X-ray laser shows possible route to substantially increased precision time measurement
An international research team has taken a decisive step toward a new generation of atomic clocks. At the European XFEL X-ray laser, the researchers have created a much more precise pulse generator based on the element scandium, which enables an accuracy of one second in 300 billion years – that is about a thousand times more precise than the current standard atomic clock based on caesium. The team presents its success in the journal Nature.
Atomic clocks are currently the world’s most accurate timekeepers. These clocks have used electrons in the atomic shell of chemical elements, such as caesium, as a pulse generator in order to define the time. These electrons can be raised to a higher energy level with microwaves of a known frequency. In the process, they absorb the microwave radiation. An atomic clock shines microwaves at caesium atoms and regulates the frequency of the radiation such that the absorption of the microwaves is maximised; experts call this a resonance. The quartz oscillator that generates the microwaves can be kept so stable with the help of resonance that caesium clocks will be accurate to within one second within 300 million years.
Crucial to the accuracy of an atomic clock is the width of the resonance used. Current caesium atomic clocks already use a very narrow resonance; strontium atomic clocks achieve a higher accuracy with only one second in 15 billion years. Further improvement is practically impossible to achieve with this method of electron excitation. Therefore, teams around the world have been working for several years on the concept of a “nuclear” clock, which uses transitions in the atomic nucleus as the pulse generator rather than in the atomic shell. Nuclear resonances are much more acute than the resonances of electrons in the atomic shell, but also much harder to excite.
At the European XFEL the team could now excite a promising transition in the nucleus of the element scandium, which is readily available as a high-purity metal foil or as the compound scandium dioxide This resonance requires X-rays with an energy of 12.4 kiloelectronvolts (keV, which is about 10,000 times the energy of visible light) and has a width of only 1.4 femtoelectronvolts (feV). This is 1.4 quadrillionths of an electronvolt, which is only about one tenth of a trillionth of the excitation energy (10-19). This makes an accuracy of 1:10,000,000,000,000 possible. “This corresponds to one second in 300 billion years,” says DESY researcher Ralf Röhlsberger, who works at the Helmholtz Institute Jena, a joint facility of the GSI Helmholtz Centre for Heavy Ion Research, the Helmholtz Zentrum Dresden-Rossendorf (HZDR), and DESY.
Read more on the DESY website
Image: An artist’s rendition of the scandium nuclear clock: scientists used the X-ray pulses of the European XFEL to excite in the atomic nucleus of scandium the sort of processes that can generate a clock signal – at an unprecedented precision of one second in 300 billion years.
Credit: European XFEL/Helmholtz Institute Jena, Tobias Wüstefeld/Ralf Röhlsberger