In a remarkable stride for precision measurement and fundamental physics, an international team led by researchers at TU Wien has uncovered groundbreaking details about the thorium-229 nuclear transition, a development that unlocks vast potential not only for innovative timekeeping but also for probing the immutable laws of nature. This breakthrough, publicized in a recent Nature Communications article, substantiates the prospect of using thorium atomic nuclei as ultra-precise nuclear clocks, surpassing the sensitivity of existing methods by several orders of magnitude. The achievement heralds a new era where the foundational constants of physics, long assumed fixed, might finally be scrutinized with unprecedented acuity.
For decades, physicists have speculated about the use of certain atomic nuclei in high-precision experiments that could challenge and extend our understanding of fundamental physics. Thorium-229, an isotope with a uniquely low-energy nuclear excited state, emerged as a promising candidate for such measurements. However, the precise nature and measurement of its nuclear transition remained elusive until the TU Wien group’s decisive discovery in 2024, which definitively identified this long-theorized transition. The ability to manipulate and measure the thorium nuclear states opened new avenues for crafting exotic nuclear clocks with remarkable stability and accuracy.
What sets the thorium-229 nuclear transition apart is its direct dependence on subtle changes in the nucleus’s shape and electromagnetic properties. When the thorium nucleus shifts from its ground state to its excited state, it undergoes a slight reshaping from a more spherical form to a subtly elongated elliptical shape. This shape deformation alters the distribution of protons within the nucleus, thereby modulating the electric field it produces. The sensitivity of this electric field’s shape, particularly its quadrupole moment, to the fine-structure constant makes the thorium nucleus an extraordinary probe for detecting potential variations in fundamental constants.
The fine-structure constant, approximately valued at 1/137, encapsulates the intrinsic strength of electromagnetic interactions. It governs how charged particles engage, dictates the nature of chemical bonding, and influences the interaction between light and matter. Conventionally, physicists have assumed this constant remains invariant across time and space, forming a cornerstone of modern physics. However, theoretical models allowing for minute, slow drifts or even periodic oscillations in this constant could revolutionize our conceptions of fundamental physics and cosmology. Detecting such variations demands instruments of unparalleled sensitivity—tools that the thorium nuclear clock now promises to deliver.
The experimental methodology hinges on leveraging the thorium-containing crystals meticulously fabricated at TU Wien. These crystals embed thorium-229 atoms within a solid lattice, stabilizing them for laser spectroscopy measurements. Conducted in conjunction with international partners in Boulder, Colorado, the spectroscopy experiments assess minute changes in nuclear energy levels and their corresponding electric fields. By monitoring the transition with exquisite precision, the researchers could discern variations in the quadrupole moment of the nuclear electric field and thereby infer changes in the fine-structure constant with significantly enhanced sensitivity.
This monumental advance in measurement precision—three orders of magnitude, or roughly a factor of six thousand, greater than established methods—affords physicists a new window into the constancy of nature’s fundamental parameters. It could potentially reveal dynamic changes in the fine-structure constant, reshaping our understanding of forces that govern the universe. Beyond fundamental physics, this mechanistic understanding of the thorium nucleus’s behavior and electric field variations forms the technical foundation for developing nuclear clocks that outperform the best atomic clocks based on electron transitions.
The development of nuclear clocks utilizing thorium-229’s unique transition represents a paradigm shift in metrology. Unlike electron-based atomic clocks, these nuclear clocks rely on transitions deep within the nucleus rather than the electron cloud, which provides inherently superior stability against environmental disturbances such as magnetic and electric field fluctuations. This renders the thorium clock particularly suited for tests of fundamental constants, gravitational effects, and even dark matter detection, where minute shifts in measurement standards are critical.
Prof. Thorsten Schumm, leading the research at the Institute of Atomic and Subatomic Physics at TU Wien, highlights the exquisite interplay between nuclear structure and fundamental constants. By measuring not just the energy difference between nuclear states but analyzing the accompanying subtle changes in the nuclear electric field geometry, the team can sensitively probe whether constants like the fine-structure constant truly hold universal invariance or fluctuate over time and space.
The international collaboration exemplifies the synergy between material science, quantum optics, and nuclear physics. The material scientists’ expertise in growing thorium-doped crystals with exacting purity and lattice structure enables stable spectroscopic interrogation. Laser physicists in Boulder deploy ultra-stable lasers that interact coherently with the nuclear transition, facilitating fine-tuned resonance measurements. Together, this alliance harnesses the nucleus’s nuclear properties in ways unimaginable until now, culminating in a measurement technique sensitive enough to test the constancy of electromagnetic interaction strength to unprecedented precision.
Beyond offering profound insights into fundamental physics, these thorium nuclear clocks have vast practical implications. Ultra-stable clocks underpin global positioning systems, telecommunications, and synchronization across scientific infrastructures. Advancements in clock precision ripple through technologies, enabling more accurate geodesy, improved navigation systems, and refined tests of general relativity. The nuclear clock’s capabilities may soon allow the detection of gravitational waves or exotic physics phenomena that subtly perturb spacetime or fundamental constants.
This research also opens potential pathways towards new physics that have remained experimentally inaccessible. If future measurements indicate variations in the fine-structure constant, it could suggest physics beyond the Standard Model, hinting at dynamic scalar fields or interactions coupling to fundamental forces. Such results could offer empirical footholds for theories uniting gravity with quantum mechanics or shed light on dark energy and dark matter’s nature.
Essentially, thorium-229’s nuclear transition provides a unique quantum system where nuclear physics meets precision metrology and cosmology. The method’s sensitivity to the fine-structure constant encourages profound reflections on whether the laws of physics remain constant or evolve over cosmic epochs. This work exemplifies the power of combining state-of-the-art experimental physics with advanced nuclear theory to probe the deepest questions about our universe’s fabric.
With the first brilliant demonstrations already underway, the scientific community eagerly anticipates further explorations leveraging thorium nuclear clocks. Enhanced measurement campaigns will refine constraints on temporal or spatial variances of fundamental constants and could transform how we conceive natural laws’ universality. In parallel, optimizing nuclear clock designs may soon yield compact, portable devices with transformative applications in navigation, communication networks, and fundamental science.
This discovery stands as a testament to innovative physics research’s vast potential, marrying atomic and nuclear physics with precision laser spectroscopy and crystal growth techniques. TU Wien’s leadership in this domain heralds an exciting future where atomic nuclei illuminate the dark corners of cosmology, particle physics, and the eternal quest to understand whether the universe’s rules themselves are indeed constant or subtly shifting beneath our gaze.
Subject of Research: Fine-structure constant and thorium-229 nuclear clock transition
Article Title: Fine-structure constant sensitivity of the Th-229 nuclear clock transition
News Publication Date: 15-Oct-2025
Web References: 10.1038/s41467-025-64191-7
Image Credits: TU Wien
Keywords
Atomic clocks, Metrology, Basic research, Physics, Experimental physics, Quantum mechanics
