An international research consortium has marked a significant stride towards a new era of atomic clocks. Operating at the European XFEL X-ray laser, these scientists have engineered a remarkably precise pulse generator using the element scandium. This groundbreaking development achieves an unprecedented level of accuracy, equivalent to one second in 300 billion years—approximately a thousand times more precise than the prevailing standard atomic clock based on cesium. Their achievement is detailed in the prestigious journal Nature.
Atomic clocks stand as the pinnacle of timekeeping precision today. Conventionally, these clocks have utilized electrons in the atomic shell of certain chemical elements, such as cesium, as the source of pulses to measure time. By subjecting these electrons to microwaves with a known frequency, they absorb the radiation.
In essence, an atomic clock directs microwaves towards cesium atoms, meticulously adjusting the radiation frequency to maximize microwave absorption—a phenomenon referred to as resonance. This stability, facilitated by the quartz oscillator producing the microwaves, allows cesium clocks to maintain an accuracy of one second within 300 million years.
The crux of atomic clock accuracy hinges on the width of the resonance used. Existing cesium atomic clocks already employ a remarkably narrow resonance, while strontium atomic clocks push the limits further, achieving an accuracy of one second in 15 billion years. However, reaching even greater precision with electron excitation seems nearly impossible using this approach.
Consequently, research teams worldwide have been diligently working on the concept of a “nuclear” clock. In this novel approach, atomic nucleus transitions, rather than atomic shell transitions, serve as the pulse generator. Nuclear resonances are considerably sharper than electron resonances in the atomic shell but present a greater challenge in terms of excitation.
At the European XFEL, the research team has managed to excite a promising nuclear transition in the element scandium. Scandium is readily available in the form of high-purity metal foil or scandium dioxide compound. This particular resonance necessitates X-rays with a staggering energy of 12.4 kiloelectronvolts (keV), approximately 10,000 times greater than visible light energy, and boasts an astonishingly narrow width of just 1.4 femtoelectronvolts (feV).
To put this into perspective, 1.4 feV is a minuscule fraction, equivalent to 1.4 quadrillionths of an electronvolt, making it just one-tenth of a trillionth of the excitation energy (10-19). This breakthrough enables an unprecedented level of precision—1:10,000,000,000,000, or one second in 300 billion years, as explained by DESY researcher Ralf Röhlsberger.
The applications of atomic clocks span a wide array of fields, with precise positioning through satellite navigation being just one example. The potential of the scandium resonance was identified over 30 years ago, but the lack of a sufficiently intense X-ray source, until now, has hindered progress in harnessing its capabilities, as described by Yuri Shvyd'ko, the project leader at Argonne National Laboratory.
In a groundbreaking experiment, the research team exposed a 0.025-millimeter-thick scandium foil to X-ray laser light and successfully detected a distinctive afterglow emitted by the excited atomic nuclei—a compelling demonstration of scandium's exceptionally narrow resonance line.
Another critical factor in building atomic clocks is knowing the precise resonance energy—essentially, the energy of the X-ray laser radiation that triggers the resonance. Advanced noise suppression techniques and high-resolution crystal optics enabled the determination of scandium's resonance energy with unprecedented precision, accurate to five decimal places at 12.38959 keV—a value 250 times more precise than previously achievable.
The precise knowledge of this transition energy holds immense significance for realizing an atomic clock based on scandium, emphasizing the importance of this advancement, as highlighted by Jörg Evers of the Max Planck Institute for Nuclear Physics in Heidelberg, who oversaw data analysis.
The researchers are now embarking on further research to bring atomic nuclear clocks closer to reality, with vast potential implications not only for timekeeping but also for ultrahigh-precision spectroscopy and the precise measurement of fundamental physical phenomena, as elucidated by Yuri Shvyd'ko.
Initiator and project leader Olga Kocharovskaya of Texas A&M University envisions possibilities such as probing gravitational time dilation at sub-millimeter distances, enabling studies of relativistic effects on length scales previously inaccessible.
This groundbreaking endeavor involved collaborative efforts from distinguished institutions and experts worldwide, including Argonne National Laboratory in the U.S., the Helmholtz Institute Jena, Friedrich Schiller University Jena, Texas A&M University in the U.S., the Max Planck Institute for Nuclear Physics in Heidelberg, the Polish synchrotron radiation source SOLARIS in Kraków, the European XFEL, and DESY.
Source: Deutsches Elektronen-Synchrotron