment to be determined to within five decimal places at 12.38959keV, which is 250 times more accurate than before.
Jörg Evers, head of data analysis at the Max Planck Institute for Nuclear Physics in Heidelberg, emphasizes: "The precise determination of the transition energy marks a major advance. An accurate knowledge of this energy is crucial for the realization of scandium-based atomic clocks."
The researchers are now exploring further steps towards realizing such a nuclear clock. Shvyd’ko explains: “Breakthroughs in scandium resonance excitation and precise measurement of its energy open up new avenues not only for nuclear clocks, but also for ultra-high-precision spectroscopy and precise measurement of fundamental physical effects.”
Olga Kocharovskaya of Texas A&M University, USA, the initiator and leader of the National Science Foundation-funded project, added: "Such high accuracy could allow, for example, the detection of gravitational time dilation at submillimeter distances. This would help study relativistic effects on hitherto unachievable length scales."