Science

The Lutetium-176 atomic clock is a record-breaker. But the Thorium-229 one is hot on its heels

New technologies that use rare isotopes to measure time with unprecedented precision are opening up new frontiers in science

by Leopoldo Benacchio

18 October 2026

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6' min read

Translated by AI

Versione italiana

Key points

From the pendulum to the laser
The solution: the atom as a sort of tuning fork
What is the point of such extreme precision
Another technology is making headway between Vienna and Beijing

6' min read

Translated by AI

Versione italiana

‘Wait a second’, ‘I’ll be there in a second’. We all say it, even when we know full well that it will take a little longer. In everyday language, a ‘second’ has come to mean a very brief interval. In science, however, it is a unit of measurement that must be defined and reproduced with the utmost rigour. It is the final stage in a story that began with the observation of the alternation of day and night and the movement of the stars: the search for a regular rhythm by which to organise our lives.

Today, this research also involves a little-known chemical element, Lutetium. Some of its isotopes – different forms in which the element’s atom occurs – are used in nuclear medicine; whilst another, lutetium-176, is at the heart of the new optical clock developed at the Centre for Quantum Technologies at the National University of Singapore. This new record-breaking atomic clock has achieved a relative uncertainty of approximately 1.2 parts per ten billion billion: to put this into perspective – assuming we can even imagine it – this is equivalent to a deviation of just one second in around 260 billion years. It is clear that this is not a verified operating lifespan, but rather a way of expressing an uncertainty estimate in a more visualised form. It is currently the best of the best; indeed, the authors – perhaps exaggerating slightly – claim that it cannot be improved upon. We shall see.

From the pendulum to the laser

To understand how it works, we can start with the pendulum. A watch requires a phenomenon that repeats with great regularity and a system to count those repetitions. As precision increases, in a pendulum the mechanical oscillations – which are, as far as possible, always identical – are counted; in mechanical watches, even very expensive ones, the harmonic motion of a balance wheel is used, whilst in quartz watches, even those costing just a few tens of euros, the frequencies of the quartz crystal itself are utilised. The new instrument from Singapore uses laser light, the rhythm of which is continuously controlled thanks to the properties of a single atom.

More precisely, it is an ion: in other words, it consists of a lutetium atom from which an outer electron has been removed, causing it to acquire a positive charge. It is suspended in an electromagnetic trap using electric fields, without coming into contact with any surface. Lasers and control systems operate around this microscopic component: as we have already realised, this is a laboratory instrument, not a device to be worn on the wrist.

Il cristallo di Torio-229 e il laser nell’orologio nucleare

The solution: the atom as a sort of tuning fork

The electrons in an atom can only occupy certain energy levels, like the steps on a long staircase. To move from one to another, they must absorb just the right amount of energy, which corresponds to a specific frequency of light. The researchers tune the laser to the chosen transition and use the ion’s response to fine-tune it; this is the solution: the atom acts as a sort of tuning fork with which to tune the instrument. It is the oscillations of light, not the movements of the atom, that mark the time! In the case of lutetium, the light used is in the near-infrared region, at a wavelength of approximately 848 nanometres. It sounds complex, but conceptually it isn’t: we isolate an electron from its atom; the atom becomes positively charged and is no longer neutral. We then trap it in a special cage, shine a laser beam on it and determine what frequency is needed to make the electron vibrate as it moves from one energy level to another.

Lutetium is an interesting element to use because the system is relatively insensitive to external disturbances, including the effects of ambient temperature and magnetic fields, which are the real bane of these devices.

It is important because the problem to be solved has two aspects: finding an extremely fast ‘pendulum’ – in this case, the electron as it jumps between one energy level and another – but also identifying and controlling anything that might alter its rhythm. To test its performance, the Singapore-based team compared two clocks for over two hundred hours.

What is the point of such extreme precision

One might quite rightly ask: what is the point of such precision? It is, quite frankly, understandable; the point is that time measurement is now used in a myriad of applications, for example to measure distances: systems such as GPS and the European Galileo determine position by comparing the travel times of signals from the satellites. One billionth of a second corresponds to about thirty centimetres in the path of light; let’s do the maths and we’ll find that with an error of one thousandth of a second, we’d go completely off course!! Atomic clocks are also essential for keeping the artificial satellites in orbit that we rely on 24 hours a day in all sorts of ways, from telecommunications and the internet to financial transactions.

That is true, but what is the point of these time-measuring devices? First and foremost, they are used to develop new technologies and enable us to carry out measurements that were previously impossible; in doing so, they help us to venture into uncharted territory – in this case, the realm of time itself – to understand its nature, if that is ever possible.

Torio 229, il laser riesce a leggere il nucleo gli orologi nucleari

One of the most surprising facts concerns the Earth. According to Einstein’s theory of relativity, time does not pass at the same rate everywhere: a clock situated higher up on Earth runs slightly faster. By comparing sufficiently accurate instruments, differences in altitude of the order of a centimetre can be detected. Incredible. The chronometer thus also becomes a tool for studying the Earth’s gravitational field, comparing the altitudes of distant locations and, in the long term, monitoring changes in the landscape.

Then there is fundamental research. Comparing clocks based on different atoms makes it possible to look for any variations in the constants of nature and for signals predicted by certain hypotheses regarding dark matter which, if it exists, may alter atomic clocks in an imperceptible but measurable way. The ultimate goal of these highly sophisticated technologies, one might say, is greater precision, not a different concept of time.

Another technology is making headway between Vienna and Beijing

Meanwhile, between Vienna and Beijing, another technology is gaining ground. Two independent research teams have unveiled new clocks that derive their reference not from electron transitions, but from those of the atomic nucleus. The findings, published in Nature on 7 October, mark an important step forward in the development of the nuclear clock. Note that ‘nuclear’ here refers to the atom’s nucleus, not the entire atom; and we know that the atom, in its infinitesimal size, is much larger than its own nucleus – at least 100,000 times larger.

This time, the star of the show is Torio-229. Its nucleus possesses an exceptional property: an energy transition low enough to be excited by ultraviolet light, whereas nuclear transitions normally require much greater energies. The principle remains the same: tuning a laser to a precise frequency and keeping it locked to the reference. The ‘conductor’ has changed: it is no longer the electrons, but the nucleus.

The nucleus is also much less affected by certain external disturbances that disrupt the electrons. This allows for a particularly interesting solution: in the new instruments , thorium is incorporated into calcium fluoride crystals . In the Viennese device, the crystal measures just a few millimetres and operates at room temperature. Rather than measuring a single atom missing an electron – the ion suspended in a vacuum that we mentioned earlier – it is possible to detect the signal from a vast number of nuclei contained within a small solid object.

Let’s not mislead watch enthusiasts: this does not mean that the whole thing, including the ‘hands’, fits in a pocket. Power supplies, lasers, control and communication instruments remain complex and essential, but, as always, the aim is to produce compact, robust instruments that can also be used outside the laboratory. The potential for greater simplicity paves the way for this. However, it must be added that the compactness and simplicity of nuclear watches are, in part, aspirations rather than characteristics already achieved by the entire system; nevertheless, the hope of achieving them is well-founded.

For the time being, nuclear prototypes are still a long way from matching the performance of the best optical atomic clocks. For the Viennese prototype, researchers report a stability which, after a day of measurement, is close to one part in a million billion – a figure that is already unimaginable to us mere mortals, but certainly very low compared to the atomic clock we mentioned earlier. In short, the thorium nucleus has begun its pursuit of the lutetium atom.