This atomic clock is so precise it can detect how gravity changes time. Scientists now want to take it outside the lab and measure tiny shifts across Earth

Just a couple of millimetres of difference in height can be measured due to increased accuracy of the clock.

This is the precision that was achieved by scientists at Centre for Quantum Technologies (CQT) in Singapore. They have created a clock based on lutetium atoms, which reportedly has a frequency uncertainty of only 1 × 10⁻¹⁹.

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As published in Nature, this clock belongs to the group of the most precise optical atomic clocks.

In addition, they were able to compare two of their clocks and find agreement at 5.7 × 10⁻¹⁹ uncertainty, which was stated to be the most precise clock comparison made so far.

Here, the atomic clock starts to become sensitive to very small variations in gravitational potential.

The clock that is situated even slightly higher in altitude will have a small variation in the flow of time relative to the clock that is situated lower in altitude.

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According to the Singapore team, their two atomic clocks were sufficiently sensitive to detect a difference in height of 5 millimetres.

The researchers now wish to take things out of the laboratory environment and design a portable device that can be used for further experiments on gravity. This may help in making the highly accurate clocks useful tools for the study of gravity and basic physics.

Why scientists needed something better than a traditional clock

The world's standard for measuring time has historically been based on caesium atoms.

The functioning of atomic clocks depends on the highly stable nature of energy transitions in atoms.

As the electron transitions from one energy level to another, there is interaction with electromagnetic waves at certain frequencies. The scientist can tune the laser to achieve this transition and utilise the oscillations as the most accurate standard for time measurement.

Since the 1960s, caesium clocks have been used internationally as the basis of a second. The accuracy of these clocks has contributed to different technologies such as navigation and telecommunications.

But scientists have spent decades looking for ways to improve on that standard.

Newer optical atomic clocks use transitions that occur at much higher frequencies than those used by cesium clocks. Because the oscillations happen more rapidly, researchers can measure time intervals with greater precision.

Strontium, ytterbium and aluminium have all been used in some of the world's leading optical clocks.

The push is now becoming relevant to the future of the international time system.

Scientists are considering whether optical atomic clocks should eventually replace the current cesium-based definition of the second, with a redefinition being considered for 2030 or later.

The Singapore team's work adds lutetium to that competition.

What makes lutetium so useful for measuring time

The CQT researchers began investigating lutetium more than a decade ago.

Their interest comes from properties of the atom that make it unusually resistant to environmental disturbances that can affect precision measurements.

Temperature and magnetic fields can slightly alter the frequency of an atomic transition. If the reference frequency shifts, the clock can lose accuracy.

The lutetium system is comparatively insensitive to these effects.

The researchers developed a technique called hyperfine averaging to define the clock transition while reducing the influence of environmental effects.

That matters because an extremely accurate clock cannot simply be accurate under carefully controlled laboratory conditions. Researchers ultimately want a clock that can maintain its performance when exposed to changes in its surroundings.

The CQT team says the properties of lutetium could allow high accuracy across a relatively broad range of environments.

The clock uses a single charged lutetium-176 ion (¹⁷⁶Lu⁺). Its clock transition is interrogated with a laser operating at a wavelength of 848 nanometres.

The researchers measured the frequency of the transition to 19 decimal places and reported an uncertainty of 1 × 10⁻¹⁹.

That does not mean the clock is simply displaying 19 useful digits on a screen. The figure describes the uncertainty in the measured frequency, showing how closely researchers can determine the clock transition.

The strange point where gravity starts changing the clock

However, the most surprising thing in this experiment will be the effect that occurs when two clocks are placed at varying altitudes.

According to Einstein's theory of relativity, there is an effect that gravity has on the speed of time.

A clock placed in an area with a strong gravity field will behave differently from another clock placed further away from the gravitational pull.

However, for ordinary clocks, the effects are so tiny as to be noticed.

For optical atomic clocks operating at the 10⁻¹⁹ precision level, it becomes measurable.

The Singapore researchers compared two lutetium clocks for about 200 hours using a technique called correlation spectroscopy.

Both of the clocks were placed on the same lab bench. They were so precise that scientists had to take into account even a few-millimetre difference in height.

Scientists claim that this measurement will be able to determine the height difference of 5 millimetres between both clocks.

To make sure that this physical height difference did not compromise the comparison, the researchers independently measured the relative positions of the lutetium ions to within about a millimetre.

This illustrates an unusual problem created by increasingly accurate clocks: Earth's gravity is no longer merely an environmental nuisance. It becomes part of what the clocks can measure.

That opens another scientific possibility.

If a clock can detect tiny differences in gravitational potential, networks of such clocks could potentially provide new information about Earth's surface and changes occurring beneath it.

From a laboratory instrument to a portable gravity sensor

The next challenge is mobility.

The Singapore clock currently occupies a laboratory-scale setup. To compare it with other high-performance clocks or use it for measurements in different locations, researchers need to make the system smaller and easier to transport.

The team plans to develop a transportable lutetium clock while attempting to preserve its exceptional accuracy.

That could make comparisons possible in environments where the gravitational difference between locations becomes scientifically useful.

The applications are not limited to geophysics. Ultra-precise optical clocks could help scientists investigate fundamental questions in physics, including tests of theories describing gravity and the behaviour of matter.

They could also contribute to the international effort to establish a future optical definition of the second.

But there is an important practical limitation.

Comparing clocks this precise across different parts of Earth is difficult because gravitational differences must themselves be known with extraordinary accuracy.

The Singapore researchers note that existing knowledge of Earth's gravitational field is not yet precise enough everywhere to make comparisons at the 10⁻¹⁹ level straightforward.

That is one reason making the clock transportable matters.

Instead of bringing two clocks into the same laboratory, researchers could eventually move the same high-performance system to different locations and compare measurements under controlled conditions.

The work also highlights how the meaning of “keeping time” has changed in modern physics.

The earliest clocks were designed to divide a day into smaller intervals.

Modern atomic clocks use the behaviour of individual atoms as a reference. At the frontier of precision, those clocks are sensitive enough to detect effects predicted by relativity and subtle differences in Earth's gravitational environment.

The Singapore team's lutetium clock is another step in that progression.

Whether it remains the world's most accurate clock for long is another question. The field is moving quickly, and competing groups are continually improving their optical clock systems.

For now, however, the achievement demonstrates just how far atomic timekeeping has advanced—and why the world's best clocks are increasingly becoming scientific sensors rather than devices built simply to tell us what time it is.

FAQ

1. How accurate is the new lutetium atomic clock?
Researchers at Singapore's Centre for Quantum Technologies measured its clock transition with a reported uncertainty of 1 × 10⁻¹⁹. A comparison between two lutetium clocks reached an uncertainty of 5.7 × 10⁻¹⁹.

2. How does an atomic clock measure gravity?

Based on Einstein’s theory of relativity, gravity influences how fast time moves. Atomic clocks that measure extremely accurately can measure changes in the rate of time movement due to different gravitational potentials. The Singapore experiment was able to distinguish a height difference of about 5 mm.

3. Why did scientists choose lutetium?
Lutetium has atomic properties that make its clock transition relatively resistant to environmental effects such as temperature and magnetic-field changes. The researchers also developed a technique called hyperfine averaging to improve the stability of the clock transition.

4. What could this atomic clock be used for?
Beyond precision timekeeping, future versions could help study gravity, test fundamental physics and make sensitive measurements of Earth's gravitational environment. The researchers also hope to develop a transportable version and contribute to the possible future redefinition of the second.