![]() ![]() The network consisted of an aluminum ion clock and a ytterbium clock, housed at the National Institute of Standards and Technology (NIST), and a strontium clock, located at the JILA research institute. The BACON collaboration 1 operated a network of three atomic clocks in Boulder, Colorado. They found that the two types of connections produced a similar level of uncertainty, except when no data could be obtained from the free-space connection during a snowstorm.įigure 1 | Compare a network of optical clocks. And in what they say is a world first, they compared the clocks using a 1.5 km ‘free space’ connection by sending laser pulses through the air between NIST and JILA along a straight line connecting the two institutions. They used a 3.6 kilometer fiber optic link – a proven method of connecting signals between optical clocks with the required uncertainty level. In particular, the authors related the clocks at NIST and JILA in two ways (Figure 1). The BACON Collaboration used redundant elements across the network of optical clocks to check for sources of bias. Nevertheless, the authors developed a statistical model to account for these uncertainties and to allow a thorough assessment of the overall measurement uncertainty. This observation suggests the existence of unknown effects, which are intrinsically difficult to quantify. They found that the daily variation in the ratios was slightly greater than expected, taking into account all known effects. ![]() The authors compared the aluminum ion and ytterbium clocks at NIST and the strontium clock at JILA over several months to check reproducibility and reduce statistical uncertainties. Therefore, exceptional care is required to control all sources of frequency hopping. Such frequency ratio measurements are not easy and correspond to determining the distance from the Earth to the Moon to within a few nanometers. So far the best comparisons between optical clocks based on different atoms 6 11 are at the level of parts in 10 17The BACON Collaboration presents measurements of optical frequency ratios that reach uncertainties at the level of parts in 10 18, bringing the redefinition of the SI second one step closer. The target accuracy for these equations is at the level of parts in 10 18 to clearly demonstrate the superiority of optical clocks over cesium clocks 5Ĭlock comparisons are performed by measuring the ratios of the optical clock frequencies using instruments called femtosecond frequency combs. But before such a redefinition is possible, scientists must build confidence in the reproducibility of optical clocks through a series of clock equations. There is therefore a desire to redefine the SI second in terms of an optical clock frequency and to move away from the current cesium-based definition. Such optical clocks are said to be 100 times more accurate than cesium clocks 5 The measured frequencies of all three clocks are estimated to be correct within a fractional uncertainty of 2 parts out of 10 18 or better 2 4In principle, this level of uncertainty would allow the clocks to keep time so accurately that they would not gain or lose more than a second over the age of the Universe. Three of the world’s best optical clocks are the aluminum ion and ytterbium clocks at NIST and the strontium clock at JILA. Clocks based on different atoms run at different speeds, and the term ‘optical clock’ refers to a clock that operates at an optical frequency. The authors show how their clock equations provide insight into fundamental physics and represent significant progress in redefining the second in the International System of Units (SI).Ītomic clocks ‘tick’ at a rate determined by the frequency of light emitted or absorbed when an atom changes from one energy state to another. Register in Nature, the Boulder Atomic Clock Optical Network (BACON) Collaboration 1 reports highly accurate comparisons of three world-leading clocks in Boulder, Colorado, housed at the National Institute of Standards and Technology (NIST) and the JILA research institute. The remarkable accuracy of atomic clocks makes them excellent tools for timekeeping and other precision measurements.
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