In the European XFEL X-ray laser, researchers have created a more precise pulse generator based on the element scandium, with an accuracy of one second over 300 billion years, which is roughly a thousand times better than that of the current caesium-based standard atomic clock. The team presented its results on September 27 in the journal Nature.
Atomic clocks are currently the most accurate timepieces in the world. These clocks use electrons in the atomic layers of chemical elements such as caesium as pulse generators to define time. Using microwaves of known frequency these electrons can be boosted to higher energy levels. In the process, they absorb microwave radiation.
Atomic clocks emit microwaves to the cesium atoms and modulate the frequency of the radiation to maximize the absorption of the microwaves; experts call this resonance. The quartz oscillator that generates the microwaves can be stabilized with the help of the resonance, allowing the cesium clock to be accurate to within one second over 300 million years.
Crucial to the accuracy of atomic clocks is the width of the resonance used. Current cesium clocks already use a very narrow resonance; strontium clocks are even more accurate, to within a second of 15 billion years. Further improvements are practically impossible to achieve using this method of electronic excitation. Teams around the world have therefore been working for many years on the concept of a “nuclear” clock, which uses the leaps in the nucleus of an atom as a pulse generator, rather than the leaps in the atomic shell. Nuclear resonances are much more violent than the resonances of electrons in the shell layers of atoms, but they are also much harder to excite.
At XFEL Europe, the team can now excite promising transitions in the nucleus of the element scandium, which is readily available as a high-purity metal foil or the compound scandium dioxide. This resonance requires X-rays with an energy of 12.4 kiloelectronvolts (keV, about 10,000 times the energy of visible light) and a width of just 1.4 femto-electronvolts (feV). This is 1.4 trillionths of an electron volt, which is about one-tenth of the excitation energy (10-19). This makes an accuracy of 1:10,000,000,000,000,000 possible.
“This corresponds to an error of one second in 300 billion years for this clock,” says Ralf Röhlsberger, a researcher at DESY working at the Helmholtz Institute in Jena, a joint institution of the GSI Helmholtz Center for Heavy Ion Research (HCIHI), the Helmholtz Zentrum für Dresden-Rossendorf (HZDR) and the Helmholtz Center for Heavy Ion Research (HZDR). Center.
There are many applications for atomic clocks that benefit from increased precision, such as precise positioning using satellite navigation. “The scientific potential of scandium resonance was discovered more than 30 years ago,” reports Yuri Shvyd’ko of the Argonne National Laboratory in the USA, the project leader of the experiment. “However, to date, no X-ray source has been able to emit light bright enough within the 1.4 feV narrow line of scandium,” says Anders Madsen, chief scientist at the European XFEL MID Experiment Station, where the experiment was carried out. Only X-ray lasers such as the European XFEL have changed this.”
In this pioneering experiment, the team irradiated a 0.025-millimeter-thick scandium foil with an X-ray laser and was able to detect a characteristic afterglow from the excited nucleus, which is clear evidence of scandium’s extremely narrow resonance line.
Equally 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 enabled the experimental scandium resonance energy value at 12.38959 keV to be determined to within five decimal places, a factor of 250 more accurate than before.
Jörg Evers, head of data analysis at the Max Planck Institute for Nuclear Physics in Heidelberg, emphasized: “The precise determination of the jump energy marks a major advance. An accurate knowledge of this energy is crucial for realizing a scandium-based atomic clock.”
The researchers are now exploring further steps towards the realization of such a nuclear clock. shvyd’ko explains, “The breakthrough in scandium resonance excitation and the precise measurement of its energy opens new avenues not only for nuclear clocks, but also for ultra-high-precision spectroscopy and the precise measurement of fundamental physical effects.