Physicists worldwide are rapidly advancing toward the development of a revolutionary "nuclear clock," a timekeeping device that promises to surpass the precision of existing atomic clocks by measuring energy transitions within the nuclei of atoms. This innovation could transform the field of timekeeping, enabling unprecedented accuracy and stability with applications ranging from fundamental physics to commercial technologies.
The concept of a nuclear clock has intrigued scientists for decades, largely centered on the unique properties of the isotope thorium-229 (229Th). Unlike typical atomic clocks, which rely on electron transitions between energy levels, a nuclear clock would measure transitions within the atomic nucleus itself. These nuclear transitions occur at energy levels remarkably stable and less susceptible to external disturbances, potentially allowing the clock to keep time with greater accuracy.
For half a century, the exact energy of the critical nuclear transition in thorium-229 remained elusive, hindering efforts to build a functional nuclear clock. This changed in 2024 when a team led by physicists Chuankun Zhang and Jun Ye employed a sophisticated laser technique known as a frequency comb to precisely identify this transition. The frequency comb, which produces millions of laser frequencies simultaneously, enabled them to pinpoint the nuclear energy change with ultra-high precision. This breakthrough effectively launched the countdown to realizing a working nuclear clock.
Today, numerous research groups spanning China, Europe, Japan, and the United States are racing to assemble the necessary components for these clocks. Key among these components are a stable source of 229Th and a powerful continuous-wave ultraviolet laser capable of exciting the thorium nuclei at a wavelength near 148 nanometers - a challenging feat given the extreme ultraviolet range involved.
At the recent American Physical Society (APS) Global Physics Summit in Denver, Colorado, researchers shared progress reports on their respective projects. Among the most notable advances is the development of ultraviolet lasers capable of reaching the precise wavelength needed to excite the thorium nuclei. For example, a team at Tsinghua University in Beijing demonstrated a laser delivering 100 nanowatts of power at 148.4 nanometers. While this is a promising achievement, some experts expressed caution regarding the laser's long-term viability because it relies on heating toxic cadmium vapor to very high temperatures (about 550 degrees Celsius), which presents technical challenges.
An alternative approach pursued by Jun Ye's group involves converting the wavelength of a more conventional optical laser to the required ultraviolet range using specialized crystals. Preliminary tests have yielded a stable output of about 40 microwatts, and although the exact crystal material remains undisclosed, Ye described it as "tremendously promising." His team collaborates with IPG Photonics, a Massachusetts-based laser manufacturer that holds a patent related to growing specialized strontium tetraborate crystals for this purpose. Despite these advances, no definitive solution for producing the ideal laser yet exists, but experts remain confident the technical hurdles will be overcome.
Beyond the laser, another critical challenge lies in sourcing and handling thorium-229. Because 229Th is radioactive and rare, researchers must devise ways to incorporate it into the clock's architecture effectively. Two main strategies have emerged: embedding trillions of thorium ions into a solid crystal matrix or isolating and trapping a handful of thorium ions individually using ion traps.
The solid-state approach benefits from a stronger clock signal due to the vast number of nuclei involved, but it faces stability issues. Specifically, researchers require a narrow linewidth for the nuclear transition - meaning the signal's frequency range must be tightly confined to maintain accuracy. So far, using calcium fluoride crystals doped with 229Th ions, Jun Ye's group has recorded a linewidth of about 30 kilohertz, which is too broad for a stable nuclear clock. The broadening may result from impurities within the crystal matrix, prompting scientists to investigate alternative materials. Thorium tetrafluoride and thorium oxide, once used as coatings in camera lenses, are among the promising candidates due to their potentially cleaner crystal environments.
Conversely, ion trap experiments offer the possibility of much higher precision by isolating and cooling individual thorium ions to temperatures near a millionth of a degree above absolute zero. This environment can minimize disturbances that broaden the transition linewidth, thereby allowing for extremely stable frequency measurements. However, trapping and manipulating 229Th ions remain technically challenging, and no team has yet demonstrated a nuclear clock using this method. Nonetheless, experts at the APS meeting expressed confidence that this milestone is achievable in the near future.
The potential impact of nuclear clocks is profound. Currently, the most advanced optical atomic clocks lose only about one second every 40 billion years, an extraordinary feat. Nuclear clocks promise to surpass this precision by orders of magnitude and offer greater resilience to environmental noise. Moreover, they could be made compact and robust enough for deployment outside specialized laboratories, opening possibilities for improved navigation systems, synchronization of telecommunications, tests of fundamental physical constants, and even exploration of physics beyond the Standard Model.
Claire Cramer, executive director of quantum science at the University of California, Berkeley, attending the APS summit, emphasized the promise of solid-state nuclear clocks for commercial applications. Their expected durability and compactness could revolutionize industries requiring ultra-precise timing.
While the realization of a fully operational nuclear clock still faces significant technical obstacles, the pace of progress and recent breakthroughs suggest it may arrive sooner than many anticipate. Eric Hudson, a physicist at the University of California, Los Angeles, involved in nuclear clock research, confidently predicts that measurable nuclear clock demonstrations will emerge in 2026.
In sum, the development of nuclear clocks represents a landmark advancement in the science of timekeeping. By harnessing nuclear energy transitions in thorium-229, scientists aim to create clocks of unparalleled precision and stability, with the potential to impact a wide array of scientific and technological fields. As laser technology improves and sources of thorium-229 are refined, the era of nuclear clocks may soon begin, offering a new standard in how humanity measures time.
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This summary is based on a report first published on March 20, 2026, by Dan Garisto for Nature magazine. The original article highlights ongoing efforts and challenges in creating the world's first nuclear clock and outlines the significance of this emerging technology.