Singapore lutetium clock measures the second about four times more accurately than any before it

The headline figure is an uncertainty of under a ten-billion-billionth of a second on each second the clock measures ^[2]. ScienceAlert restates it as a clock that would be out by less than a second after 300 billion years of running, a span it puts at roughly ten billion-billion seconds ^[3]. The conversion checks out. At about 31.6 million seconds a year, 300 billion years comes to about 9.5 billion-billion seconds ^[18]. "I am confident that what we have now is the most accurate clock in the world," physicist Murray Barrett said ^[4]. He went further: "In the future, I just don't see how this clock can be beat." ^[5]

That confidence rests on a comparison. The team built a second lutetium clock, corrected for general relativity because the two did not sit in exactly the same place, and measured one against the other ^[8]. "It basically tells you that the only way to test the accuracy of a standard is to compare clocks and demonstrate reproducibility," physicist Kyle Arnold said ^[10]. The 200 hours of measurement fell inside a 12-day window, about 69 percent of the 288 hours available ^{[9] [17]}.

The thing this doesn't tell you is whether both clocks share an error. Two units of one design, built by one group, would carry a common offset into both, and it would cancel when one reading is subtracted from the other ^[19]. I think the near-zero difference is solid evidence that the design reproduces, and that the fourfold accuracy claim will be on firmer ground once a clock of another kind sits on the other side of the comparison. The account does not say whether either clock was checked against one built on a different element, or whether lutetium is being weighed for any future definition of the second. The group has worked on lutetium for a decade and, according to ScienceAlert, is so far the only team to realize its timekeeping potential ^[12].

Cesium has been the traditional reference element for atomic clocks, and newer clocks built on elements such as strontium oscillate faster, packing more ticks into each second ^[11]. Lutetium's advantage is its insensitivity: its atomic structure is well shielded from heat, magnetism and motion, all of which disturb the electron transitions a clock counts ^[6]. "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 ^[7].

An accuracy record does not measure the first obstacle to field use. The clock currently takes up an entire lab, and the team wants it small enough to move, so it can be tested in more locations and put to work on jobs such as monitoring magma underground ^[14]. Clocks this precise also feed physics such as measuring how gravity changes across Earth ^[16]. Lutetium helps with the shrinking: the clock works at room temperature on off-the-shelf lasers, with no bulky cooling system ^[13]. "The next step is to take the lab-scale clock and miniaturize it into a transportable system," physicist Michael Lee said ^[15].