The story begins with a very special property that physicists had long suspected thorium atomic nuclei to possess. Most atomic nuclei have energy states that are separated by enormous gaps, far too large to be bridged by laser light, which is why nuclei have never been usable for clockwork. Thorium-229 is the exception. It has two different energy states whose energies lie extremely close together, separated by an exceptionally small gap. Because of this, it is possible to use a laser to deliberately switch the atomic nucleus from one state to the other, exciting it in a controlled and repeatable way. This near-degenerate pair of states is precisely what makes thorium an ideal tool for extremely precise measurements, and it is the reason researchers have spent so many years searching for and characterizing this unique nuclear transition.

The breakthrough came in stages. In April 2024, the team led by Prof. Thorsten Schumm at the Institute of Atomic and Subatomic Physics at TU Wien, together with the team led by Prof. Ekkehard Peik at PTB Braunschweig, succeeded for the first time in finding this long-suspected nuclear transition, showing that thorium nuclei can be excited with laser beams. Later that same year, in the autumn, the team demonstrated that the discovery could indeed be used to build a high-precision clock. In that first demonstration, the thorium nucleus excitation apparatus was coupled to a conventional optical atomic clock, and the thorium nuclei served as a timekeeper only with the help of that external reference. It was a proof of principle, but not yet the self-contained instrument that researchers had envisioned.

The crucial difference lies in the ability to self-stabilize. As Prof. Thorsten Schumm explains, what you really want is a self-stabilizing nuclear clock. The basic idea, he notes, is simple: you have a laser and you have thorium. The laser changes the energy state of the thorium nuclei, and the thorium nuclei are used to stabilize the frequency of the laser. In other words, the clock’s two essential components, an oscillator and a reference, are contained within a single closed system. The laser provides the ticking, and the nuclei provide the discipline that keeps the ticking honest. This closed feedback loop is what elevates the Vienna prototype from a laboratory demonstration into the first true stand-alone nuclear clock.

At the heart of the new clock is a crystal containing thorium atoms, produced at TU Wien, which is irradiated with a laser. The oscillation of that laser light is what can be used for timekeeping, but a laser frequency is never perfectly fixed. It can shift slightly from time to time, for example due to temperature fluctuations, and for high-precision measurements even the smallest drift matters. A mechanism is therefore needed to keep the laser frequency exactly stable so that the clock continues to tick with precisely the same rhythm. In ordinary atomic clocks, atoms and the energy states of their electrons perform this stabilizing role. Schumm’s team has now replaced them with thorium nuclei, exploiting the nuclear transition as the frequency reference in a continuously operating feedback system.

The mechanism works because the thorium nuclei absorb laser light only when the laser frequency is exactly right. If the frequency drifts even slightly away from the optimum value, the absorption decreases measurably. The system detects this reduction and automatically readjusts the laser frequency, locking it back onto the nuclear transition so that the clock continues to tick precisely. This is the same principle that underlies the operation of optical atomic clocks, but here the reference is a transition inside the atomic nucleus rather than in the electron shell. The result is the first self-regulating nuclear clock, one that no longer necessarily has to rely on a conventional atomic clock to keep itself in check. The Vienna instrument is, in the most literal sense, a clock that keeps its own time.

The advantage of using atomic nuclei rather than atoms is fundamental. Atomic nuclei are more than ten thousand times smaller than the atoms that contain them, and because of their compactness they react much more weakly to external disturbances such as stray electric and magnetic fields. A timekeeper that is largely indifferent to its environment is a more reliable timekeeper, and in principle much higher precision is possible with nuclei than with the electron-based transitions used in today’s atomic clocks. This is the promise that has driven the field for decades: a clock whose accuracy is limited not by environmental noise but by the intrinsic quality of the nuclear reference itself, opening the door to a new kind of high-performance metrology in which a wide range of physical quantities can be measured with previously unattainable precision.

The precision of the new nuclear clock was investigated over the course of a full day and was found to be approximately 10 to the power of minus 15. In practical terms, that corresponds to an error of roughly one second in 30 million years. Schumm is candid about where the prototype stands relative to the state of the art. This is not yet at the level of the world’s best optical atomic clocks, he says, but for a first prototype it is a fantastic result. The comparison is instructive: the best optical atomic clocks are the product of decades of refinement, while the nuclear clock has existed in a self-stabilizing form for only a short time. Reaching one part in ten to the fifteen within the first generation of the technology is a strong indication of how much headroom remains.

That headroom is exactly where the team’s attention now turns. The precision is to be drastically improved through several measures, including stronger lasers and better thorium crystals, the two components that currently limit how much signal the feedback loop can extract from the nuclei. More powerful excitation lasers would strengthen the interaction with the thorium nuclei, while improved crystals would provide a cleaner and more homogeneous environment for the embedded atoms, reducing the disturbances that currently blur the nuclear transition. Each improvement feeds directly into the stability of the locked laser and therefore into the accuracy of the clock itself. The path from the current prototype to a record-setting instrument is, in principle, clearly mapped.

The significance of the Vienna result extends well beyond timekeeping. Published in the journal Nature under the title A thorium-229 optical nuclear clock with feedback loop, the work establishes nuclear timekeeping as an independent discipline within precision metrology. A clock based on a nuclear transition is sensitive to physics in a different way than any electron-based clock, which means that comparisons between nuclear and atomic clocks could probe whether the fundamental constants of nature change over time, and could enable measurements of quantities that no existing instrument can resolve. For now, the prototype in Vienna is ticking steadily, holding its laser to the rhythm of the thorium nucleus hour after hour without intervention. It is the first of its kind, and if the promised improvements deliver, it will not remain the most precise of its kind for long.

Subject of Research: Development of the first self-stabilizing thorium-229 nuclear clock

Article Title: The first stand-alone nuclear clock is ticking in Vienna

Article References: The first stand-alone nuclear clock is ticking in Vienna. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: nuclear clock, thorium-229, TU Wien, atomic clocks, precision metrology, laser stabilization, nuclear transition, Thorsten Schumm, Nature, timekeeping, optical clock, thorium crystal