Singapore Physicists Say Their Single-Ion Lutetium Clock Is the Most Accurate Ever Built

Physicists in Singapore say they have built the most accurate clock in the world. It is an optical atomic clock that uses a single charged atom of lutetium, an element no other group uses for timekeeping.

The team at the Centre for Quantum Technologies at the National University of Singapore reported in Nature on Sept. 23 that it measured its clock's frequency to 19 decimal places, with an uncertainty of 1 × 10⁻¹⁹. That is the lowest uncertainty reported for any optical atomic clock. The researchers also built a second clock and compared the two over 200 hours using a technique called correlation spectroscopy. The pair agreed to within 5.7 × 10⁻¹⁹, which the team says is the most precise comparison between two clocks ever made.

"I am confident that what we have now is the most accurate clock in the world," said team leader Murray Barrett, an associate professor of physics at the National University of Singapore. His group started working with lutetium more than a decade ago on a hunch that it had the right properties to compete with the best clocks.

An atomic clock keeps time by tying a laser to a "clock transition," the fixed frequency at which one of an atom's electrons jumps between energy levels. The light's rapid oscillations work like a pendulum's swings. Cesium clocks have defined the second since the 1960s and underpin GPS and global communications networks. Optical clocks built from ytterbium, strontium and aluminum tick far faster than cesium and have held recent accuracy records. International timekeeping authorities are weighing data from these newer clocks toward a redefinition of the second expected in or after 2030.

Lutetium's edge comes from the atom itself. Its clock transition barely shifts with changes in temperature or magnetic field, two of the main sources of error in other clocks. Each Singapore clock holds a single lutetium-176 ion whose transition is matched to a laser with a wavelength of 848 nanometers. The team spent years on precision engineering and developed a method it calls "hyperfine averaging" to define the transition cleanly. "The lutetium clock would be stable even if you went from the hottest place recorded on Earth in Death Valley to the coldest place in the Antarctic plateau," Barrett said. "In the future, I just don't see how this clock can be beat."

The two-clock test is what gives the team confidence. "There is a humorous saying that 'A man with a watch knows what time it is. A man with two watches is never sure,'" said Kyle Arnold, a senior research scientist and joint first author of the paper. "It basically tells you that the only way to test the accuracy of a standard is to compare clocks and demonstrate reproducibility."

Clocks this good do more than keep time. Because gravity slows time slightly, ultra-precise clocks can detect tiny height and mass changes across Earth's surface. They also let physicists test whether the constants of nature truly stay constant, and they can hunt for signs of dark matter and other new physics. The researchers say more measurements could push the uncertainty lower still.