5/1/2023 0 Comments Atomic clocksOne issue the road map does not address, Ludlow says, is how to choose which atomic transition-and hence type of clock-will ultimately come to define the second. Ludlow says the measurements are not far off the desired accuracy, and that once they've finished they can compare their results with labs in Europe or Asia. The NIST scientists and colleagues at JILA, a research institute down the road in Boulder, are doing just that, comparing the ytterbium clocks to others that rely on strontium atoms and aluminum ions. Another check involves comparing the ticking of different types of atomic clocks. The BIPM road map calls for a number of cross-checks, including reaching the required accuracy with clocks in different labs. Jérôme Lodewyck, a physicist working on strontium lattice clocks at the Paris Observatory's Time-Space Reference Systems lab, says the NIST result "is probably correct," but that when it comes to changing the second, "probably correct is not good enough." He notes that something unexpected-perhaps a stray electric field-could throw off two clocks in the same lab by the same amount, making the drift undetectable. "It would be the first time that two clocks of the same species have been shown to agree at that level," Ludlow says.īeing a cautious bunch, metrologists will not accept the NIST result at face value. The researchers found that the two clocks ticked at the same rate to within 1.4 parts in 10 18-just over 100 times better than the top cesium devices. The NIST team operated two optical clocks, using several lasers to cool and trap a few thousand ytterbium atoms in an "optical lattice" and then excite a particular energy transition in those atoms. And in unpublished work, Andrew Ludlow and colleagues at NIST appear to have reached the accuracy stipulated by BIPM's first milestone. On 14 February it published a paper in its journal, Metrologia, setting out five milestones that should be met before the second can be redefined based on visible light. Improvements in laser technology led BIPM a few years ago to begin reviewing the accuracy of optical clocks. However, the lasers needed to cool atoms and provide a stable reference at these frequencies are a challenge to build. The frequency of visible light is about 100,000 times higher than that of microwaves, promising even more precision. Today, the best cesium clocks have accuracies of 1.6 parts in 10 16. In 1967, the second was defined as 9,192,631,770 cycles of a beam tuned to the cesium standard. Current atomic clocks depend on the oscillations of a microwave beam at the precise frequency needed to excite atoms of cesium-133 to a higher energy level. A grandfather clock relies on the regular swings of a pendulum, and the original definition of the second was based on the length of a day as fixed by Earth's spin. Already, physicists at the National Institute of Standards and Technology (NIST) Boulder Laboratories in Colorado appear to have satisfied one of the road map's key requirements-a 100-fold improvement in accuracy over the best microwave clocks-using a pair of optical clocks.Ĭlocks mark time by tracking a periodic action. In a paper out last month, a group of experts set up by the International Bureau of Weights and Measures (BIPM) in Sèvres, France, lays out a road map for the steps needed to redefine the unit of time-the metric second-in terms of optical radiation. So the push is on to replace current clocks, which are tuned to a specific microwave frequency, with even better clocks that exploit higher-frequency visible light. A more precise time standard might improve the navigation of spacecraft and help experimenters look for variations in fundamental constants that would signal new physics. The atomic clocks that mark official time lose the equivalent of just 1 second every 200 million years.
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