A lutetium ion clock in Singapore reports the lowest uncertainty yet for an optical clock

The result rests on two numbers that measure different things. One part in 10^19 is the uncertainty the Centre for Quantum Technologies reports for a single lutetium clock, whose frequency the team measured to 19 decimal places, and the paper presents it as the lowest reported for any optical atomic clock ^[3]. 5.7 parts in 10^19 is how closely two of those clocks agreed with each other ^[4]. The agreement figure is 5.7 times the single-clock uncertainty ^[19], and the team says more measurements could bring it lower ^[4]. The comparison ran for 200 hours, a little over eight days of averaging ^[5][20].

That comparison tests reproducibility between two setups built by the same group, from the same element, on the same table ^[4]. It does not test lutetium against ytterbium, strontium or aluminum, the elements that held the accuracy record before it ^[12]. The team would like to make that comparison. According to the phys.org account, the obstacle is gravity: at the 10^-19 level an optical clock detects the slowing of time across height differences of millimetres ^[17]. The Singapore measurement could resolve a 5 mm height difference between its own two clocks ^[18].

Lutetium's advantage is insensitivity. Its clock transition is hardly affected by changes in temperature or magnetic field, the environmental factors that slightly vary the transition frequency in other elements ^[7]. Each clock holds a single charged 176Lu+ ion with its clock transition matched to a laser at 848 nanometres ^[6], and the group invented a scheme it calls hyperfine averaging to define that transition ^[8]. "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," said Murray Barrett, the team leader and a principal investigator at CQT ^[9][2].

Cesium has set the global standard for time since the 1960s, and cesium clocks still support GPS and synchronise communication and transport networks ^[11]. The international body responsible for time standards is considering data from the new optical clocks toward a redefinition of the second expected in or after 2030 ^[13]. CQT started on lutetium more than a decade ago on a hunch that the atom had the right properties ^[14].

Kyle Arnold, a senior research scientist at CQT and joint first author, stated the test the field applies. "It basically tells you that the only way to test the accuracy of a standard is to compare clocks and demonstrate reproducibility," he said ^[16]. By that standard, two clocks in one laboratory is where the test begins. Barrett said, "I am confident that what we have now is the most accurate clock in the world" ^[2], and to the group's knowledge no one else is working with the element for timekeeping ^[14].