These Atomic Clocks Are Accurate Enough To Help Redefine Time Itself

Experiment Could Rewrite the Definition of a Second

Two atomic clocks built independently in the same lab just agreed with each other to within roughly 5.7 parts in ten quintillion, a gap so small it is statistically indistinguishable from zero. That match is one of the strongest experimental checks yet of a clock’s claimed accuracy.

Researchers at the Centre for Quantum Technologies at the National University of Singapore, publishing their results in the journal Nature, built the two clocks using a charged form of the element lutetium, a heavy metal. Each clock on its own reached an accuracy near one part in ten quintillion, and the team ran the two side by side for 200 hours before spotting that near-perfect match.

This might sound like a lab curiosity, but its reach goes well beyond the building where it was tested. Clocks this exact could reshape how the world defines a second, help scientists map the true shape of the Earth to fractions of an inch, and test whether the basic laws of physics, including Einstein’s theories, hold up under the toughest scrutiny yet.

Twin Clocks Confirm the Atomic Clock Precision Claims

Most of the world’s top atomic clocks are built from different atoms, such as strontium, ytterbium, or aluminum. Comparing these designs is like comparing a mechanical watch to a digital one: any mismatch could be blamed on the technologies rather than a flaw in either clock. Building two copies of the same clock removes that excuse, since matching results at an extreme predicted accuracy is strong proof the science is sound. That’s what the team did, building two separate setups around a single trapped, charged lutetium atom each, running both through an exhaustive battery of tests for every known way a clock’s rate could drift, then comparing them directly.

An atomic clock keeps time by tracking the natural vibration rate of atoms, something like an extremely steady pendulum. Magnetic fields, heat, and stray electric fields can nudge that rate off, so scientists must track down every such disturbance and calculate how much it throws off the ticking. A trustworthy check needs three things: a full list of known effects backed by real measurements rather than assumptions, agreement between two identical clocks within a pre-calculated margin of error, and the ability to dial a major disturbance up or down and watch the rate shift by exactly the predicted amount.

This lutetium clock has a built-in edge: its key atomic transition resists heat and magnetic fields better than other leading designs, and its large atomic mass makes its ticking rate less sensitive to motion of the trapped ion. That combination let the team reach this precision using equipment that runs at normal room temperature, rather than in an exotic supercooled chamber.

Two Lutetium Clocks Matched to Within 5.7 Parts in Ten Quintillion

To check the two clocks against each other, researchers pinged both with laser pulses from a shared source, then compared how the atoms in each responded. This cancels out most of the static coming from the shared laser itself, making the comparison far more sensitive to real differences between the two clocks. Observing the atoms for up to 10 seconds at a stretch, a long window in this field, let them pick out extremely faint differences: the resulting gap came out consistent with zero once every source of error was accounted for.

Researchers also stress-tested the biggest source of error: a magnetic field effect that can throw off a clock’s rate. They measured it in 2024 and again in 2025, both readings taken after a major lab renovation and the replacement of one ion trap, and got matching results each time, the check needed before trusting a clock’s stated accuracy.

This Atomic Clock Precision Could Redefine the Second

Right now, the second’s definition rests on an older clock built around microwave signals. Optical clocks, which rely on light instead, have grown so much more accurate that scientists are working toward switching to one, but competing designs must first demonstrate accuracy directly, through tests like this one, rather than relying on precision alone.

Beyond timekeeping, clocks this sensitive can detect tiny gravity differences. Under Einstein’s theory of gravity, time runs at slightly different rates depending on elevation, so comparing two such clocks in different locations can reveal height differences down to millimeters, a new tool for mapping Earth’s true shape.

These clocks could also help physicists search for cracks in the current picture of the universe: a break in Einstein’s relativity, a hint of dark matter, or a shift in the constants that govern atoms and forces. Any such discovery would first need to rule out flaws in the clocks themselves, the kind of checking demonstrated here. Comparing clocks across different labs or countries remains harder, partly because gravitational differences between distant sites aren’t known precisely enough, corresponding to uncertainties of a few centimeters in equivalent height. For now, this accuracy requires two clocks together, built from the same atom.

What stands out is that the researchers could prove their claimed precision, through a comparison rigorous enough to leave little room for doubt. Two independently built machines agreed almost perfectly, an outcome that doesn’t happen by accident or without years of unglamorous checking. For a field racing toward redefining one of science’s most basic units, that proof is what matters most.

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