Milestone For A New Type Of Atomic Clock

X-ray lasers lead the way to better precision time measurement

An international research team has taken a decisive step towards a new generation of atomic clocks. At the European X-ray laser European The team presents its success in the journal “Nature”.

Atomic clocks are currently the most accurate timepieces. So far, they have used electrons in the atomic shell of chemical elements, such as cesium, as a clock. These can be raised to a higher energy level using microwaves of a known frequency. In doing so, they absorb the microwave radiation. An atomic clock irradiates cesium atoms with microwaves and regulates the frequency of the radiation so that the microwaves are absorbed as strongly as possible; experts call this a resonance. The quartz oscillator that generates the microwaves can be kept so stable using resonance that cesium clocks will be accurate to one second in 300 million years.

The decisive factor for the accuracy of an atomic clock is the width of the resonance used. Current cesium atomic clocks already use a very narrow resonance, while strontium atomic clocks achieve greater accuracy at just one second per 15 billion years. Further improvement can practically no longer be achieved by exciting electrons. For several years now, teams around the world have been working on a nuclear clock that uses transitions in the atomic nucleus as a clock instead of in the atomic shell. These nuclear resonances are significantly sharper than the resonances of electrons in the atomic shell, but are also significantly more difficult to excite.

At the European This resonance requires X-rays with an energy of 12.4 kilo-electron volts (about 10,000 times the energy of visible light) and has a width of only 1.4 femto-electron volts (feV). That is 1.4 quadrillionths of an electron volt and therefore only about a tenth of a quadrillionth of the excitation energy (10 ^-19). This makes an accuracy of 1:10,000,000,000,000,000,000 possible. “That corresponds to one second in 300 billion years,” says DESY researcher Ralf Röhlsberger, who works at the Helmholtz Institute Jena, a joint institution of the GSI Helmholtz Center for Heavy Ion Research, the Helmholtz Center Dresden-Rossendorf (HZDR) and DESY.

Atomic clocks have numerous applications, such as precise positioning using satellite navigation, which benefit from improving accuracy. “The scientific potential of scandium resonance was recognized more than 30 years ago,” reports the experiment's project leader, Yuri Shvyd'ko from Argonne National Laboratory in the USA. “However, until now no X-ray source has been available that shines brightly enough within the 1.4 feV narrow line of scandium,” says Anders Madsen, lead scientist at the MID experimental station at the European XFEL, where the experiment took place. “That only changed with X-ray lasers like the European XFEL.” In the groundbreaking experiment, the team irradiated a 0,

It is also important for the construction of atomic clocks to have precise knowledge of the resonance energy, i.e. the energy of the X-ray laser radiation at which the resonance occurs. Through sophisticated extreme noise reduction and high-resolution crystal optics, the experiments made it possible to determine the value of the scandium resonance energy at 12.38959 keV down to the fifth decimal place, which is 250 times more precise than before. “The precise determination of the transition energy is a significant advance,” emphasizes the head of data analysis, Jörg Evers from the Max Planck Institute for Nuclear Physics in Heidelberg. “The exact knowledge of this energy is of enormous importance for the realization of an atomic clock based on scandium.” The researchers are now exploring further steps towards the realization of such an atomic clock.

“The breakthrough in the resonance excitation of scandium and the precise measurement of its energy opens up new possibilities not only for atomic nuclear clocks, but also in ultra-precision spectroscopy and for the precision measurement of fundamental physical effects,” explains Shvyd'ko. Olga Kocharovskaya from Texas A&M University in the USA, initiator and leader of the project, which was funded by the National Science Foundation of the USA, adds: “Such a high level of precision could, for example, make it possible to investigate gravitational time dilation at distances in the submillimeter range. This would enable studies of relativistic effects at length scales that were previously inaccessible.”

The work involved researchers from the Argonne National Laboratory in the USA, the Helmholtz Institute Jena, the Friedrich Schiller University Jena, the Texas A&M University in the USA, the Max Planck Institute for Nuclear Physics in Heidelberg, the Polish one Synchrotron radiation source SOLARIS in Krakow, the European XFEL and DESY are involved.