Nuclear clocks would track time by observing shifts within an atom’s nucleus. This method could make them less vulnerable to outside interference and potentially achieve greater accuracy compared to traditional atomic clocks. The development of these clocks might enhance timekeeping and navigation, enable faster internet connections, and facilitate progress in fundamental physics studies. Researchers have successfully showcased essential elements of a nuclear clock, including accurate frequency measurements related to an energy transition in the thorium-229 nucleus.
While atomic clocks currently provide the standard for global timekeeping, the emergence of nuclear clocks may transform our understanding of time measurement and fundamental physics.
Researchers from around the world, led by experts at JILA — a collaboration between the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder — have unveiled significant components of a nuclear clock. This innovative timekeeping instrument relies on signals from the atom’s nucleus. The team used a unique ultraviolet laser to measure the precise frequency of an energy transition in thorium nuclei embedded in a solid crystal. They also utilized an optical frequency comb, which functions like a highly precise light measuring instrument, to count the ultraviolet wave cycles associated with this energy shift. Although this laboratory effort is not a fully functional nuclear clock, it encompasses all necessary core technologies for one.
Nuclear clocks have the potential to exceed the accuracy of current atomic clocks, which are used for international timekeeping and are crucial for technologies like GPS, internet synchronization, and financial transactions. For everyday users, this progression could lead to even more precise navigation systems (with or without GPS), quicker internet speeds, more dependable network connections, and enhanced security in digital communications.
In addition to technological advancements, nuclear clocks could refine tests of fundamental theories regarding the universe, possibly resulting in significant discoveries in physics. They may assist in the detection of dark matter or help to confirm whether the natural constants remain constant, allowing for the validation of theories in particle physics without large particle accelerator infrastructures.
Precision of Lasers in Time Measurement
Atomic clocks track time by adjusting laser light to frequencies that prompt electrons to transition between specific energy levels. Instead, nuclear clocks operate on energy transitions within the atom’s nucleus — the small core where protons and neutrons are tightly arranged. These energy transitions can be likened to flipping a light switch. By shining laser light with the precise energy required for this transition, researchers can activate this nuclear “switch.”
Nuclear clocks promise substantial benefits in timekeeping precision. Unlike the electrons in atomic clocks, the nucleus is significantly less influenced by external factors such as stray electromagnetic fields. The laser light necessary for initiating energy transitions in nuclei operates at much higher frequencies than what is needed for atomic clocks. This increase in frequency, translating to more wave cycles per second, correlates to a higher number of “ticks” each second, resulting in superior timekeeping accuracy.
However, creating a nuclear clock is exceedingly challenging. Most atomic nuclei require coherent X-rays (a form of high-frequency light) with energies far beyond what current technology can achieve to cause energy transitions. Therefore, scientists have concentrated on thorium-229, which features a smaller energy transition compared to any other known atom and only requires ultraviolet light, which has lower energy than X-rays.
The energy transition in thorium was identified by scientists in 1976, referred to as a “nuclear transition” in physics terminology. In 2003, the idea of utilizing this transition for a clock was proposed, and it was directly observed for the first time in 2016. Earlier this year, two distinct research teams successfully employed ultraviolet lasers to trigger the nuclear “switch” and measure the required light’s wavelength.
In this latest research, the team from JILA and collaborators has assembled all critical components for a clock: the thorium-229 nuclear transition for the clock’s “ticks,” a laser to facilitate precise energy transitions among the nucleus’s individual quantum states, and a frequency comb for direct measurement of these “ticks.” This initiative has achieved a precision level one million times greater than previous measurements based on wavelengths. Additionally, they established the first direct frequency connection between a nuclear transition and one of the world’s most accurate atomic clocks, which uses strontium atoms. This direct link and heightened precision are vital advancements in the pursuit of a nuclear clock and its integration with current timekeeping systems.
Notable outcomes from this research include previously unobserved details regarding the shape of the thorium nucleus, akin to spotting individual blades of grass from an airplane view.
The findings are detailed in the Sept. 4 issue of the journal Nature, featured as the cover story.
Towards a Future in Nuclear Timekeeping
Although this research hasn’t yet culminated in a functional nuclear clock, it represents a pivotal move toward creating a clock that is both portable and stable. The incorporation of thorium within a solid crystal, coupled with the nucleus’s lesser sensitivity to external disturbances, suggests the potential for compact and durable timekeeping devices.
“Consider a wristwatch that would not lose even a second if it were kept running for billions of years,” remarked Jun Ye, a physicist from NIST and JILA. “While we haven’t entirely reached that point, this research brings us nearer to achieving such a level of accuracy.”
This research team included members from JILA, a joint institute of NIST and the University of Colorado Boulder; the Vienna Center for Quantum Science and Technology; and IMRA America, Inc.
